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Phase differences in direct conversion receivers
I'm curious... with the current popularity of simple (e.g., QRP usage)
direct conversion receivers, whatever happened to the problem of having to synchronize the cariier phases? I'm looking at Experimental Methods in RF Design, and they just use an LC oscillator for the input to the mixer. If input carrier is cos(f*t) and the LC oscillator is generating cos(f*t+phi), where phi is the phase offset between them, you end up with a cos(phi) term coming out of the mixer. If the frequencies are ever-so-slightly off, phi essentially varies slowly and cos(phi) should slowly cause the signal to fade in and out. Why isn't this a problem in practice? Thanks, ---Joel Kolstad |
This is indeed what happens only if the VFO and an incoming single are
at almost the same frequency ("zero beat"). However, in practice, if the signal is a cw signal, we listen to a signal that is 600 Hz or so away from the VFO so that we hear the 600 Hz tone difference. For a SSB signal, we listen to the audio content contained in the sideband, which is 300 Hz to 3 KHz away from the VFO signal when it is tuned in correctly. - Dan, N7VE Joel Kolstad wrote: I'm curious... with the current popularity of simple (e.g., QRP usage) direct conversion receivers, whatever happened to the problem of having to synchronize the cariier phases? I'm looking at Experimental Methods in RF Design, and they just use an LC oscillator for the input to the mixer. If input carrier is cos(f*t) and the LC oscillator is generating cos(f*t+phi), where phi is the phase offset between them, you end up with a cos(phi) term coming out of the mixer. If the frequencies are ever-so-slightly off, phi essentially varies slowly and cos(phi) should slowly cause the signal to fade in and out. Why isn't this a problem in practice? Thanks, ---Joel Kolstad |
This is indeed what happens only if the VFO and an incoming single are
at almost the same frequency ("zero beat"). However, in practice, if the signal is a cw signal, we listen to a signal that is 600 Hz or so away from the VFO so that we hear the 600 Hz tone difference. For a SSB signal, we listen to the audio content contained in the sideband, which is 300 Hz to 3 KHz away from the VFO signal when it is tuned in correctly. - Dan, N7VE Joel Kolstad wrote: I'm curious... with the current popularity of simple (e.g., QRP usage) direct conversion receivers, whatever happened to the problem of having to synchronize the cariier phases? I'm looking at Experimental Methods in RF Design, and they just use an LC oscillator for the input to the mixer. If input carrier is cos(f*t) and the LC oscillator is generating cos(f*t+phi), where phi is the phase offset between them, you end up with a cos(phi) term coming out of the mixer. If the frequencies are ever-so-slightly off, phi essentially varies slowly and cos(phi) should slowly cause the signal to fade in and out. Why isn't this a problem in practice? Thanks, ---Joel Kolstad |
Dan Tayloe wrote:
This is indeed what happens only if the VFO and an incoming single are at almost the same frequency ("zero beat"). However, in practice, if the signal is a cw signal, we listen to a signal that is 600 Hz or so away from the VFO so that we hear the 600 Hz tone difference. ....or at least, say, 595-605Hz is the local oscillator tends to drift +/-5Hz over time, eh? Good enough. With SSB, presumably you have the same 'problem' -- the entire voice signal is shifted in pitch by the difference between the LO and the real carrier. In fact, with SSB and direct conversion, how do you even decide you have the correct LO frequency? Just when people sound 'most natural?' Thanks, ---Joel Kolstad |
Dan Tayloe wrote:
This is indeed what happens only if the VFO and an incoming single are at almost the same frequency ("zero beat"). However, in practice, if the signal is a cw signal, we listen to a signal that is 600 Hz or so away from the VFO so that we hear the 600 Hz tone difference. ....or at least, say, 595-605Hz is the local oscillator tends to drift +/-5Hz over time, eh? Good enough. With SSB, presumably you have the same 'problem' -- the entire voice signal is shifted in pitch by the difference between the LO and the real carrier. In fact, with SSB and direct conversion, how do you even decide you have the correct LO frequency? Just when people sound 'most natural?' Thanks, ---Joel Kolstad |
This is a problem in general in direct conversion schemes: and this is the
reason that "quadrature" detectors are made, with two mixing channels 90 degrees apart, so that the phasing is no longer a problem. (the sqrt of sum of squares of the signal out of the two channels (or "magnitude") is not sensitive to phase.) Cliff "Joel Kolstad" wrote in message ... I'm curious... with the current popularity of simple (e.g., QRP usage) direct conversion receivers, whatever happened to the problem of having to synchronize the cariier phases? I'm looking at Experimental Methods in RF Design, and they just use an LC oscillator for the input to the mixer. If input carrier is cos(f*t) and the LC oscillator is generating cos(f*t+phi), where phi is the phase offset between them, you end up with a cos(phi) term coming out of the mixer. If the frequencies are ever-so-slightly off, phi essentially varies slowly and cos(phi) should slowly cause the signal to fade in and out. Why isn't this a problem in practice? Thanks, ---Joel Kolstad |
This is a problem in general in direct conversion schemes: and this is the
reason that "quadrature" detectors are made, with two mixing channels 90 degrees apart, so that the phasing is no longer a problem. (the sqrt of sum of squares of the signal out of the two channels (or "magnitude") is not sensitive to phase.) Cliff "Joel Kolstad" wrote in message ... I'm curious... with the current popularity of simple (e.g., QRP usage) direct conversion receivers, whatever happened to the problem of having to synchronize the cariier phases? I'm looking at Experimental Methods in RF Design, and they just use an LC oscillator for the input to the mixer. If input carrier is cos(f*t) and the LC oscillator is generating cos(f*t+phi), where phi is the phase offset between them, you end up with a cos(phi) term coming out of the mixer. If the frequencies are ever-so-slightly off, phi essentially varies slowly and cos(phi) should slowly cause the signal to fade in and out. Why isn't this a problem in practice? Thanks, ---Joel Kolstad |
Cliff Curry wrote:
This is a problem in general in direct conversion schemes: and this is the reason that "quadrature" detectors are made, with two mixing channels 90 degrees apart, so that the phasing is no longer a problem. (the sqrt of sum of squares of the signal out of the two channels (or "magnitude") is not sensitive to phase.) Hmm... I went through the math, and indeed, this is the case! But this then begs the question: Since the quadrature detector obtains the correct magnitude of the transmitted signal for ANY phase difference between the carrier and the LO, and if we model the phase difference as a function of time that slowly changes due to the fact that, in actuality, our LO isn't _quite_ the same frequency as the carrier, will the system still work? This almost seems too good to be true... Thanks, ---Joel Kolstad |
Cliff Curry wrote:
This is a problem in general in direct conversion schemes: and this is the reason that "quadrature" detectors are made, with two mixing channels 90 degrees apart, so that the phasing is no longer a problem. (the sqrt of sum of squares of the signal out of the two channels (or "magnitude") is not sensitive to phase.) Hmm... I went through the math, and indeed, this is the case! But this then begs the question: Since the quadrature detector obtains the correct magnitude of the transmitted signal for ANY phase difference between the carrier and the LO, and if we model the phase difference as a function of time that slowly changes due to the fact that, in actuality, our LO isn't _quite_ the same frequency as the carrier, will the system still work? This almost seems too good to be true... Thanks, ---Joel Kolstad |
Of course, if you were demodulating DSB suppressed carrier and you
injected the carrier at the wrong phase, you indeed would get the two sidebands going through constructive and destructive phases. If you're 90 degrees out with your LO, it looks a lot like narrowband FM, though very slightly different as I posted in the thread on SSB-FM. If you do the quadrature detector thing with DSB-suppressed carrier, then when one of the two is just the wrong phase (and you get no output from that one), the other will be just the right phase, and vice-versa. When it's in between, does it work out right to just sum the two? I suppose so, though it's worth going through the math to make sure. And of course, with quadrature mixers, you can combine the outputs with audio phase shifting to select just one of the two sidebands (or just CW signals on one side of the LO). In fact, the mixer LO inputs don't have to be exactly in quadratu it's possible to apply a calibration to account for a phase error (and also an amplitude error, where the gain through one mixer path is slightly different from the gain through the other). That's all practical to do digitally...we do that sort of thing at 100 megasamples per second with some custom chips. Cheers, Tom "Joel Kolstad" wrote in message ... Dan Tayloe wrote: This is indeed what happens only if the VFO and an incoming single are at almost the same frequency ("zero beat"). However, in practice, if the signal is a cw signal, we listen to a signal that is 600 Hz or so away from the VFO so that we hear the 600 Hz tone difference. ...or at least, say, 595-605Hz is the local oscillator tends to drift +/-5Hz over time, eh? Good enough. With SSB, presumably you have the same 'problem' -- the entire voice signal is shifted in pitch by the difference between the LO and the real carrier. In fact, with SSB and direct conversion, how do you even decide you have the correct LO frequency? Just when people sound 'most natural?' Thanks, ---Joel Kolstad |
Of course, if you were demodulating DSB suppressed carrier and you
