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Frequency accuracy in older RXs
Being somewhat of a polymath (just spent all day fence judging
at a horse trial) I find that I have a string of ideas faster than I could ever implement them (rather unkindly described in one area as vapourware), but I think it to be useful to punt them for a wider discussion. Musing upon the Huff-and-Puff technique, I wondered if there was a better way to improve the frequency stability of older RXs, because the Huff-and-Puff necessarily brings about a punctuated frequency span (eg, multiples of 32 Hz), and this is what I came up with ... Using the ubiquitous timers to be found en masse in most micros that seem to sell for only a few pence / cents these days, implement a frequency counter to measure the local oscillator. Then, when the user presses a Lock button (yet to be provided) the same micro can program an si570 to generate the same frequency indefinitely and to switch the mixer stage from the original to this new oscillator. |
Frequency accuracy in older RXs
On Sun, 05 Oct 2014 19:30:43 +0100, gareth wrote:
Being somewhat of a polymath (just spent all day fence judging at a horse trial) I find that I have a string of ideas faster than I could ever implement them (rather unkindly described in one area as vapourware), but I think it to be useful to punt them for a wider discussion. Musing upon the Huff-and-Puff technique, I wondered if there was a better way to improve the frequency stability of older RXs, because the Huff-and-Puff necessarily brings about a punctuated frequency span (eg, multiples of 32 Hz), and this is what I came up with ... Using the ubiquitous timers to be found en masse in most micros that seem to sell for only a few pence / cents these days, implement a frequency counter to measure the local oscillator. Then, when the user presses a Lock button (yet to be provided) the same micro can program an si570 to generate the same frequency indefinitely and to switch the mixer stage from the original to this new oscillator. If you are going to go to those lengths, why not program the micro to recognise the difference between rapid changes of frequency (tuning) and slower changes (drift) and apply a correctional voltage to a varactor in the case of the latter. Capture range may be a problem if the radio is a determined drifter. |
Frequency accuracy in older RXs
On 10/5/2014 3:15 PM, Radiohead70 wrote:
On Sun, 05 Oct 2014 19:30:43 +0100, gareth wrote: Being somewhat of a polymath (just spent all day fence judging at a horse trial) I find that I have a string of ideas faster than I could ever implement them (rather unkindly described in one area as vapourware), but I think it to be useful to punt them for a wider discussion. Musing upon the Huff-and-Puff technique, I wondered if there was a better way to improve the frequency stability of older RXs, because the Huff-and-Puff necessarily brings about a punctuated frequency span (eg, multiples of 32 Hz), and this is what I came up with ... Using the ubiquitous timers to be found en masse in most micros that seem to sell for only a few pence / cents these days, implement a frequency counter to measure the local oscillator. Then, when the user presses a Lock button (yet to be provided) the same micro can program an si570 to generate the same frequency indefinitely and to switch the mixer stage from the original to this new oscillator. If you are going to go to those lengths, why not program the micro to recognise the difference between rapid changes of frequency (tuning) and slower changes (drift) and apply a correctional voltage to a varactor in the case of the latter. Capture range may be a problem if the radio is a determined drifter. I'm not sure I understand the issue. But if you want a digitally controlled oscillator for your mixer why not just make a digitally controlled oscillator for your mixer? That is already there with the Si570. Why the complex usage of using the existing oscillator and then switching? -- Rick |
Frequency accuracy in older RXs
"rickman" wrote in message
... I'm not sure I understand the issue. But if you want a digitally controlled oscillator for your mixer why not just make a digitally controlled oscillator for your mixer? That is already there with the Si570. Why the complex usage of using the existing oscillator and then switching? The older RX, with its mechanical tuning, perhaps loaded with a flywheel, is a much more ergonomic tuning mechanism than anything that has come out of the semiconductor world. Therefore, use the existing tuning dial in the first place, and then lock with the 2nd oscillator. |
Frequency accuracy in older RXs
On 10/5/2014 6:34 PM, gareth wrote:
"rickman" wrote in message ... I'm not sure I understand the issue. But if you want a digitally controlled oscillator for your mixer why not just make a digitally controlled oscillator for your mixer? That is already there with the Si570. Why the complex usage of using the existing oscillator and then switching? The older RX, with its mechanical tuning, perhaps loaded with a flywheel, is a much more ergonomic tuning mechanism than anything that has come out of the semiconductor world. Therefore, use the existing tuning dial in the first place, and then lock with the 2nd oscillator. There are controls which can be just as nice to spin as any existing tuning dial and can control a digital circuit. What does the existing knob control, an air capacitor? That dial knob can be connected to another type of control which can be read by the MCU to control the Si570. -- Rick |
Frequency accuracy in older RXs
"rickman" wrote in message
... There are controls which can be just as nice to spin as any existing tuning dial and can control a digital circuit. What does the existing knob control, an air capacitor? That dial knob can be connected to another type of control which can be read by the MCU to control the Si570. How would you move the dial pointer in such cases? |
Frequency accuracy in older RXs
On 10/5/2014 7:28 PM, gareth wrote:
"rickman" wrote in message ... There are controls which can be just as nice to spin as any existing tuning dial and can control a digital circuit. What does the existing knob control, an air capacitor? That dial knob can be connected to another type of control which can be read by the MCU to control the Si570. How would you move the dial pointer in such cases? Why change that? -- Rick |
Frequency accuracy in older RXs
On Sun, 5 Oct 2014, rickman wrote:
On 10/5/2014 3:15 PM, Radiohead70 wrote: On Sun, 05 Oct 2014 19:30:43 +0100, gareth wrote: Being somewhat of a polymath (just spent all day fence judging at a horse trial) I find that I have a string of ideas faster than I could ever implement them (rather unkindly described in one area as vapourware), but I think it to be useful to punt them for a wider discussion. Musing upon the Huff-and-Puff technique, I wondered if there was a better way to improve the frequency stability of older RXs, because the Huff-and-Puff necessarily brings about a punctuated frequency span (eg, multiples of 32 Hz), and this is what I came up with ... Using the ubiquitous timers to be found en masse in most micros that seem to sell for only a few pence / cents these days, implement a frequency counter to measure the local oscillator. Then, when the user presses a Lock button (yet to be provided) the same micro can program an si570 to generate the same frequency indefinitely and to switch the mixer stage from the original to this new oscillator. If you are going to go to those lengths, why not program the micro to recognise the difference between rapid changes of frequency (tuning) and slower changes (drift) and apply a correctional voltage to a varactor in the case of the latter. Capture range may be a problem if the radio is a determined drifter. I'm not sure I understand the issue. But if you want a digitally controlled oscillator for your mixer why not just make a digitally controlled oscillator for your mixer? That is already there with the Si570. Why the complex usage of using the existing oscillator and then switching? He wants a big Eddystone dial, likely connected to a Command Set or BC221 variable capacitor. Michael |
Frequency accuracy in older RXs
On Sun, 5 Oct 2014, gareth wrote:
Being somewhat of a polymath (just spent all day fence judging at a horse trial) I find that I have a string of ideas faster than I could ever implement them (rather unkindly described in one area as vapourware), but I think it to be useful to punt them for a wider discussion. Musing upon the Huff-and-Puff technique, I wondered if there was a better way to improve the frequency stability of older RXs, because the Huff-and-Puff necessarily brings about a punctuated frequency span (eg, multiples of 32 Hz), and this is what I came up with ... It's worth pointing out that the "Huff and Puff" seems to have been done originally without a frequency counter. The circuitry is similar, but it's just done with counters and gates, no inherent need for the latches and decoders and readouts. I'm pretty sure that came first, unless the concept came from two different places about the same. So you could take any old receiver, so long as it had a decent dial, and add some stability. it was later that the same concept appeared (or so I remember) as an extension of a frequency counter. That makes sense, since there was a transition to frequency counters, and the "lock" circuit was just a bit more. Using the ubiquitous timers to be found en masse in most micros that seem to sell for only a few pence / cents these days, implement a frequency counter to measure the local oscillator. Then, when the user presses a Lock button (yet to be provided) the same micro can program an si570 to generate the same frequency indefinitely and to switch the mixer stage from the original to this new oscillator. As a standalone, the odd lock points are because of the way the counters are arranged, I don't think it was anything to do with some design need (other than simplicity). Once you are adapting a frequency counter, you end up with decade type steps. But (and I've never tried the circuitry) I don't think these tune the way you are expecting. It takes time to count, so you aren't snapping between steps like with an FM broadcast receiver with AFC or a synthesizer of the type in the HRO-500. The notion of the circuit is to keep it locked, with in the nearest step. So only if it drifts beyond the specified step does it jump back (and "jump" is probably the wrong word here). Michael |
Frequency accuracy in older RXs
On 10/5/2014 9:42 PM, Michael Black wrote:
On Sun, 5 Oct 2014, gareth wrote: Being somewhat of a polymath (just spent all day fence judging at a horse trial) I find that I have a string of ideas faster than I could ever implement them (rather unkindly described in one area as vapourware), but I think it to be useful to punt them for a wider discussion. Musing upon the Huff-and-Puff technique, I wondered if there was a better way to improve the frequency stability of older RXs, because the Huff-and-Puff necessarily brings about a punctuated frequency span (eg, multiples of 32 Hz), and this is what I came up with ... It's worth pointing out that the "Huff and Puff" seems to have been done originally without a frequency counter. The circuitry is similar, but it's just done with counters and gates, no inherent need for the latches and decoders and readouts. I'm pretty sure that came first, unless the concept came from two different places about the same. So you could take any old receiver, so long as it had a decent dial, and add some stability. it was later that the same concept appeared (or so I remember) as an extension of a frequency counter. That makes sense, since there was a transition to frequency counters, and the "lock" circuit was just a bit more. Using the ubiquitous timers to be found en masse in most micros that seem to sell for only a few pence / cents these days, implement a frequency counter to measure the local oscillator. Then, when the user presses a Lock button (yet to be provided) the same micro can program an si570 to generate the same frequency