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Sync detectors and fading
Greetings,
I have just got a sony ICF sw7600GR and it is a very nice radio. The sync detector seems to take care of a lot of the distortion, but the audio continues fading in and out and is quite annoying. Could the fading be mitigated to any extent by using another stage of agc? I am going to be doing some experiments with the 455kc if out on my Red Sun RP2100 whenever it gets here. Detectors, filters, SSB, etc... I thought that along with other experiments I might want to try some outboard agc. regards, NEO |
Sync detectors and fading
N9NEO wrote: Greetings, I have just got a sony ICF sw7600GR and it is a very nice radio. The sync detector seems to take care of a lot of the distortion, but the audio continues fading in and out and is quite annoying. Could the fading be mitigated to any extent by using another stage of agc? I am going to be doing some experiments with the 455kc if out on my Red Sun RP2100 whenever it gets here. Detectors, filters, SSB, etc... I thought that along with other experiments I might want to try some outboard agc. regards, NEO AGC systems don't lend themselves to easy modification. There are many issues, attack time and release time, are the two most significant. Too fast the audio gets "chopy", too slow not enough effect. I think I can say that I really understand the Kenwood R2000. And while I have tried several "improved" AGC designs, none made enough difference to be worth mentioning. The DX398 has some published modifications that you may want to track down. The main effect as I recall was to slow the release time. The DX398's AGC is a tad too fast. Good luck and let us know how it works. Synch detectors can mitigate the some of the effects caused by fading. But in my experience are not the "do all- end all" that I had hoped. At this point I have decided to concentrate on improving my antenna to get the most signal and as little noise as possible. A much better pay off for the energy required. Terry |
Sync detectors and fading
"N9NEO" ) writes:
Greetings, I have just got a sony ICF sw7600GR and it is a very nice radio. The sync detector seems to take care of a lot of the distortion, but the audio continues fading in and out and is quite annoying. Could the fading be mitigated to any extent by using another stage of agc? I am going to be doing some experiments with the 455kc if out on my Red Sun RP2100 whenever it gets here. Detectors, filters, SSB, etc... I thought that along with other experiments I might want to try some outboard agc. Synchronous detectors have never been about dealing with fading. They are about ensuring there is enough "carrier" to beat the sidebands down to audio. So there's fading on the incoming signal. That means the amplitude of the sidebands is varying with that fading. A locally generated "carrier" at the receiver ensures that there is something to beat those sidebands down to audio, even if the transmitter's carrier has faded too much to do the proper job. But a constant level "carrier" at the receiver beats the sideband down to audio intact, ie an ideal mixer would not add anything to the signal. So if the sideband is fading, of course the audio output of the receiver will vary with that fading. What the sync detector brings you is the ability to decode that signal even if the carrier goes missing, because of selective fading. Dealing with the fading of the sidebands is in a different realm, and obviously a miraculous receiver that eliminates fading has long been sought after. Armstrong dealt with it in part, by moving to FM and using limiters in the receiver, but that only works when the signal is above a certain level. Below it, the signal levels are too low for the limiters to kick in, and that fading is obvious. Beyond a certain point, you get conflict. Have a scheme that does a really good job of eliminating the fading, and likely that starts affecting the "fidelity" of the signal, because how do you discriminate between the voice at the transmitter end varying in amplitude, because the speaker starts talking more quietly or even just because sounds are made up of varying levels, and the signal fading as it travels to the receiver? It's easy to counter some of the fading, but it gets harder the more you try to conquer it. Michael |
Sync detectors and fading
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Sync detectors and fading
The purpose of the sync detector is not to reduce fading, but to reduce the
distortion during the fades. A good sync detector does a remarkable job of that, even when listening to music. A sync detector that allows one side band to be selected adds the ability to reduce adjacent channel inteference. -- Brian Denley http://home.comcast.net/~b.denley/index.html wrote in message oups.com... N9NEO wrote: Synch detectors can mitigate the some of the effects caused by fading. But in my experience are not the "do all- end all" that I had hoped. Terry |
Sync detectors and fading