injected the carrier at the wrong phase, you indeed would get the two sidebands going through constructive and destructive phases. If you're 90 degrees out with your LO, it looks a lot like narrowband FM, though very slightly different as I posted in the thread on SSB-FM. If you do the quadrature detector thing with DSB-suppressed carrier, then when one of the two is just the wrong phase (and you get no output from that one), the other will be just the right phase, and vice-versa. When it's in between, does it work out right to just sum the two? I suppose so, though it's worth going through the math to make sure. And of course, with quadrature mixers, you can combine the outputs with audio phase shifting to select just one of the two sidebands (or just CW signals on one side of the LO). In fact, the mixer LO inputs don't have to be exactly in quadratu it's possible to apply a calibration to account for a phase error (and also an amplitude error, where the gain through one mixer path is slightly different from the gain through the other). That's all practical to do digitally...we do that sort of thing at 100 megasamples per second with some custom chips. Cheers, Tom "Joel Kolstad" wrote in message ... Dan Tayloe wrote: This is indeed what happens only if the VFO and an incoming single are at almost the same frequency ("zero beat"). However, in practice, if the signal is a cw signal, we listen to a signal that is 600 Hz or so away from the VFO so that we hear the 600 Hz tone difference. ...or at least, say, 595-605Hz is the local oscillator tends to drift +/-5Hz over time, eh? Good enough. With SSB, presumably you have the same 'problem' -- the entire voice signal is shifted in pitch by the difference between the LO and the real carrier. In fact, with SSB and direct conversion, how do you even decide you have the correct LO frequency? Just when people sound 'most natural?' Thanks, ---Joel Kolstad |
With all this discussion of phasing fun... could someone answer the
following question for me? Say I'm transmitting binaural audio, with I being L and Q being R. I receive this signal and generate my own I' and Q' outputs. However, if the RF carrier and my LO have a phase difference, the entire IQ (phasor) diagram is rotated by that difference and, e.g., a 90 degree difference will result in the left and right channels I receive being swapped. How do IQ-binaural receivers recover a phase lock to present this? If you do the quadrature detector thing with DSB-suppressed carrier, then when one of the two is just the wrong phase (and you get no output from that one), the other will be just the right phase, and vice-versa. When it's in between, does it work out right to just sum the two? I suppose so, though it's worth going through the math to make sure. I went through the math and you end up with the magnitude of the original signal. What's unclear to me is how to recover the phase offset between your signal and the original -- although adding a DC component (or some other unique frequency component) to either I or Q (or placed at some strategic angle between them) would allow you to synchronize the phases. Have any suggestions for a nice simple mixer (ala the NE602) that retains both the I and Q signals at the output? ---Joel Kolstad |
With all this discussion of phasing fun... could someone answer the
following question for me? Say I'm transmitting binaural audio, with I being L and Q being R. I receive this signal and generate my own I' and Q' outputs. However, if the RF carrier and my LO have a phase difference, the entire IQ (phasor) diagram is rotated by that difference and, e.g., a 90 degree difference will result in the left and right channels I receive being swapped. How do IQ-binaural receivers recover a phase lock to present this? If you do the quadrature detector thing with DSB-suppressed carrier, then when one of the two is just the wrong phase (and you get no output from that one), the other will be just the right phase, and vice-versa. When it's in between, does it work out right to just sum the two? I suppose so, though it's worth going through the math to make sure. I went through the math and you end up with the magnitude of the original signal. What's unclear to me is how to recover the phase offset between your signal and the original -- although adding a DC component (or some other unique frequency component) to either I or Q (or placed at some strategic angle between them) would allow you to synchronize the phases. Have any suggestions for a nice simple mixer (ala the NE602) that retains both the I and Q signals at the output? ---Joel Kolstad |
"Joel Kolstad" wrote in message ...
With all this discussion of phasing fun... could someone answer the following question for me? Say I'm transmitting binaural audio, with I being L and Q being R. I receive this signal and generate my own I' and Q' outputs. However, if the RF carrier and my LO have a phase difference, the entire IQ (phasor) diagram is rotated by that difference and, e.g., a 90 degree difference will result in the left and right channels I receive being swapped. How do IQ-binaural receivers recover a phase lock to present this? For standard broadcast, don't they always put L+R on I and L-R on Q, so standard receivers get L+R? All this is not my forte; I know only enough to be dangerous with it. But I assume that in any transmission standard, there is something transmitted that lets you recover the right phase at the receiver. If you're sending symbols, presumably there can be some symbol you transmit periodically to insure things stay synched, a complex version of the old RS-232 start and stop bits if you will. For analog signals, you can transmit some sort of pilot tone, perhaps the carrier itself, which of course must be done for compatible AM anyway. If you do the quadrature detector thing with DSB-suppressed carrier, then when one of the two is just the wrong phase (and you get no output from that one), the other will be just the right phase, and vice-versa. When it's in between, does it work out right to just sum the two? I suppose so, though it's worth going through the math to make sure. I went through the math and you end up with the magnitude of the original signal. What's unclear to me is how to recover the phase offset between your signal and the original -- although adding a DC component (or some other unique frequency component) to either I or Q (or placed at some strategic angle between them) would allow you to synchronize the phases. Have any suggestions for a nice simple mixer (ala the NE602) that retains both the I and Q signals at the output? One easy way is with a "Tayloe mixer" -- you should be able to find info on that on the web, but it's basically just a commutating switch that switches the signal (through its source resistance) among four different capacitors. The I and Q outputs are V(C1)-V(C3) and V(C2)-v(C4) respectively. It must be driven with a "LO" at four times the detected frequency: that is, the switch must rotate through all four positions in one cycle of what would normally be considered the LO frequency. If you use care in its construction, you should be able to get very good balance. The size of the capacitors determines the bandwidth. There are commercial quadrature active mixers, too, but they typically cover a modest frequency range in the VHF region or higher. Cheers, Tom |
"Joel Kolstad" wrote in message ...
With all this discussion of phasing fun... could someone answer the following question for me? Say I'm transmitting binaural audio, with I being L and Q being R. I receive this signal and generate my own I' and Q' outputs. However, if the RF carrier and my LO have a phase difference, the entire IQ (phasor) diagram is rotated by that difference and, e.g., a 90 degree difference will result in the left and right channels I receive being swapped. How do IQ-binaural receivers recover a phase lock to present this? For standard broadcast, don't they always put L+R on I and L-R on Q, so standard receivers get L+R? All this is not my forte; I know only enough to be dangerous with it. But I assume that in any transmission standard, there is something transmitted that lets you recover the right phase at the receiver. If you're sending symbols, presumably there can be some symbol you transmit periodically to insure things stay synched, a complex version of the old RS-232 start and stop bits if you will. For analog signals, you can transmit some sort of pilot tone, perhaps the carrier itself, which of course must be done for compatible AM anyway. If you do the quadrature detector thing with DSB-suppressed carrier, then when one of the two is just the wrong phase (and you get no output from that one), the other will be just the right phase, and vice-versa. When it's in between, does it work out right to just sum the two? I suppose so, though it's worth going through the math to make sure. I went through the math and you end up with the magnitude of the original signal. What's unclear to me is how to recover the phase offset between your signal and the original -- although adding a DC component (or some other unique frequency component) to either I or Q (or placed at some strategic angle between them) would allow you to synchronize the phases. Have any suggestions for a nice simple mixer (ala the NE602) that retains both the I and Q signals at the output? One easy way is with a "Tayloe mixer" -- you should be able to find info on that on the web, but it's basically just a commutating switch that switches the signal (through its source resistance) among four different capacitors. The I and Q outputs are V(C1)-V(C3) and V(C2)-v(C4) respectively. It must be driven with a "LO" at four times the detected frequency: that is, the switch must rotate through all four positions in one cycle of what would normally be considered the LO frequency. If you use care in its construction, you should be able to get very good balance. The size of the capacitors determines the bandwidth. There are commercial quadrature active mixers, too, but they typically cover a modest frequency range in the VHF region or higher. Cheers, Tom |
Tom Bruhns wrote:
"Joel Kolstad" wrote in message ... For standard broadcast, don't they always put L+R on I and L-R on Q, so standard receivers get L+R? They may well -- I'm not sure which 'standard' we're talking about here (there seem to be several out there...). :-) But note that with L+R on I and L-R on Q, you only get L+R if you manage to synchronize with the phase (the original problem)! If you use a quadrature detector and extract the magnitude, you of course get sqrt(L^2+R^2) -- good, but not exactly L+R either. The Motorola C-QUAM AM stereo system forces the magnitude of the I-Q phasor to be L+R, which makes it compatible with both envelope detectors and quadrature detectors. (I probably sound like a Motorola sales guy these day... I just think it's clever and the end goal here is to build a direct conversion receiver to decode it...) But I assume that in any transmission standard, there is something transmitted that lets you recover the right phase at the receiver. For AM, it would appear that the carrier itself is what people use for that purpose. If it very clear to me now why TV transmission need to send the colorburst sequence these days -- otherwise they'd have no way to synchronize the chroma decoder, which is taking in a DSB-SC modulated in both I and Q. One easy way is with a "Tayloe mixer" -- you should be able to find info on that on the web, but it's basically just a commutating switch that switches the signal (through its source resistance) among four different capacitors. I was just thinking of doing something like this -- from a square wave, get a 4 bit shift register (that always has one output active) and feed the outputs to a 4066 such that I sample the 'I' channel during, say, clocks #1 and #4 and the 'Q' channel during clocks #1 and #2. Add some filtering and -- presto chango! -- we've got I and Q. Thanks for all the help -- this just might end up working after all. ---Joel Kolstad |
Tom Bruhns wrote:
"Joel Kolstad" wrote in message ... For standard broadcast, don't they always put L+R on I and L-R on Q, so standard receivers get L+R? They may well -- I'm not sure which 'standard' we're talking about here (there seem to be several out there...). :-) But note that with L+R on I and L-R on Q, you only get L+R if you manage to synchronize with the phase (the original problem)! If you use a quadrature detector and extract the magnitude, you of course get sqrt(L^2+R^2) -- good, but not exactly L+R either. The Motorola C-QUAM AM stereo system forces the magnitude of the I-Q phasor to be L+R, which makes it compatible with both envelope detectors and quadrature detectors. (I probably sound like a Motorola sales guy these day... I just think it's clever and the end goal here is to build a direct conversion receiver to decode it...) But I assume that in any transmission standard, there is something transmitted that lets you recover the right phase at the receiver. For AM, it would appear that the carrier itself is what people use for that purpose. If it very clear to me now why TV transmission need to send the colorburst sequence these days -- otherwise they'd have no way to synchronize the chroma decoder, which is taking in a DSB-SC modulated in both I and Q. One easy way is with a "Tayloe mixer" -- you should be able to find info on that on the web, but it's basically just a commutating switch that switches the signal (through its source resistance) among four different capacitors. I was just thinking of doing something like this -- from a square wave, get a 4 bit shift register (that always has one output active) and feed the outputs to a 4066 such that I sample the 'I' channel during, say, clocks #1 and #4 and the 'Q' channel during clocks #1 and #2. Add some filtering and -- presto chango! -- we've got I and Q. Thanks for all the help -- this just might end up working after all. ---Joel Kolstad |
In article , "Joel Kolstad"
writes: With all this discussion of phasing fun... could someone answer the following question for me? Say I'm transmitting binaural audio, with I being L and Q being R. I receive this signal and generate my own I' and Q' outputs. However, if the RF carrier and my LO have a phase difference, the entire IQ (phasor) diagram is rotated by that difference and, e.g., a 90 degree difference will result in the left and right channels I receive being swapped. How do IQ-binaural receivers recover a phase lock to present this? You are using an example of separation of conventional AM sidebands. Phase synchronization to the carrier can be done separately or by using parts of the multi-mixer I-Q circuitry. Phase synchronization is not absolutely necessary for listening and still hearing separate sidebands. Relative phase in mixers is NOT disturbed. (basic fact) Single-sideband phasing systems use at least two mixers, the LO of one in quadrature (90 degrees) phase with the other LO. Since the LO frequency is the same, the two mixers' output will have a relative phase difference of 90 degrees. In addition to that, the mixer outputs are put through an audio- frequency-range wideband phasing network. The Gingell 4-phase network is ideal for this (it works fine with just two phase inputs). With the Yoshida value optimization, the Gingell network can be made with excellent constant-relative-phase-quadrature over a broad audio frequency range without using precision tolerance parts. The "trick" now is to linearly combine two of the four audio phases such that the TOTAL relative phase shift is 0 degrees (or very nearly so). With the LO having a relative phase differential of 90 degrees the audio output of the mixers will have a differential phase of 90 degrees. Since the audio polyphase network provides additional 90 degrees relative phase difference, the total is 180 degrees...or 0 degrees if an inverting unity gain amplifier is used. So, what happens if the LO isn't "locked" to the incoming carrier? Actually, very little. If the 2 LOs remain in quadrature phase realation- ship, the two mixer output relative phase relationships are STILL in quadrature. The only thing that has changed is the slight frequency difference in the mixer outputs relative to the original modulation frequency. This has no effect on any broadband audio phasing network following the mixer outputs...those maintain the additional quadrature relative phase and linear addition and subtraction will be the same. Unwanted sideband AND carrier suppression in demodulation will be essentially unaffected. In going back through messages after a short absence, I detect some worry about a slow "beat" effect if the LO isn't synchronized. That's not a real worry if you've gone through the full expansion of the basic AM equation and shifted the whole series in phase by 90 degrees, then did a linear comparison with the same series unshifted in phase, then taking the TWO audio frequency components from the series and did a linear addition or subtraction with an additional 90 degree relative difference. Synchronization-to-the-carrier-frequency-and-phase is necessary only with conventional AM and the audio circuitry being broadband all the way down to DC. There are several ways to make the DC component represented by the carrier mixed down to baseband either disappear or reduce greatly in value. Hint: Using the full series expansion, use a small phase shift error and note the comparisons of all series components, including the carrier. With SSB, there's no real worry since the transmitting end carriers are already reduced reduced in amplitude. If the LO isn't quite in sync or even not on-frequency, all that will be noticed is the slight change in demodulated audio frequency relative to original frequency. The amount of rejection of RF in the unwanted side of the carrier will vary by the error of exact LO quadrature and the error of the phasing network quadrature. On conventional AM, those errors are the same as the isolation between sideband modulation content. If the AM has left ear content on lower sideband and right ear content on upper, that isolation is the same as "left-ear v. right-ear separation of stereo." Have any suggestions for a nice simple mixer (ala the NE602) that retains both the I and Q signals at the output? "A" mixer, no. You must use at least two to get an In-phase and Quadrature output. The Tayloe detector has several advantages. First, the CMOS switch can be driven with 4 phases, not just 2, and the equivalent conversion transconductance is far higher than any passive mixer; mixer noise is also reduced relative to active mixers due to the nature of the CMOS switch structure. With four phases in the output, all at 90 degree multiples, it fits the Gingell-Yoshida polyphase network just dandy such that quadrature errors from network components are greatly diminished. The original Gingell polyphase network as described by Peter Martinez* in RSGB's Radio Communication magazine in 1973 had only 0 and 180 degree audio phase differences at the input. The network outputs were still at 90 degree multiples over a wide audio bandwidth. That bandwidth will increase and with less error when inputs are already at four phases of relative quadrature. The only disadvantage of the Tayloe mixer is the need to use a 4x frequency master LO if the four phases are derived digitally for broad tuning range. *G3PLX, the same that inovated PSK31 some years later. Len Anderson retired (from regular hours) electronic engineer person |
In article , "Joel Kolstad"
writes: With all this discussion of phasing fun... could someone answer the following question for me? Say I'm transmitting binaural audio, with I being L and Q being R. I receive this signal and generate my own I' and Q' outputs. However, if the RF carrier and my LO have a phase difference, the entire IQ (phasor) diagram is rotated by that difference and, e.g., a 90 degree difference will result in the left and right channels I receive being swapped. How do IQ-binaural receivers recover a phase lock to present this? You are using an example of separation of conventional AM sidebands. Phase synchronization to the carrier can be done separately or by using parts of the multi-mixer I-Q circuitry. Phase synchronization is not absolutely necessary for listening and still hearing separate sidebands. Relative phase in mixers is NOT disturbed. (basic fact) Single-sideband phasing systems use at least two mixers, the LO of one in quadrature (90 degrees) phase with the other LO. Since the LO frequency is the same, the two mixers' output will have a relative phase difference of 90 degrees. In addition to that, the mixer outputs are put through an audio- frequency-range wideband phasing network. The Gingell 4-phase network is ideal for this (it works fine with just two phase inputs). With the Yoshida value optimization, the Gingell network can be made with excellent constant-relative-phase-quadrature over a broad audio frequency range without using precision tolerance parts. The "trick" now is to linearly combine two of the four audio phases such that the TOTAL relative phase shift is 0 degrees (or very nearly so). With the LO having a relative phase differential of 90 degrees the audio output of the mixers will have a differential phase of 90 degrees. Since the audio polyphase network provides additional 90 degrees relative phase difference, the total is 180 degrees...or 0 degrees if an inverting unity gain amplifier is used. So, what happens if the LO isn't "locked" to the incoming carrier? Actually, very little. If the 2 LOs remain in quadrature phase realation- ship, the two mixer output relative phase relationships are STILL in quadrature. The only thing that has changed is the slight frequency difference in the mixer outputs relative to the original modulation frequency. This has no effect on any broadband audio phasing network following the mixer outputs...those maintain the additional quadrature relative phase and linear addition and subtraction will be the same. Unwanted sideband AND carrier suppression in demodulation will be essentially unaffected. In going back through messages after a short absence, I detect some worry about a slow "beat" effect if the LO isn't synchronized. That's not a real worry if you've gone through the full expansion of the basic AM equation and shifted the whole series in phase by 90 degrees, then did a linear comparison with the same series unshifted in phase, then taking the TWO audio frequency components from the series and did a linear addition or subtraction with an additional 90 degree relative difference. Synchronization-to-the-carrier-frequency-and-phase is necessary only with conventional AM and the audio circuitry being broadband all the way down to DC. There are several ways to make the DC component represented by the carrier mixed down to baseband either disappear or reduce greatly in value. Hint: Using the full series expansion, use a small phase shift error and note the comparisons of all series components, including the carrier. With SSB, there's no real worry since the transmitting end carriers are already reduced reduced in amplitude. If the LO isn't quite in sync or even not on-frequency, all that will be noticed is the slight change in demodulated audio frequency relative to original frequency. The amount of rejection of RF in the unwanted side of the carrier will vary by the error of exact LO quadrature and the error of the phasing network quadrature. On conventional AM, those errors are the same as the isolation between sideband modulation content. If the AM has left ear content on lower sideband and right ear content on upper, that isolation is the same as "left-ear v. right-ear separation of stereo." Have any suggestions for a nice simple mixer (ala the NE602) that retains both the I and Q signals at the output? "A" mixer, no. You must use at least two to get an In-phase and Quadrature output. The Tayloe detector has several advantages. First, the CMOS switch can be driven with 4 phases, not just 2, and the equivalent conversion transconductance is far higher than any passive mixer; mixer noise is also reduced relative to active mixers due to the nature of the CMOS switch structure. With four phases in the output, all at 90 degree multiples, it fits the Gingell-Yoshida polyphase network just dandy such that quadrature errors from network components are greatly diminished. The original Gingell polyphase network as described by Peter Martinez* in RSGB's Radio Communication magazine in 1973 had only 0 and 180 degree audio phase differences at the input. The network outputs were still at 90 degree multiples over a wide audio bandwidth. That bandwidth will increase and with less error when inputs are already at four phases of relative quadrature. The only disadvantage of the Tayloe mixer is the need to use a 4x frequency master LO if the four phases are derived digitally for broad tuning range. *G3PLX, the same that inovated PSK31 some years later. Len Anderson retired (from regular hours) electronic engineer person |
This gets to the question of whether DC receivers can be used to copy
DSB and SSB: By Goodman, W1DX, explained the problem in the 1965 edition of "Single Sideband for the Radio Amateur" (page 11): "Unfortunately, if both sidebands are received at the detector where the carrier is introduced, the carrier has to have exactly the correct phase relationship with the sidebands if distortion is to be avoided. Since exact phase relationship precludes even the slightest frequency error, such a system is workable only with very complicated receiving techniques. However, if only one sideband is present at the detector, there is no need for an exact phase relationship and there can be some frequency error without destroying intelligibility. " Modern SSB transcievers send only one of the sidebands to the detector, so this distortion problem only occurs when receiving a DSB signal on a receiver that sends both sidebands to the detector. 73 Bill M0HBR "Joel Kolstad" wrote in message ... I'm curious... with the current popularity of simple (e.g., QRP usage) direct conversion receivers, whatever happened to the problem of having to synchronize the cariier phases? I'm looking at Experimental Methods in RF Design, and they just use an LC oscillator for the input to the mixer. If input carrier is cos(f*t) and the LC oscillator is generating cos(f*t+phi), where phi is the phase offset between them, you end up with a cos(phi) term coming out of the mixer. If the frequencies are ever-so-slightly off, phi essentially varies slowly and cos(phi) should slowly cause the signal to fade in and out. Why isn't this a problem in practice? Thanks, ---Joel Kolstad |
This gets to the question of whether DC receivers can be used to copy
DSB and SSB: By Goodman, W1DX, explained the problem in the 1965 edition of "Single Sideband for the Radio Amateur" (page 11): "Unfortunately, if both sidebands are received at the detector where the carrier is introduced, the carrier has to have exactly the correct phase relationship with the sidebands if distortion is to be avoided. Since exact phase relationship precludes even the slightest frequency error, such a system is workable only with very complicated receiving techniques. However, if only one sideband is present at the detector, there is no need for an exact phase relationship and there can be some frequency error without destroying intelligibility. " Modern SSB transcievers send only one of the sidebands to the detector, so this distortion problem only occurs when receiving a DSB signal on a receiver that sends both sidebands to the detector. 73 Bill M0HBR "Joel Kolstad" wrote in message ... I'm curious... with the current popularity of simple (e.g., QRP usage) direct conversion receivers, whatever happened to the problem of having to synchronize the cariier phases? I'm looking at Experimental Methods in RF Design, and they just use an LC oscillator for the input to the mixer. If input carrier is cos(f*t) and the LC oscillator is generating cos(f*t+phi), where phi is the phase offset between them, you end up with a cos(phi) term coming out of the mixer. If the frequencies are ever-so-slightly off, phi essentially varies slowly and cos(phi) should slowly cause the signal to fade in and out. Why isn't this a problem in practice? Thanks, ---Joel Kolstad |
Bill Meara wrote:
This gets to the question of whether DC receivers can be used to copy DSB and SSB: By Goodman, W1DX, explained the problem in the 1965 edition of "Single Sideband for the Radio Amateur" (page 11): "Unfortunately, if both sidebands are received at the detector where the carrier is introduced, the carrier has to have exactly the correct phase relationship with the sidebands if distortion is to be avoided. Since exact phase relationship precludes even the slightest frequency error, such a system is workable only with very complicated receiving techniques. In 1965 I can imagine that a Costas loop, two mixers, etc. was considered 'very complicated.' It doesn't seem all that horribly fancy by today's standards, however. But of course it's not like I've actually _built_ such a thing yet! :-) However, if only one sideband is present at the detector, there is no need for an exact phase relationship and there can be some frequency error without destroying intelligibility. " Modern SSB transcievers send only one of the sidebands to the detector, so this distortion problem only occurs when receiving a DSB signal on a receiver that sends both sidebands to the detector. It's ironic that DSB, which came about due to the ease of detection with diode (envelope detectors) turns out to be somewhat challenging to recover with a more sophisticated synchronous detection scheme. Experimental Methods in RF Design points out that direct conversion receivers have become highly popular in the past couple of decades... this seems somewhat surprising; I would have guessed people back in the, e.g., '60s, would have gone to great lengths to avoid image reject filters and long IF chains. ---Joel Kolstad |
Bill Meara wrote:
This gets to the question of whether DC receivers can be used to copy DSB and SSB: By Goodman, W1DX, explained the problem in the 1965 edition of "Single Sideband for the Radio Amateur" (page 11): "Unfortunately, if both sidebands are received at the detector where the carrier is introduced, the carrier has to have exactly the correct phase relationship with the sidebands if distortion is to be avoided. Since exact phase relationship precludes even the slightest frequency error, such a system is workable only with very complicated receiving techniques. In 1965 I can imagine that a Costas loop, two mixers, etc. was considered 'very complicated.' It doesn't seem all that horribly fancy by today's standards, however. But of course it's not like I've actually _built_ such a thing yet! :-) However, if only one sideband is present at the detector, there is no need for an exact phase relationship and there can be some frequency error without destroying intelligibility. " Modern SSB transcievers send only one of the sidebands to the detector, so this distortion problem only occurs when receiving a DSB signal on a receiver that sends both sidebands to the detector. It's ironic that DSB, which came about due to the ease of detection with diode (envelope detectors) turns out to be somewhat challenging to recover with a more sophisticated synchronous detection scheme. Experimental Methods in RF Design points out that direct conversion receivers have become highly popular in the past couple of decades... this seems somewhat surprising; I would have guessed people back in the, e.g., '60s, would have gone to great lengths to avoid image reject filters and long IF chains. ---Joel Kolstad |
In article , "Joel Kolstad"
writes: Bill Meara wrote: This gets to the question of whether DC receivers can be used to copy DSB and SSB: By Goodman, W1DX, explained the problem in the 1965 edition of "Single Sideband for the Radio Amateur" (page 11): "Unfortunately, if both sidebands are received at the detector where the carrier is introduced, the carrier has to have exactly the correct phase relationship with the sidebands if distortion is to be avoided. Since exact phase relationship precludes even the slightest frequency error, such a system is workable only with very complicated receiving techniques. In 1965 I can imagine that a Costas loop, two mixers, etc. was considered 'very complicated.' It doesn't seem all that horribly fancy by today's standards, however. But of course it's not like I've actually _built_ such a thing yet! :-) Most of you guys are knocking yourself out on what is little more than an "intellectual experiment." Get a receiver and do a practical experiment. An old receiver with a "BFO" isn't "sophisticated" and will receive DSB and SSB just dandy, very "workable" if the LO is warmed up and stable and the fine-tuning ("bandspread") can zero- beat where the carrier is (or was). Do the same thing with a newer receiver that has a "product detector" (nothing more than a mixer, the same as what a DC receiver front end has but at IF, not HF). Very "workable" and done all over in everyday HF comm, both ham and maritime radio. Been done for decades. The only "distortion" comes from not being able to set the tuning precisely without some AFC. With AM and a "product detector" (or BFO on), there's the carrier beat, strong, and won't go away unless there's a terrific lowpass audio filter in there. If using a more modern, basically-SSB receiver, it probably has a "RIT" or Receiver Incremental Tuning that allows making the carrier beat almost to DC. That's a frequency distortion still and manual tweaking can't get the low-frequency, absolutely non-phase (or rapidly changing phase) all the way out. Can one get separated sidebands on AM DSB with a DC receiver? Absolutely! No problem with a manual tuning DC receiver that has TWO audio networks out of the mixer. A stereo-like effect (amazing to hear for the first time) with lower SB = left ear, upper SB = right ear is possible, even if the tuning doesn't hit right on carrier zero beat. The "phase distortion" manifests itself solely in the amount of rejection of the unwanted side of tuning...too great a phase from ideal results in poor unwanted side rejection...very close phase and the the rejection of unwanted side is