Michael Black wrote: Beyond a certain point, you get conflict. Have a scheme that does a really good job of eliminating the fading, and likely that starts affecting the "fidelity" of the signal, because how do you discriminate between the voice at the transmitter end varying in amplitude, because the speaker starts talking more quietly or even just because sounds are made up of varying levels, and the signal fading as it travels to the receiver? It's easy to counter some of the fading, but it gets harder the more you try to conquer it. Michael Has anyone tried to measure the fading of the carrier and use that as a guide as to whether the level change in the sideband is fading related or program content related? Bob |
Sync detectors and fading
Michael Black wrote: "N9NEO" ) writes: Greetings, I have just got a sony ICF sw7600GR and it is a very nice radio. The sync detector seems to take care of a lot of the distortion, but the audio continues fading in and out and is quite annoying. Could the fading be mitigated to any extent by using another stage of agc? I am going to be doing some experiments with the 455kc if out on my Red Sun RP2100 whenever it gets here. Detectors, filters, SSB, etc... I thought that along with other experiments I might want to try some outboard agc. Synchronous detectors have never been about dealing with fading. They are about ensuring there is enough "carrier" to beat the sidebands down to audio. Narrow band signal have less fading, thus sync demod will have less fading. However, the result isn't all that significant since all you have done is cut the bandwidth in half. So there's fading on the incoming signal. That means the amplitude of the sidebands is varying with that fading. A locally generated "carrier" at the receiver ensures that there is something to beat those sidebands down to audio, even if the transmitter's carrier has faded too much to do the proper job. But a constant level "carrier" at the receiver beats the sideband down to audio intact, ie an ideal mixer would not add anything to the signal. So if the sideband is fading, of course the audio output of the receiver will vary with that fading. With an envelope detector, the carrier isn't beating down the sideband. If you just look at the math of AM modulation, you would see that the carrier is just there for the ride. What the sync detector brings you is the ability to decode that signal even if the carrier goes missing, because of selective fading. Dealing with the fading of the sidebands is in a different realm, and obviously a miraculous receiver that eliminates fading has long been sought after. Armstrong dealt with it in part, by moving to FM and using limiters in the receiver, but that only works when the signal is above a certain level. Below it, the signal levels are too low for the limiters to kick in, and that fading is obvious. Beyond a certain point, you get conflict. Have a scheme that does a really good job of eliminating the fading, and likely that starts affecting the "fidelity" of the signal, because how do you discriminate between the voice at the transmitter end varying in amplitude, because the speaker starts talking more quietly or even just because sounds are made up of varying levels, and the signal fading as it travels to the receiver? It's easy to counter some of the fading, but it gets harder the more you try to conquer it. Michael |
Sync detectors and fading
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Sync detectors and fading
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Sync detectors and fading
Telamon wrote: In article . com, wrote: Michael Black wrote: "N9NEO" ) writes: Greetings, I have just got a sony ICF sw7600GR and it is a very nice radio. The sync detector seems to take care of a lot of the distortion, but the audio continues fading in and out and is quite annoying. Could the fading be mitigated to any extent by using another stage of agc? I am going to be doing some experiments with the 455kc if out on my Red Sun RP2100 whenever it gets here. Detectors, filters, SSB, etc... I thought that along with other experiments I might want to try some outboard agc. Synchronous detectors have never been about dealing with fading. They are about ensuring there is enough "carrier" to beat the sidebands down to audio. Narrow band signal have less fading, thus sync demod will have less fading. However, the result isn't all that significant since all you have done is cut the bandwidth in half. Narrow band signals do not have less fading. So there's fading on the incoming signal. That means the amplitude of the sidebands is varying with that fading. A locally generated "carrier" at the receiver ensures that there is something to beat those sidebands down to audio, even if the transmitter's carrier has faded too much to do the proper job. But a constant level "carrier" at the receiver beats the sideband down to audio intact, ie an ideal mixer would not add anything to the signal. So if the sideband is fading, of course the audio output of the receiver will vary with that fading. With an envelope detector, the carrier isn't beating down the sideband. If you just look at the math of AM modulation, you would see that the carrier is just there for the ride. Selective fading occurs when conditions cause a very narrow band of frequencies to be received at very low amplitudes where most of the side band