best. However, if only one sideband is present at the detector, there is no need for an exact phase relationship and there can be some frequency error without destroying intelligibility. " Modern SSB transcievers send only one of the sidebands to the detector, so this distortion problem only occurs when receiving a DSB signal on a receiver that sends both sidebands to the detector. Pfui. The old receivers with BFOs could "work" SSB. Problem is, those old receivers were so finicky and unstable, had such wide final bandwidths that those faults predominated. I have a nice 1948 National NC-57 gathering dust as proof of that. :-) It's ironic that DSB, which came about due to the ease of detection with diode (envelope detectors) turns out to be somewhat challenging to recover with a more sophisticated synchronous detection scheme. Nooo...AM "came about" with absurdly SIMPLE components first, not even using any vacuum tubes! Case in point: Reginald Fessenden's famous Christmas Eve, 1906, voice transmission from Brant Rock, MA. Used a rotary alternator LF generator with a special (probably carbon) microphone in series with the antenna lead. The few who heard it along the east coast used galena crystal (the first point-contact diode?) or "coherer" or "liquid barreter" detectors. Technologically primitive by today's standards. The existance of two sidebands in AM wasn't known for sure until the first Johnny Carson (John R. Carson, AT&T) published his modulation equations to show the presence of identical-information sidebands. Few labs had the equivalent of spectrum analyzers and vacuum tubes were still rare in the 1915-1922 era. "Detectors" of that early time were still the simple "rectifying" types...a regenerative detector still does "rectifying" (averaging of amplified signal input) to recover the modulation audio. Long-distance telephony was the birthplace of SSB. Frequency multiplexing was the only practical way to cram four telephone circuits on a single pair of wires running many miles way back when. Frequency multiplexing uses SSB techniques. When RF amplifiers using tubes got going, the first SSB was four-voice-channel long-lines "carrier" frequency multiplexed modulation with radio replacing the wire pairs. Those applications needed AFC for unattended operation. If you want synchronous detection of AM DSB, then you concentrate on getting a carrier reinsertion oscillator locked to the received carrier. Primary object is to get that lock. Worry about "phase differences due to distortion" in intellectual experiments. Lock guarantees that the synchronous detection will hold. There won't be any noticeable recovered audio distortion EXCEPT from unusual selective fading propagation on very long-distance radio circuits; you can hear that over old receivers with "rectifying" detectors. Can you get a synchronous detection of AM SSB? Difficult unless the transmitter at the other end has sloppy carrier suppression. The commercial HF SSB stuff uses "pilot carriers" and the like to provide an AFC lock...deliberate steady tones at unused sideband frequencies. Experimental Methods in RF Design points out that direct conversion receivers have become highly popular in the past couple of decades... this seems somewhat surprising; I would have guessed people back in the, e.g., '60s, would have gone to great lengths to avoid image reject filters and long IF chains. DC receivers (also called "Zero-IF") came into popularity in Europe THREE decades ago. RSGB's Radio Communication magazines of 1973 were showing stuff in Pat Hawker's monthly column. I got interested in the Mike Gingell polyphase R-C network by seeing it first in there. The UK ham who was experimenting with it was Peter Martinez, G3PLX. Hams of today will know him as the innovator of PSK31. To get good sensitivity with DC receivers you need ultra-low-noise mixers and following audio stages. Since the input side selectivity is poor (compared to IF xtal filters), those mixers need a terrifically high intermodulation specification which precludes low-noise operation. The Tayloe Mixer handles both superbly, the "mixer" being a CMOS switch IC with very low conversion loss as a mixer (all other passive mixers have at least 6 db loss). The CMOS switch IC has very low internal noise. Absolutely the best of both worlds. In order to achieve selectivity and unwanted frequency side rejection the Tayloe Mixer system needs a basic LO at four times the carrier frequency to get wideband phase quadrature using digital devices. The In-phase and quadrature mixer output is in the audio range. It seems to me that a modification of the Tayloe circuit would suit a synchronous detector application. How to go about it is another matter. Planning for THAT can start with an "intellectual experiment" but trying to implement it requires bench experimentation. There won't be any "distortion due to phase" once carrier lock has been achieved. Carrier lock methods have to concentrate on the narrow frequency region (tolerance of tuning offset) of the carrier. In practical reception the carrier of AM DSB is relatively constant (within a 20 db spread of amplitude if some AGC exists elsewhere). Once the carrier is locked, the remainder of the detection process (recovering modulation audio) is straightforward. Any "phase distortion" is due to phasing network errors...which can be checked and trimmed independently prior to applying them. Go for it! :-) Len Anderson retired (from regular hours) electronic engineer person |
In article , "Joel Kolstad"
writes: Bill Meara wrote: This gets to the question of whether DC receivers can be used to copy DSB and SSB: By Goodman, W1DX, explained the problem in the 1965 edition of "Single Sideband for the Radio Amateur" (page 11): "Unfortunately, if both sidebands are received at the detector where the carrier is introduced, the carrier has to have exactly the correct phase relationship with the sidebands if distortion is to be avoided. Since exact phase relationship precludes even the slightest frequency error, such a system is workable only with very complicated receiving techniques. In 1965 I can imagine that a Costas loop, two mixers, etc. was considered 'very complicated.' It doesn't seem all that horribly fancy by today's standards, however. But of course it's not like I've actually _built_ such a thing yet! :-) Most of you guys are knocking yourself out on what is little more than an "intellectual experiment." Get a receiver and do a practical experiment. An old receiver with a "BFO" isn't "sophisticated" and will receive DSB and SSB just dandy, very "workable" if the LO is warmed up and stable and the fine-tuning ("bandspread") can zero- beat where the carrier is (or was). Do the same thing with a newer receiver that has a "product detector" (nothing more than a mixer, the same as what a DC receiver front end has but at IF, not HF). Very "workable" and done all over in everyday HF comm, both ham and maritime radio. Been done for decades. The only "distortion" comes from not being able to set the tuning precisely without some AFC. With AM and a "product detector" (or BFO on), there's the carrier beat, strong, and won't go away unless there's a terrific lowpass audio filter in there. If using a more modern, basically-SSB receiver, it probably has a "RIT" or Receiver Incremental Tuning that allows making the carrier beat almost to DC. That's a frequency distortion still and manual tweaking can't get the low-frequency, absolutely non-phase (or rapidly changing phase) all the way out. Can one get separated sidebands on AM DSB with a DC receiver? Absolutely! No problem with a manual tuning DC receiver that has TWO audio networks out of the mixer. A stereo-like effect (amazing to hear for the first time) with lower SB = left ear, upper SB = right ear is possible, even if the tuning doesn't hit right on carrier zero beat. The "phase distortion" manifests itself solely in the amount of rejection of the unwanted side of tuning...too great a phase from ideal results in poor unwanted side rejection...very close phase and the the rejection of unwanted side is best. However, if only one sideband is present at the detector, there is no need for an exact phase relationship and there can be some frequency error without destroying intelligibility. " Modern SSB transcievers send only one of the sidebands to the detector, so this distortion problem only occurs when receiving a DSB signal on a receiver that sends both sidebands to the detector. Pfui. The old receivers with BFOs could "work" SSB. Problem is, those old receivers were so finicky and unstable, had such wide final bandwidths that those faults predominated. I have a nice 1948 National NC-57 gathering dust as proof of that. :-) It's ironic that DSB, which came about due to the ease of detection with diode (envelope detectors) turns out to be somewhat challenging to recover with a more sophisticated synchronous detection scheme. Nooo...AM "came about" with absurdly SIMPLE components first, not even using any vacuum tubes! Case in point: Reginald Fessenden's famous Christmas Eve, 1906, voice transmission from Brant Rock, MA. Used a rotary alternator LF generator with a special (probably carbon) microphone in series with the antenna lead. The few who heard it along the east coast used galena crystal (the first point-contact diode?) or "coherer" or "liquid barreter" detectors. Technologically primitive by today's standards. The existance of two sidebands in AM wasn't known for sure until the first Johnny Carson (John R. Carson, AT&T) published his modulation equations to show the presence of identical-information sidebands. Few labs had the equivalent of spectrum analyzers and vacuum tubes were still rare in the 1915-1922 era. "Detectors" of that early time were still the simple "rectifying" types...a regenerative detector still does "rectifying" (averaging of amplified signal input) to recover the modulation audio. Long-distance telephony was the birthplace of SSB. Frequency multiplexing was the only practical way to cram four telephone circuits on a single pair of wires running many miles way back when. Frequency multiplexing uses SSB techniques. When RF amplifiers using tubes got going, the first SSB was four-voice-channel long-lines "carrier" frequency multiplexed modulation with radio replacing the wire pairs. Those applications needed AFC for unattended operation. If you want synchronous detection of AM DSB, then you concentrate on getting a carrier reinsertion oscillator locked to the received carrier. Primary object is to get that lock. Worry about "phase differences due to distortion" in intellectual experiments. Lock guarantees that the synchronous detection will hold. There won't be any noticeable recovered audio distortion EXCEPT from unusual selective fading propagation on very long-distance radio circuits; you can hear that over old receivers with "rectifying" detectors. Can you get a synchronous detection of AM SSB? Difficult unless the transmitter at the other end has sloppy carrier suppression. The commercial HF SSB stuff uses "pilot carriers" and the like to provide an AFC lock...deliberate steady tones at unused sideband frequencies. Experimental Methods in RF Design points out that direct conversion receivers have become highly popular in the past couple of decades... this seems somewhat surprising; I would have guessed people back in the, e.g., '60s, would have gone to great lengths to avoid image reject filters and long IF chains. DC receivers (also called "Zero-IF") came into popularity in Europe THREE decades ago. RSGB's Radio Communication magazines of 1973 were showing stuff in Pat Hawker's monthly column. I got interested in the Mike Gingell polyphase R-C network by seeing it first in there. The UK ham who was experimenting with it was Peter Martinez, G3PLX. Hams of today will know him as the innovator of PSK31. To get good sensitivity with DC receivers you need ultra-low-noise mixers and following audio stages. Since the input side selectivity is poor (compared to IF xtal filters), those mixers need a terrifically high intermodulation specification which precludes low-noise operation. The Tayloe Mixer handles both superbly, the "mixer" being a CMOS switch IC with very low conversion loss as a mixer (all other passive mixers have at least 6 db loss). The CMOS switch IC has very low internal noise. Absolutely the best of both worlds. In order to achieve selectivity and unwanted frequency side rejection the Tayloe Mixer system needs a basic LO at four times the carrier frequency to get wideband phase quadrature using digital devices. The In-phase and quadrature mixer output is in the audio range. It seems to me that a modification of the Tayloe circuit would suit a synchronous detector application. How to go about it is another matter. Planning for THAT can start with an "intellectual experiment" but trying to implement it requires bench experimentation. There won't be any "distortion due to phase" once carrier lock has been achieved. Carrier lock methods have to concentrate on the narrow frequency region (tolerance of tuning offset) of the carrier. In practical reception the carrier of AM DSB is relatively constant (within a 20 db spread of amplitude if some AGC exists elsewhere). Once the carrier is locked, the remainder of the detection process (recovering modulation audio) is straightforward. Any "phase distortion" is due to phasing network errors...which can be checked and trimmed independently prior to applying them. Go for it! :-) Len Anderson retired (from regular hours) electronic engineer person |
Joel: Image reject filters? Long IF chains? My DC recievers have
neither. And they work just fine. I use one on 17 meters (phone). JFET front end, diode ring mixer, VXO at the operating freq. Three BJT transistors in the audio amp. That's it. 73 Bill M0HBR http://planeta.clix.pt/n2cqr "Filters? We don't need no stinkin' filters!" :-0 "Joel Kolstad" wrote in message ... Bill Meara wrote: This gets to the question of whether DC receivers can be used to copy DSB and SSB: By Goodman, W1DX, explained the problem in the 1965 edition of "Single Sideband for the Radio Amateur" (page 11): "Unfortunately, if both sidebands are received at the detector where the carrier is introduced, the carrier has to have exactly the correct phase relationship with the sidebands if distortion is to be avoided. Since exact phase relationship precludes even the slightest frequency error, such a system is workable only with very complicated receiving techniques. In 1965 I can imagine that a Costas loop, two mixers, etc. was considered 'very complicated.' It doesn't seem all that horribly fancy by today's standards, however. But of course it's not like I've actually _built_ such a thing yet! :-) However, if only one sideband is present at the detector, there is no need for an exact phase relationship and there can be some frequency error without destroying intelligibility. " Modern SSB transcievers send only one of the sidebands to the detector, so this distortion problem only occurs when receiving a DSB signal on a receiver that sends both sidebands to the detector. It's ironic that DSB, which came about due to the ease of detection with diode (envelope detectors) turns out to be somewhat challenging to recover with a more sophisticated synchronous detection scheme. Experimental Methods in RF Design points out that direct conversion receivers have become highly popular in the past couple of decades... this seems somewhat surprising; I would have guessed people back in the, e.g., '60s, would have gone to great lengths to avoid image reject filters and long IF chains. ---Joel Kolstad |
Joel: Image reject filters? Long IF chains? My DC recievers have
neither. And they work just fine. I use one on 17 meters (phone). JFET front end, diode ring mixer, VXO at the operating freq. Three BJT transistors in the audio amp. That's it. 73 Bill M0HBR http://planeta.clix.pt/n2cqr "Filters? We don't need no stinkin' filters!" :-0 "Joel Kolstad" wrote in message ... Bill Meara wrote: This gets to the question of whether DC receivers can be used to copy DSB and SSB: By Goodman, W1DX, explained the problem in the 1965 edition of "Single Sideband for the Radio Amateur" (page 11): "Unfortunately, if both sidebands are received at the detector where the carrier is introduced, the carrier has to have exactly the correct phase relationship with the sidebands if distortion is to be avoided. Since exact phase relationship precludes even the slightest frequency error, such a system is workable only with very complicated receiving techniques. In 1965 I can imagine that a Costas loop, two mixers, etc. was considered 'very complicated.' It doesn't seem all that horribly fancy by today's standards, however. But of course it's not like I've actually _built_ such a thing yet! :-) However, if only one sideband is present at the detector, there is no need for an exact phase relationship and there can be some frequency error without destroying intelligibility. " Modern SSB transcievers send only one of the sidebands to the detector, so this distortion problem only occurs when receiving a DSB signal on a receiver that sends both sidebands to the detector. It's ironic that DSB, which came about due to the ease of detection with diode (envelope detectors) turns out to be somewhat challenging to recover with a more sophisticated synchronous detection scheme. Experimental Methods in RF Design points out that direct conversion receivers have become highly popular in the past couple of decades... this seems somewhat surprising; I would have guessed people back in the, e.g., '60s, would have gone to great lengths to avoid image reject filters and long IF chains. ---Joel Kolstad |
Experimental Methods in RF Design points out that direct conversion receivers have become highly popular in the past couple of decades... this seems somewhat surprising; I would have guessed people back in the, e.g., '60s, would have gone to great lengths to avoid image reject filters and long IF chains. The nice thing about DC IQ receivers (apart from their zero image problem) is that, any kind of demodulation can be solved in software, and is fully updatable .... whereas if it's done in hardware, you'd need new hardware for each mode required etc. The lack of image problems, simplicity of hardware and fully updatable modulation schemes is what makes DC IQ so nice -. so it's not surprising to me at all why it's becoming so popular. Clive |
Experimental Methods in RF Design points out that direct conversion receivers have become highly popular in the past couple of decades... this seems somewhat surprising; I would have guessed people back in the, e.g., '60s, would have gone to great lengths to avoid image reject filters and long IF chains. The nice thing about DC IQ receivers (apart from their zero image problem) is that, any kind of demodulation can be solved in software, and is fully updatable .... whereas if it's done in hardware, you'd need new hardware for each mode required etc. The lack of image problems, simplicity of hardware and fully updatable modulation schemes is what makes DC IQ so nice -. so it's not surprising to me at all why it's becoming so popular. Clive |
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Avery Fineman wrote:
Can one get separated sidebands on AM DSB with a DC receiver? Absolutely! That's good to know. At present I'm going to quit performing these "intellectual experiments" and start building something, and while I'm after C-QUAM AM stereo (rather than upper/lower sideband stereo), it's good to know what else _could_ be received. BTW, if anyone wants to see the block diagram of what I'm planning to do, see he http://oregonstate.edu/~kolstadj/RadioProj.gif . Keep in mind it's designed primarily for simplicity, not for phoenomenally good noise performance, sensitivity, selectivity, etc. (I'd be particularly interested in comments on how to implement the low pass filters -- it seems one would want phase preserving filters such as Bessels or a cascade of a Chebyshev followed by an all-pass phase restoration filter.) Nooo...AM "came about" with absurdly SIMPLE components first, not even using any vacuum tubes! Wow... I realize now there's a large gap in my knowledge of the history of the progression of radio inbetween "spark gap transmitter" and "diode-based envelope detector!" I have read of coherers before in Lee's book, "Design of CMOS Radio-Frequency Integrated Circuits" where he claims that nobody ever really did figure out _how_ they worked -- interest wanted as better detectors were available before they were around long enough for someone to do so. You have a phoenomenal memory, Len... I wish I could recall the details as well as you have! Long-distance telephony was the birthplace of SSB. Frequency multiplexing was the only practical way to cram four telephone circuits on a single pair of wires running many miles way back when. If you tell me people were already using IQ modulation back then as well I'll be quite impressed... If you want synchronous detection of AM DSB, then you concentrate on getting a carrier reinsertion oscillator locked to the received carrier. Primary object is to get that lock. I'm planning to write a (software) quadrature detector, and once that works, start worrying about obtaining phase lock so that stereo can be decoded. Can you get a synchronous detection of AM SSB? Difficult unless the transmitter at the other end has sloppy carrier suppression. Without a carrier of some pilot tone (as a reference) it seems as though it's difficult to even claim there could be such a thing as 'synchronous detection.' DC receivers (also called "Zero-IF") came into popularity in Europe THREE decades ago. RSGB's Radio Communication magazines of 1973 were showing stuff in Pat Hawker's monthly column. I got interested in the Mike Gingell polyphase R-C network by seeing it first in there. I took a quick look at the Gingell networks and they seem quite novel -- even made their way into a Real Commerical Product (a Maxim IC). (Interestingly enough, Dr. Gabor Temes -- who spent a long time designing telephone network filters before going into academia, where he is now, all of about 500' away from me here -- says there is still some black magic involved in making them work. :-) ) Thanks for all the advice Len... I'd be offering to take you to dinner by now if you were halfway local! ---Joel |