information is present at levels that your receiver can ordinarily demodulate properly. When part of the side band is being notched out it does not sound all that bad but when the carrier gets weakened then the AM demodulator can't process the side band information properly and there is horrendous distortion. The carrier which is at the right frequency and phase relative to the side band information keeps the detector in the linear region so distortion is minimized. A sync detector uses a local oscillator in a similar to the way SSB is detected with the difference that it is phase locked to the signal carrier and mixed with it so when the carrier fades out this near perfect copy of the carrier allows the demodulator to continue to detect the side band or bands without distortion during a carrier fading condition. Here this necessary frequency and phase information carried by the "carrier" is retained by the sync circuitry. What the sync detector brings you is the ability to decode that signal even if the carrier goes missing, because of selective fading. Snip Michael has it right. -- Telamon Ventura, California Why do you insist that the atmosphere treats the carrier differently from the rest of the signal? Geez. You have a spectrum produced by modulation. If the modulation is AM, then a carrier is present. Now you are saying the atmosphere is sucking out the narrow band carrier and leaving the wideband spectrum untouched. Fiction at best. |
Sync detectors and fading
In article . com,
wrote: Telamon wrote: In article . com, wrote: Michael Black wrote: "N9NEO" ) writes: Greetings, I have just got a sony ICF sw7600GR and it is a very nice radio. The sync detector seems to take care of a lot of the distortion, but the audio continues fading in and out and is quite annoying. Could the fading be mitigated to any extent by using another stage of agc? I am going to be doing some experiments with the 455kc if out on my Red Sun RP2100 whenever it gets here. Detectors, filters, SSB, etc... I thought that along with other experiments I might want to try some outboard agc. Synchronous detectors have never been about dealing with fading. They are about ensuring there is enough "carrier" to beat the sidebands down to audio. Narrow band signal have less fading, thus sync demod will have less fading. However, the result isn't all that significant since all you have done is cut the bandwidth in half. Narrow band signals do not have less fading. So there's fading on the incoming signal. That means the amplitude of the sidebands is varying with that fading. A locally generated "carrier" at the receiver ensures that there is something to beat those sidebands down to audio, even if the transmitter's carrier has faded too much to do the proper job. But a constant level "carrier" at the receiver beats the sideband down to audio intact, ie an ideal mixer would not add anything to the signal. So if the sideband is fading, of course the audio output of the receiver will vary with that fading. With an envelope detector, the carrier isn't beating down the sideband. If you just look at the math of AM modulation, you would see that the carrier is just there for the ride. Selective fading occurs when conditions cause a very narrow band of frequencies to be received at very low amplitudes where most of the side band information is present at levels that your receiver can ordinarily demodulate properly. When part of the side band is being notched out it does not sound all that bad but when the carrier gets weakened then the AM demodulator can't process the side band information properly and there is horrendous distortion. The carrier which is at the right frequency and phase relative to the side band information keeps the detector in the linear region so distortion is minimized. A sync detector uses a local oscillator in a similar to the way SSB is detected with the difference that it is phase locked to the signal carrier and mixed with it so when the carrier fades out this near perfect copy of the carrier allows the demodulator to continue to detect the side band or bands without distortion during a carrier fading condition. Here this necessary frequency and phase information carried by the "carrier" is retained by the sync circuitry. What the sync detector brings you is the ability to decode that signal even if the carrier goes missing, because of selective fading. Snip Michael has it right. Why do you insist that the atmosphere treats the carrier differently from the rest of the signal? Geez. You have a spectrum produced by modulation. If the modulation is AM, then a carrier is present. Now you are saying the atmosphere is sucking out the narrow band carrier and leaving the wideband spectrum untouched. Fiction at best. What makes you interpret my remarks as the "atmosphere" treats the carrier differently than the rest of the signal? You mean ionosphere. Phase cancelation is a frequency dependent phenomenon and the modulation scheme or part of it does not matter. When phase cancelation is narrow and right at the carrier frequency then the carrier level goes down where the rest of the side band signal is still good. You been talking to John Smith or something? -- Telamon Ventura, California |