Avery Fineman wrote:
Can one get separated sidebands on AM DSB with a DC receiver? Absolutely! That's good to know. At present I'm going to quit performing these "intellectual experiments" and start building something, and while I'm after C-QUAM AM stereo (rather than upper/lower sideband stereo), it's good to know what else _could_ be received. BTW, if anyone wants to see the block diagram of what I'm planning to do, see he http://oregonstate.edu/~kolstadj/RadioProj.gif . Keep in mind it's designed primarily for simplicity, not for phoenomenally good noise performance, sensitivity, selectivity, etc. (I'd be particularly interested in comments on how to implement the low pass filters -- it seems one would want phase preserving filters such as Bessels or a cascade of a Chebyshev followed by an all-pass phase restoration filter.) Nooo...AM "came about" with absurdly SIMPLE components first, not even using any vacuum tubes! Wow... I realize now there's a large gap in my knowledge of the history of the progression of radio inbetween "spark gap transmitter" and "diode-based envelope detector!" I have read of coherers before in Lee's book, "Design of CMOS Radio-Frequency Integrated Circuits" where he claims that nobody ever really did figure out _how_ they worked -- interest wanted as better detectors were available before they were around long enough for someone to do so. You have a phoenomenal memory, Len... I wish I could recall the details as well as you have! Long-distance telephony was the birthplace of SSB. Frequency multiplexing was the only practical way to cram four telephone circuits on a single pair of wires running many miles way back when. If you tell me people were already using IQ modulation back then as well I'll be quite impressed... If you want synchronous detection of AM DSB, then you concentrate on getting a carrier reinsertion oscillator locked to the received carrier. Primary object is to get that lock. I'm planning to write a (software) quadrature detector, and once that works, start worrying about obtaining phase lock so that stereo can be decoded. Can you get a synchronous detection of AM SSB? Difficult unless the transmitter at the other end has sloppy carrier suppression. Without a carrier of some pilot tone (as a reference) it seems as though it's difficult to even claim there could be such a thing as 'synchronous detection.' DC receivers (also called "Zero-IF") came into popularity in Europe THREE decades ago. RSGB's Radio Communication magazines of 1973 were showing stuff in Pat Hawker's monthly column. I got interested in the Mike Gingell polyphase R-C network by seeing it first in there. I took a quick look at the Gingell networks and they seem quite novel -- even made their way into a Real Commerical Product (a Maxim IC). (Interestingly enough, Dr. Gabor Temes -- who spent a long time designing telephone network filters before going into academia, where he is now, all of about 500' away from me here -- says there is still some black magic involved in making them work. :-) ) Thanks for all the advice Len... I'd be offering to take you to dinner by now if you were halfway local! ---Joel |
Bill Meara wrote:
Joel: Image reject filters? Long IF chains? My DC recievers have neither. That's what I meant -- DC receivers don't need them, and therefore it's surprising DC receivers have only really been gaining steam here in the U.S. in the past decade or two. Then again, there's no arguing with performance: The GE "super radio" is, I believe, a triple conversion receiver! ---Joel Kolstad |
Bill Meara wrote:
Joel: Image reject filters? Long IF chains? My DC recievers have neither. That's what I meant -- DC receivers don't need them, and therefore it's surprising DC receivers have only really been gaining steam here in the U.S. in the past decade or two. Then again, there's no arguing with performance: The GE "super radio" is, I believe, a triple conversion receiver! ---Joel Kolstad |
In article , "Joel Kolstad"
writes: Avery Fineman wrote: Can one get separated sidebands on AM DSB with a DC receiver? Absolutely! That's good to know. At present I'm going to quit performing these "intellectual experiments" and start building something, and while I'm after C-QUAM AM stereo (rather than upper/lower sideband stereo), it's good to know what else _could_ be received. BTW, if anyone wants to see the block diagram of what I'm planning to do, see he http://oregonstate.edu/~kolstadj/RadioProj.gif . Keep in mind it's designed primarily for simplicity, not for phoenomenally good noise performance, sensitivity, selectivity, etc. (I'd be particularly interested in comments on how to implement the low pass filters -- it seems one would want phase preserving filters such as Bessels or a cascade of a Chebyshev followed by an all-pass phase restoration filter.) OK, I got the block diagram. If you are using even a rudimentary R-C lowpass following the two mixers, you need the parts rather well matched in order to preserve identical relative phases. It is important to HOLD the relative phase error at audio to a very small number in order to do the In-phase/Quadrature thing. DSP will work with BOTH magnitude and phase regardless of the kind of modulation going into the mixers. You CAN realize a lowpass function in DSP but the TI chip inputs probably needs some sort of hardware lowpass filtering...? A simple R-C lowpass can be checked out independently just from equal parts values to assure minimum relative phase error. Here's a good hint on melding hardware with software using DSP: "Scientist's and Engineer's Guide to Digital Signal Processing," by Stephen W. Smith, PhD, California Technical Publishing. Despite the title, this is a good text on DSP from the beginner's point of view on to the more advanced. What is special is that ALL the chapters can be downloaded absolutely FREE! :-) [or pay about $68 for the hardcover] http://www.DSPguide.com Well organized book and a good "teaching style" to the writing. Once you have the hardware fairly well in shape, it's time to go nuts with the programming. This book ought to help whatever it is you are going to code. Nooo...AM "came about" with absurdly SIMPLE components first, not even using any vacuum tubes! Wow... I realize now there's a large gap in my knowledge of the history of the progression of radio inbetween "spark gap transmitter" and "diode-based envelope detector!" I have read of coherers before in Lee's book, "Design of CMOS Radio-Frequency Integrated Circuits" where he claims that nobody ever really did figure out _how_ they worked -- interest wanted as better detectors were available before they were around long enough for someone to do so. Actually, that's irrelevant and a historical curiosity. The galena crystal and "cat's whisker" formed a rudimentary point- contact diode. I had one of those in 1946, a Philmore Crystal Set my Dad got for me (el cheapo quality, but it worked after a fashion). A half year later a new electronics store opened up in town and they were selling surplus WW2 radar set silicon mixer diodes, type 1N21 and 1N23. Put one of those in the Philmore and really "souped up" the audio. :-) Long-distance telephony was the birthplace of SSB. Frequency multiplexing was the only practical way to cram four telephone circuits on a single pair of wires running many miles way back when. If you tell me people were already using IQ modulation back then as well I'll be quite impressed... As far as I've seen, the old telephony "carrier" equipment used ordinary 4-diode ring mixers, usually copper-oxide stacked plate types, the small ones the size of old multimeter AC rectifiers. That was pre-1930. I'm not sure when the In-phase/Quadrature demod/mod sub-systems were first used other than probably just before 1940...or maybe in the WW2 years. I know the beginning applications were there in the late 1940s. If you want synchronous detection of AM DSB, then you concentrate on getting a carrier reinsertion oscillator locked to the received carrier. Primary object is to get that lock. I'm planning to write a (software) quadrature detector, and once that works, start worrying about obtaining phase lock so that stereo can be decoded. Good luck on that. Can you get a synchronous detection of AM SSB? Difficult unless the transmitter at the other end has sloppy carrier suppression. Without a carrier of some pilot tone (as a reference) it seems as though it's difficult to even claim there could be such a thing as 'synchronous detection.' I've seen it claimed in text, but no details, that a quasi-lock could be obtained via voice, working on the harmonics of speech tones. I'm not going to buy that until I see a demo. DC receivers (also called "Zero-IF") came into popularity in Europe THREE decades ago. RSGB's Radio Communication magazines of 1973 were showing stuff in Pat Hawker's monthly column. I got interested in the Mike Gingell polyphase R-C network by seeing it first in there. I took a quick look at the Gingell networks and they seem quite novel -- even made their way into a Real Commerical Product (a Maxim IC). (Interestingly enough, Dr. Gabor Temes -- who spent a long time designing telephone network filters before going into academia, where he is now, all of about 500' away from me here -- says there is still some black magic involved in making them work. :-) ) The polyphase network was the subject of Michael Gingell's PhD thesis in the UK. Material on that was seen on the Internet. Mike is a USA resident now (or was a couple years ago when he had a website...has a US ham callsign, too). A Japanese ham got busy on that polyphase network and came up with an optimum set of component values. That was published in QEX. Hans Summers' website has links to all those. Gabor Temes is a familiar name to textbook thumbers. :-) There isn't a lot of black magic associated with the Gingell network, but it is a thorough #$%^!!!! to try and analyze with its busy interconnections. I stole a few minutes of CPU time on the RCA corporate computer, using their LECAP (a frequency-domain version of IBM's ECAP...the SPICE thing hadn't been developed yet) back in the 1970s. It worked as advertised with only 0 and 180 degree audio input, producing nice relative quadrature phases on all four outputs. Was surprised! Some scrounged parts, not well measured as to values, and a quick open-air toss-together showed excellent broadband relative quadrature with less than 1 degree error in the 'voice' bandspace. Checked that with a time-interval function on a homebuilt frequency counter. Thanks for all the advice Len... I'd be offering to take you to dinner by now if you were halfway local! Thank you but I'll just wave as my wife and I roll through Oregon along I-5 about once a year from southern California to Puget Sound area of Washington. :-) Want to bypass the usual clogging just before crossing the river into WA and vice-versa. Len Anderson retired (from regular hours) electronic engineer person |
In article , "Joel Kolstad"