Sync detectors and fading
N9NEO wrote:
Greetings, I have just got a sony ICF sw7600GR and it is a very nice radio. The sync detector seems to take care of a lot of the distortion, but the audio continues fading in and out and is quite annoying. Could the fading be mitigated to any extent by using another stage of agc? I am going to be doing some experiments with the 455kc if out on my Red Sun RP2100 whenever it gets here. Detectors, filters, SSB, etc... I thought that along with other experiments I might want to try some outboard agc. regards, NEO The sync' is doing what it's designed for by reducing the distortion caused by selective fading, but you need a longer time constant (release time) for the AGC, to help smooth out the fading. You could lengthen the time constant of the AGC circuit in the 7600GR but then it wouldn't work well for other conditions where a shorter time constant is needed. This is why a good table-top receiver has more than one AGC rate, which can be selected by the user. The AGC in the Drake-R8 series uses a decay rate of 300-ms for the fast setting and a much longer rate of about 2-seconds in the slow mode. The latter really aides in reducing the effects of rapid fading. |
Sync detectors and fading
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Sync detectors and fading
Carter-k8vt wrote: wrote: Why do you insist that the atmosphere treats the carrier differently from the rest of the signal? Because it does. See below... Geez. You have a spectrum produced by modulation. If the modulation is AM, then a carrier is present. Now you are saying the atmosphere is sucking out the narrow band carrier and leaving the wideband spectrum untouched. Fiction at best. Bzzzzt. Wrong! Yes, the "atmosphere" [ionosphere] DOES "suck out a narrow band" or even a single frequency. Ask any amateur radio operator that has used RTTY (radio teletype). The RTTY "modulation mode" used is FSK or frequency shift keying. At any given instant, the transmitter is sending either a "mark" or "space", essentially two carriers if you will, 170 Hertz apart that represent the 5-level Baudot code as used in ham RTTY. As an aid to tuning, an oscilloscope is used as a tuning indicator; the mark signal from your RTTY decoder is connected to the horizontal plates of the 'scope, the space signal to the vertical plates. On the screen of the CRT (due to the persistence of the CRT phosphors and your eyes), this shows what appears to be a "+" sign, also known as the classic "cross display". When you see the cross on your screen, you know you are tuned in properly. So, "What does this have to do with the discussion above?" you ask. Remember, you are looking at essentially two "carriers", 170 Hz apart, one on the horizontal axis and one displayed on the vertical axis. During disturbed ionospheric conditions, many times you will see one signal or the other disappear; i.e., the cross turns into a single line, either a "-" or a "|", depending if the mark or space faded--and yes, sometimes both fade, but it is more common to see one or the other disappear. This phenomenon is known to hams as "selective fading", is quite common and is interesting to observe. So, yes, the ionosphere CAN suck out one signal separated from another by as little as 170 Hz. But, is it 'sucking it out' or merely propagating it somewhere else other than that particular spot where your antenna is? And that 'somewhere else' might not be very far away, but merely a few wavelengths in distance. dxAce Michigan USA |
Sync detectors and fading
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Sync detectors and fading
dxAce wrote:
But, is it 'sucking it out' or merely propagating it somewhere else other than that particular spot where your antenna is? And that 'somewhere else' might not be very far away, but merely a few wavelengths in distance. dxAce Michigan USA Before satellites carried most of the milcom they used "diversity receivers". Two, or more, receivers tuned to the same frequency but located some distance apart. The logic being that when the singal faded at one location, the other didn't fade at the same time. The more important a comm cicuit the more receivers spread over a wider area. A friend and I played with our receivers feeding phone patches and since we live 30 miles apart it was clear this approach was workable. With signals that experienced deep fades we were able to listen to nearly all of the time. Real (commercial or military) had AGC based voting systems to decided which signal to pass. We ran into issues of our audio phases shifting producing very odd sounding "flanging" effects. I have often thought about trying this with receivers whose antennas are only a few hundred to thosand feet apart. I never have gotten around to it. The military also used freqeuncy diversity, sending the same singal on more then one frequency. Kind of like listening to WWV on 5 10 and 15MHz at the same time. Terry |
Sync detectors and fading
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Sync detectors and fading
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Sync detectors and fading
bpnjensen wrote: wrote: dxAce wrote: But, is it 'sucking it out' or merely propagating it somewhere else other than that particular spot where your antenna is? And that 'somewhere else' might not be very far away, but merely a few wavelengths in distance. dxAce Michigan USA Before satellites carried most of the milcom they used "diversity receivers". Two, or more, receivers tuned to the same frequency but located some distance apart. The logic being that when the singal faded at one location, the other didn't fade at the same time. The more important a comm cicuit the more receivers spread over a wider area. A friend and I played with our receivers feeding phone patches and since we live 30 miles apart it was clear this approach was workable. With signals that experienced deep fades we were able to listen to nearly all of the time. Real (commercial or military) had AGC based voting systems to decided which signal to pass. We ran into issues of our audio phases shifting producing very odd sounding "flanging" effects. I have often thought about trying this with receivers whose antennas are only a few hundred to thosand feet apart. I never have gotten around to it. The military also used freqeuncy diversity, sending the same singal on more then one frequency. Kind of like listening to WWV on 5 10 and 15MHz at the same time. Terry Fascinating. It sounds like a couple of antennae, maybe even on the same property but spaced some modest distance apart, maybe a few hundred feet, and phased into the same radio, might also be a solution to the problem. Anyone try this with a 50-acre lot and a phasing harness? I don't think that would work properly. In practice, I think you need the 'voting machine' that works on two receivers AGC to pick the best signal. I do recall some folks trying to emulate this to a certain degree by having two receivers, two antennas widely seperated (more than a wavelength), and feeding the audio to headphones (one receiver in the right ear, one in the left). dxAce Michigan USA |
Sync detectors and fading
bpnjensen wrote:
wrote: dxAce wrote: But, is it 'sucking it out' or merely propagating it somewhere else other than that particular spot where your antenna is? And that 'somewhere else' might not be very far away, but merely a few wavelengths in distance. dxAce Michigan USA Before satellites carried most of the milcom they used "diversity receivers". Two, or more, receivers tuned to the same frequency but located some distance apart. The logic being that when the singal faded at one location, the other didn't fade at the same time. The more important a comm cicuit the more receivers spread over a wider area. A friend and I played with our receivers feeding phone patches and since we live 30 miles apart it was clear this approach was workable. With signals that experienced deep fades we were able to listen to nearly all of the time. Real (commercial or military) had AGC based voting systems to decided which signal to pass. We ran into issues of our audio phases shifting producing very odd sounding "flanging" effects. I have often thought about trying this with receivers whose antennas are only a few hundred to thosand feet apart. I never have gotten around to it. The military also used freqeuncy diversity, sending the same singal on more then one frequency. Kind of like listening to WWV on 5 10 and 15MHz at the same time. Terry Fascinating. It sounds like a couple of antennae, maybe even on the same property but spaced some modest distance apart, maybe a few hundred feet, and phased into the same radio, might also be a solution to the problem. Anyone try this with a 50-acre lot and a phasing harness? Bruce Jensen Diversity reception has been a well established practice since the early days. Hallicrafters produced a diversity receiver, which was actually two receivers diplexed into a single audio stage, fed by separate antennae. May have been a bit of overkill. Separate antennae, if electically isolated from one another, diplexed into a single input can produce similar results: reducing selective fading before it reaches the receiver. When I lived in Rockford, I rented a two bedroom home on a private estate west of town. The rental property included several acres on a hilltop, and access to the private lake on the estate. Of course, I went antenna crazy. And using multiple antennae into the BC-794, was able to mitigate a good deal of the selective fading throughout most of the HF spectra. Each antenna was connected to an RF preamp with a gain of 2-6db. The outputs of the preamps were combined through resistive pads (for isolation) into the RF input of the BC-794. The result was nothing short of amazing, with fading distortions dramatically reduced, and program listening, was quite pleasant. Even my wife was no longer critical of SW listening. Make no mistake, it wasn't FM quality. But it was fine wideband (when conditions permitted) AM quality. And though there WAS some latent fading remaining, it was, by far, less objectionable, and often barely noticeable than a single antenna on the same receiver. Diversity, at least in this case, is something to be implemented with affirmative action. :) |
Sync detectors and fading