writes: Avery Fineman wrote: Can one get separated sidebands on AM DSB with a DC receiver? Absolutely! That's good to know. At present I'm going to quit performing these "intellectual experiments" and start building something, and while I'm after C-QUAM AM stereo (rather than upper/lower sideband stereo), it's good to know what else _could_ be received. BTW, if anyone wants to see the block diagram of what I'm planning to do, see he http://oregonstate.edu/~kolstadj/RadioProj.gif . Keep in mind it's designed primarily for simplicity, not for phoenomenally good noise performance, sensitivity, selectivity, etc. (I'd be particularly interested in comments on how to implement the low pass filters -- it seems one would want phase preserving filters such as Bessels or a cascade of a Chebyshev followed by an all-pass phase restoration filter.) OK, I got the block diagram. If you are using even a rudimentary R-C lowpass following the two mixers, you need the parts rather well matched in order to preserve identical relative phases. It is important to HOLD the relative phase error at audio to a very small number in order to do the In-phase/Quadrature thing. DSP will work with BOTH magnitude and phase regardless of the kind of modulation going into the mixers. You CAN realize a lowpass function in DSP but the TI chip inputs probably needs some sort of hardware lowpass filtering...? A simple R-C lowpass can be checked out independently just from equal parts values to assure minimum relative phase error. Here's a good hint on melding hardware with software using DSP: "Scientist's and Engineer's Guide to Digital Signal Processing," by Stephen W. Smith, PhD, California Technical Publishing. Despite the title, this is a good text on DSP from the beginner's point of view on to the more advanced. What is special is that ALL the chapters can be downloaded absolutely FREE! :-) [or pay about $68 for the hardcover] http://www.DSPguide.com Well organized book and a good "teaching style" to the writing. Once you have the hardware fairly well in shape, it's time to go nuts with the programming. This book ought to help whatever it is you are going to code. Nooo...AM "came about" with absurdly SIMPLE components first, not even using any vacuum tubes! Wow... I realize now there's a large gap in my knowledge of the history of the progression of radio inbetween "spark gap transmitter" and "diode-based envelope detector!" I have read of coherers before in Lee's book, "Design of CMOS Radio-Frequency Integrated Circuits" where he claims that nobody ever really did figure out _how_ they worked -- interest wanted as better detectors were available before they were around long enough for someone to do so. Actually, that's irrelevant and a historical curiosity. The galena crystal and "cat's whisker" formed a rudimentary point- contact diode. I had one of those in 1946, a Philmore Crystal Set my Dad got for me (el cheapo quality, but it worked after a fashion). A half year later a new electronics store opened up in town and they were selling surplus WW2 radar set silicon mixer diodes, type 1N21 and 1N23. Put one of those in the Philmore and really "souped up" the audio. :-) Long-distance telephony was the birthplace of SSB. Frequency multiplexing was the only practical way to cram four telephone circuits on a single pair of wires running many miles way back when. If you tell me people were already using IQ modulation back then as well I'll be quite impressed... As far as I've seen, the old telephony "carrier" equipment used ordinary 4-diode ring mixers, usually copper-oxide stacked plate types, the small ones the size of old multimeter AC rectifiers. That was pre-1930. I'm not sure when the In-phase/Quadrature demod/mod sub-systems were first used other than probably just before 1940...or maybe in the WW2 years. I know the beginning applications were there in the late 1940s. If you want synchronous detection of AM DSB, then you concentrate on getting a carrier reinsertion oscillator locked to the received carrier. Primary object is to get that lock. I'm planning to write a (software) quadrature detector, and once that works, start worrying about obtaining phase lock so that stereo can be decoded. Good luck on that. Can you get a synchronous detection of AM SSB? Difficult unless the transmitter at the other end has sloppy carrier suppression. Without a carrier of some pilot tone (as a reference) it seems as though it's difficult to even claim there could be such a thing as 'synchronous detection.' I've seen it claimed in text, but no details, that a quasi-lock could be obtained via voice, working on the harmonics of speech tones. I'm not going to buy that until I see a demo. DC receivers (also called "Zero-IF") came into popularity in Europe THREE decades ago. RSGB's Radio Communication magazines of 1973 were showing stuff in Pat Hawker's monthly column. I got interested in the Mike Gingell polyphase R-C network by seeing it first in there. I took a quick look at the Gingell networks and they seem quite novel -- even made their way into a Real Commerical Product (a Maxim IC). (Interestingly enough, Dr. Gabor Temes -- who spent a long time designing telephone network filters before going into academia, where he is now, all of about 500' away from me here -- says there is still some black magic involved in making them work. :-) ) The polyphase network was the subject of Michael Gingell's PhD thesis in the UK. Material on that was seen on the Internet. Mike is a USA resident now (or was a couple years ago when he had a website...has a US ham callsign, too). A Japanese ham got busy on that polyphase network and came up with an optimum set of component values. That was published in QEX. Hans Summers' website has links to all those. Gabor Temes is a familiar name to textbook thumbers. :-) There isn't a lot of black magic associated with the Gingell network, but it is a thorough #$%^!!!! to try and analyze with its busy interconnections. I stole a few minutes of CPU time on the RCA corporate computer, using their LECAP (a frequency-domain version of IBM's ECAP...the SPICE thing hadn't been developed yet) back in the 1970s. It worked as advertised with only 0 and 180 degree audio input, producing nice relative quadrature phases on all four outputs. Was surprised! Some scrounged parts, not well measured as to values, and a quick open-air toss-together showed excellent broadband relative quadrature with less than 1 degree error in the 'voice' bandspace. Checked that with a time-interval function on a homebuilt frequency counter. Thanks for all the advice Len... I'd be offering to take you to dinner by now if you were halfway local! Thank you but I'll just wave as my wife and I roll through Oregon along I-5 about once a year from southern California to Puget Sound area of Washington. :-) Want to bypass the usual clogging just before crossing the river into WA and vice-versa. Len Anderson retired (from regular hours) electronic engineer person |
Avery Fineman wrote:
You CAN realize a lowpass function in DSP but the TI chip inputs probably needs some sort of hardware lowpass filtering...? An anti-alias filter at the very least! The TI chip in question is actually a microcontroller rather than a DSP; I chose that based on desiring a low power design and only requiring one chip rather than two (although I'd grant you that there are some, e.g., SO-8 package serial interface ADCs out there that almost don't count as another chip...) and, uh, because I already have experience with it from other projects. It can sample up to ~200ksps, but I was shooting for a much lower rather (perhaps some 20-50 ksps) since there isn't much number crunching 'oomph' available. (I.e., no multipy-accumulate instruction. In fact, no hardware multiplier at all! :-) I'll build the mind-numbingly fast DSP monster receiver that can pull a signal 10dB beneath the noise floor and turn it into CD quality audio once I get the simple ones working.) Here's a good hint on melding hardware with software using DSP: "Scientist's and Engineer's Guide to Digital Signal Processing," by Stephen W. Smith, PhD, California Technical Publishing. How very interesting -- I actually have a copy of that with from Analog Devices (under a slightly different name); I always figured it was some third party and not the application engineers at Analog Devices that wrote it. Now I just have to crack the thing open... Good luck on that. It should be up and running within a month here. I've seen it claimed in text, but no details, that a quasi-lock could be obtained via voice, working on the harmonics of speech tones. I'm not going to buy that until I see a demo. I figured that's what they might have had in mind. Seems like a lot of effort to avoid sending a pilot tone (but then again, what else to use the aforementioned DSP for... oh, wait... fancy digital modulation schemes... ok...) ---Joel |
Avery Fineman wrote:
You CAN realize a lowpass function in DSP but the TI chip inputs probably needs some sort of hardware lowpass filtering...? An anti-alias filter at the very least! The TI chip in question is actually a microcontroller rather than a DSP; I chose that based on desiring a low power design and only requiring one chip rather than two (although I'd grant you that there are some, e.g., SO-8 package serial interface ADCs out there that almost don't count as another chip...) and, uh, because I already have experience with it from other projects. It can sample up to ~200ksps, but I was shooting for a much lower rather (perhaps some 20-50 ksps) since there isn't much number crunching 'oomph' available. (I.e., no multipy-accumulate instruction. In fact, no hardware multiplier at all! :-) I'll build the mind-numbingly fast DSP monster receiver that can pull a signal 10dB beneath the noise floor and turn it into CD quality audio once I get the simple ones working.) Here's a good hint on melding hardware with software using DSP: "Scientist's and Engineer's Guide to Digital Signal Processing," by Stephen W. Smith, PhD, California Technical Publishing. How very interesting -- I actually have a copy of that with from Analog Devices (under a slightly different name); I always figured it was some third party and not the application engineers at Analog Devices that wrote it. Now I just have to crack the thing open... Good luck on that. It should be up and running within a month here. I've seen it claimed in text, but no details, that a quasi-lock could be obtained via voice, working on the harmonics of speech tones. I'm not going to buy that until I see a demo. I figured that's what they might have had in mind. Seems like a lot of effort to avoid sending a pilot tone (but then again, what else to use the aforementioned DSP for... oh, wait... fancy digital modulation schemes... ok...) ---Joel |
Also regarding microcontrollers vs. DSPs: TI has an application note where
they use so-called wave filters (which I know next to nothing about other than the name and that they can be designed to only require additions/subtractions and not multiplications in their implementations and hence execute quickly) for detecting DTMF. I thought it was pretty clever... ---Joel |
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