dxAce wrote: I don't think that would work properly. In practice, I think you need the 'voting machine' that works on two receivers AGC to pick the best signal. I do recall some folks trying to emulate this to a certain degree by having two receivers, two antennas widely seperated (more than a wavelength), and feeding the audio to headphones (one receiver in the right ear, one in the left). dxAce Michigan USA I fed the output of the phone patch intop one ear and my radis audio into the other. Gave me a splitting headache that turned into a miagrane. So I cheated and just fed the patch aduio into one speaker and used the local receiver's speaker. Like I mentioned it worked very well, but the aduio sounded very odd. A "Rolling hollowness is the best discription I can come up with. Seehttp://www.harmony-central.com/Effects/Articles/Flanging/ Terry |
Sync detectors and fading
D Peter Maus wrote: Diversity reception has been a well established practice since the early days. Hallicrafters produced a diversity receiver, which was actually two receivers diplexed into a single audio stage, fed by separate antennae. May have been a bit of overkill. Separate antennae, if electically isolated from one another, diplexed into a single input can produce similar results: reducing selective fading before it reaches the receiver. When I lived in Rockford, I rented a two bedroom home on a private estate west of town. The rental property included several acres on a hilltop, and access to the private lake on the estate. Of course, I went antenna crazy. And using multiple antennae into the BC-794, was able to mitigate a good deal of the selective fading throughout most of the HF spectra. Each antenna was connected to an RF preamp with a gain of 2-6db. The outputs of the preamps were combined through resistive pads (for isolation) into the RF input of the BC-794. The result was nothing short of amazing, with fading distortions dramatically reduced, and program listening, was quite pleasant. Even my wife was no longer critical of SW listening. Make no mistake, it wasn't FM quality. But it was fine wideband (when conditions permitted) AM quality. And though there WAS some latent fading remaining, it was, by far, less objectionable, and often barely noticeable than a single antenna on the same receiver. Diversity, at least in this case, is something to be implemented with affirmative action. :) I will have to try this tonight! I have 2 very nice active dipoles seperated by about 100'. Right now I am using a "phaser" to "rotate" the beam/pattern. It will be very easy to coulple them via a Mini Circuit power divider/combiner! Thanks for the hint. I had always assumed you needed "fancy" electronics to do this. Terry |
Ok, thanks everybody
Ok, I got it. I kinda figured the band was very narrow where you would
lose carrier. The fade is probably moving around the transmitted spectrum like a game of musical chairs. I figured I was going to have to play with AGC some so I'll probably throw an agc amp on the board as well. 73 NEO HFguy wrote: N9NEO wrote: Greetings, I have just got a sony ICF sw7600GR and it is a very nice radio. The sync detector seems to take care of a lot of the distortion, but the audio continues fading in and out and is quite annoying. Could the fading be mitigated to any extent by using another stage of agc? I am going to be doing some experiments with the 455kc if out on my Red Sun RP2100 whenever it gets here. Detectors, filters, SSB, etc... I thought that along with other experiments I might want to try some outboard agc. regards, NEO The sync' is doing what it's designed for by reducing the distortion caused by selective fading, but you need a longer time constant (release time) for the AGC, to help smooth out the fading. You could lengthen the time constant of the AGC circuit in the 7600GR but then it wouldn't work well for other conditions where a shorter time constant is needed. This is why a good table-top receiver has more than one AGC rate, which can be selected by the user. The AGC in the Drake-R8 series uses a decay rate of 300-ms for the fast setting and a much longer rate of about 2-seconds in the slow mode. The latter really aides in reducing the effects of rapid fading. |
Sync detectors and fading
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Sync detectors and fading
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Sync detectors and fading
In article ,
dxAce wrote: Carter-k8vt wrote: wrote: Why do you insist that the atmosphere treats the carrier differently from the rest of the signal? Because it does. See below... Geez. You have a spectrum produced by modulation. If the modulation is AM, then a carrier is present. Now you are saying the atmosphere is sucking out the narrow band carrier and leaving the wideband spectrum untouched. Fiction at best. Bzzzzt. Wrong! Yes, the "atmosphere" [ionosphere] DOES "suck out a narrow band" or even a single frequency. Ask any amateur radio operator that has used RTTY (radio teletype). The RTTY "modulation mode" used is FSK or frequency shift keying. At any given instant, the transmitter is sending either a "mark" or "space", essentially two carriers if you will, 170 Hertz apart that represent the 5-level Baudot code as used in ham RTTY. As an aid to tuning, an oscilloscope is used as a tuning indicator; the mark signal from your RTTY decoder is connected to the horizontal plates of the 'scope, the space signal to the vertical plates. On the screen of the CRT (due to the persistence of the CRT phosphors and your eyes), this shows what appears to be a "+" sign, also known as the classic "cross display". When you see the cross on your screen, you know you are tuned in properly. So, "What does this have to do with the discussion above?" you ask. Remember, you are looking at essentially two "carriers", 170 Hz apart, one on the horizontal axis and one displayed on the vertical axis. During disturbed ionospheric conditions, many times you will see one signal or the other disappear; i.e., the cross turns into a single line, either a "-" or a "|", depending if the mark or space faded--and yes, sometimes both fade, but it is more common to see one or the other disappear. This phenomenon is known to hams as "selective fading", is quite common and is interesting to observe. So, yes, the ionosphere CAN suck out one signal separated from another by as little as 170 Hz. But, is it 'sucking it out' or merely propagating it somewhere else other than that particular spot where your antenna is? And that 'somewhere else' might not be very far away, but merely a few wavelengths in distance. The answer is multi-path signals sum or cancel based on phase as I wrote he "Phase cancelation is a frequency dependent phenomenon and the modulation scheme or part of it does not matter. When phase cancelation is narrow and right at the carrier frequency then the carrier level goes down where the rest of the side band signal is still good." This is called selective fading. This is not an absorption or refraction problem. -- Telamon Ventura, California |
Sync detectors and fading
Carter-k8vt wrote: wrote: Why do you insist that the atmosphere treats the carrier differently from the rest of the signal? Because it does. See below... Geez. You have a spectrum produced by modulation. If the modulation is AM, then a carrier is present. Now you are saying the atmosphere is sucking out the narrow band carrier and leaving the wideband spectrum untouched. Fiction at best. Bzzzzt. Wrong! Yes, the "atmosphere" [ionosphere] DOES "suck out a narrow band" or even a single frequency. Ask any amateur radio operator that has used RTTY (radio teletype). If the carrier is effected, the odds are signals nearby (i.e. the audio) will be effected. I still stand by the statement that the only advantage to sync demod is that you have cut the bandwidth in half. I never bought the "fresh carrier" story. The RTTY "modulation mode" used is FSK or frequency shift keying. At any given instant, the transmitter is sending either a "mark" or "space", essentially two carriers if you will, 170 Hertz apart that represent the 5-level Baudot code as used in ham RTTY. As an aid to tuning, an oscilloscope is used as a tuning indicator; the mark signal from your RTTY decoder is connected to the horizontal plates of the 'scope, the space signal to the vertical plates. On the screen of the CRT (due to the persistence of the CRT phosphors and your eyes), this shows what appears to be a "+" sign, also known as the classic "cross display". When you see the cross on your screen, you know you are tuned in properly. So, "What does this have to do with the discussion above?" you ask. Remember, you are looking at essentially two "carriers", 170 Hz apart, one on the horizontal axis and one displayed on the vertical axis. During disturbed ionospheric conditions, many times you will see one signal or the other disappear; i.e., the cross turns into a single line, either a "-" or a "|", depending if the mark or space faded--and yes, sometimes both fade, but it is more common to see one or the other disappear. This phenomenon is known to hams as "selective fading", is quite common and is interesting to observe. So, yes, the ionosphere CAN suck out one signal separated from another by as little as 170 Hz. Carter K8VT If the selective fading is as tight as you indicate, then there would be "holes" in the audio spectrum of the recovered AM, much like a comb filter. Sync demod won't fix that problem. There are too many people that think sync demod will cure everything. It's just not true. Now if you have a nearby signal bleeding into the desired signal, then pick the sideband the furthest away from the interfering signal. Here, sync works great. If you have fading, you can narrow band the signal by using one sideband. It helps a bit, but the signal will still fade. All this assumes your sync is decent, and not a growler. Otherwise, all bets are off. |
Sync detectors and fading
On 17 Jan 2007 20:54:27 -0800, wrote:
Telamon wrote: In article . com, wrote: Michael Black wrote: "N9NEO" ) writes: Greetings, I have just got a sony ICF sw7600GR and it is a very nice radio. The sync detector seems to take care of a lot of the distortion, but the audio continues fading in and out and is quite annoying. Could the fading be mitigated to any extent by using another stage of agc? I am going to be doing some experiments with the 455kc if out on my Red Sun RP2100 whenever it gets here. Detectors, filters, SSB, etc... I thought that along with other experiments I might want to try some outboard agc. Synchronous detectors have never been about dealing with fading. They are about ensuring there is enough "carrier" to beat the sidebands down to audio. Narrow band signal have less fading, thus sync demod will have less fading. However, the result isn't all that significant since all you have done is cut the bandwidth in half. Narrow band signals do not have less fading. So there's fading on the incoming signal. That means the amplitude of the sidebands is varying with that fading. A locally generated "carrier" at the receiver ensures that there is something to beat those sidebands down to audio, even if the transmitter's carrier has faded too much to do the proper job. But a constant level "carrier" at the receiver beats the sideband down to audio intact, ie an ideal mixer would not add anything to the signal. So if the sideband is fading, of course the audio output of the receiver will vary with that fading. With an envelope detector, the carrier isn't beating down the sideband. If you just look at the math of AM modulation, you would see that the carrier is just there for the ride. Selective fading occurs when conditions cause a very narrow band of frequencies to be received at very low amplitudes where most of the side band information is present at levels that your receiver can ordinarily demodulate properly. When part of the side band is being notched out it does not sound all that bad but when the carrier gets weakened then the AM demodulator can't process the side band information properly and there is horrendous distortion. The carrier which is at the right frequency and phase relative to the side band information keeps the detector in the linear region so distortion is minimized. A sync detector uses a local oscillator in a similar to the way SSB is detected with the difference that it is phase locked to the signal carrier and mixed with it so when the carrier fades out this near perfect copy of the carrier allows the demodulator to continue to detect the side band or bands without distortion during a carrier fading condition. Here this necessary frequency and phase information carried by the "carrier" is retained by the sync circuitry. What the sync detector brings you is the ability to decode that signal even if the carrier goes missing, because of selective fading. Snip Michael has it right. -- Telamon Ventura, California Why do you insist that the atmosphere treats the carrier differently from the rest of the signal? Geez. You have a spectrum produced by modulation. If the modulation is AM, then a carrier is present. Now you are saying the atmosphere is sucking out the narrow band carrier and leaving the wideband spectrum untouched. Fiction at best. No, it is called selective fading, and it is a real phenomenon. You can think of the two sidebands for AM as creating constructive and destructive interference with the carrier. Any time the sideband energy exceeds the carrier energy you get the equivalent of over modulation. DSB and an envelope detector isn't a good combination. As others have pointed out, what Synch detection does is to insure that the sideband energy can never exceed the carrier energy. While it is preferable to have it in phase, which a synch detector does, in human speech, phase carries no information. As a result SSB can be used to communicate. A product detector (which is used for synch detection) simply produces phase distortion if the local carrier isn't in phase with the original carrier. The phase distortion is a fact of life in SSB-SC communication. If you Independent side band, and generate the two independent side bands at quardrature, it is possible to determine, and lock the local carrier in phase with the original carrier. |
Sync detectors and fading
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Sync detectors and fading
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Sync detectors and fading
In article . com,
wrote: If the selective fading is as tight as you indicate, then there would be "holes" in the audio spectrum of the recovered AM, much like a comb filter. Sync demod won't fix that problem. But the audio spectrum comes pre-equipped with a lot of holes* so there is some benefit, as sometimes it's unimportant frequencies that are fading out. (*That's why MP3 type psycho-acoustic compression can work). There are too many people that think sync demod will cure everything. It's just not true. Now if you have a nearby signal bleeding into the desired signal, then pick the sideband the furthest away from the interfering signal. Here, sync works great. If you have fading, you can narrow band the signal by using one sideband. It helps a bit, but the signal will still fade. I find that there's a number of signals where (as close as I can get to) sync detection (by zero beating the BFO in SSB mode) cleans up the audio to the point where it's listen-able. Typically, these are weak and fluttery high latitude path signals, Europe to Western North America. Mark Zenier Googleproofaddress(account:mzenier provider:eskimo domain:com) |
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