indefinitely and to switch the mixer stage from the original to this new oscillator. As a standalone, the odd lock points are because of the way the counters are arranged, I don't think it was anything to do with some design need (other than simplicity). Once you are adapting a frequency counter, you end up with decade type steps. But (and I've never tried the circuitry) I don't think these tune the way you are expecting. It takes time to count, so you aren't snapping between steps like with an FM broadcast receiver with AFC or a synthesizer of the type in the HRO-500. The notion of the circuit is to keep it locked, with in the nearest step. So only if it drifts beyond the specified step does it jump back (and "jump" is probably the wrong word here). I don't think what you are describing and what the OP is describing are the same thing. His approach is to let the receiver tune normally, but when a button is pushed, the receiver's VFO is replaced with a digital VFO automatically tuned to the same frequency. I don't see any problems with doing that. A frequency counter takes some time to make a measurement which is related to the resolution desired. But it is not in any way limited to decades or even Hertz. But there are other ways of measuring frequency. The Si570 can be tuned in 10 ms, if I read the data sheet correctly when I scanned it. A relative measurement of frequency can be done nearly instantaneously (relatively speaking). So doing a binary search will let you tune to 1 ppm accuracy in 19 steps or about 190 ms. Actually the Si570 can tune faster than that once you get within 3500 ppm so every bit after the first 9 is much faster. At some point taking the measurement takes more time than the tuning as finer resolutions take more and more time. Hmmm, I was thinking as I typed this and I think I realize the binary search doesn't help you much if at all. The measurements are fast in the beginning, but even to determine if your synthesized frequency is high or low for the final bit takes as long as a complete measurement by frequency counter... maybe. If a phase detector with a proportional output is used, it can speed up the measurement. In essence use the Si570 as the VFO in a PLL until it is locked and then hold the settings and make the switch so the receiver is using the synthesized reference. More thinking on the fly... a two stage approach may give the fastest lock. A coarse measurement of frequency to get the VFO in the right ballpark would be fairly fast. Then use a phase detector to lock the Si570 to the VFO through the MCU. It could be used as a digital frequency readout when you are manually tuning. If anyone thinks there could be a market for this device I would be interested in making it. I can see it being sold for as little as $50 in a case with a button and a couple of BNC connectors. This unit would need to be spliced into the VFO signal path somehow. Do receivers typically have inputs and outputs for the VFO? -- Rick |
Frequency accuracy in older RXs
"Michael Black" wrote in message
news:alpine.LNX.2.02.1410052133310.11722@darkstar. example.org... He wants a big Eddystone dial, likely connected to a Command Set or BC221 variable capacitor. Partially right, but after recent posts from me on that matter, I acquired an Eddystone EA12, which is very stable, because, as its handbook claims, its thermal capacity is large. However, just before the acquisition, I finally put money where my mouth is and obtained some heavy grade aluminium plate to form the chassis of my projected retro rig, and, anticipating oscillator drift wanted to discuss various ways of circumventing that. I am fully aware of the huff-and-puff and how it functions, BTW, but the huff-and-puff is not absolutely stable to suit reception of some digital techniques. Yes, I do want RX facilities that have the human-friendly big flywheel knob and moving pointer, an ergonomic appraoch with its visual feedback that has never been equalled in any rice box. Think of the difference between using a DVM and an analogue meter such as an Avometer where visual tweaking is more usable! |
Frequency accuracy in older RXs
On 10/5/2014 2:30 PM, gareth wrote:
Being somewhat of a polymath (just spent all day fence judging at a horse trial) I find that I have a string of ideas faster than I could ever implement them (rather unkindly described in one area as vapourware), but I think it to be useful to punt them for a wider discussion. Musing upon the Huff-and-Puff technique, I wondered if there was a better way to improve the frequency stability of older RXs, because the Huff-and-Puff necessarily brings about a punctuated frequency span (eg, multiples of 32 Hz), and this is what I came up with ... Using the ubiquitous timers to be found en masse in most micros that seem to sell for only a few pence / cents these days, implement a frequency counter to measure the local oscillator. Then, when the user presses a Lock button (yet to be provided) the same micro can program an si570 to generate the same frequency indefinitely and to switch the mixer stage from the original to this new oscillator. Gareth really had me going on this one. I was actually thinking of building this. But there is only one problem with the idea. To use the Si570 as the VFO for a receiver it would need to output a sine wave. However the Si570 outputs square waves for digital circuitry. I don't think the mixer would appreciate all the harmonics produced in a square wave would it? On the other hand, the Huff-and-Puff approach can be improved by making the reference frequency adjustable. Tune the receiver in the normal way and when you press the button the MCU determines the appropriate frequency to use for the PLL reference. -- Rick |
Frequency accuracy in older RXs
"rickman" wrote in message ... Gareth really had me going on this one. I was actually thinking of building this. But there is only one problem with the idea. To use the Si570 as the VFO for a receiver it would need to output a sine wave. However the Si570 outputs square waves for digital circuitry. I don't think the mixer would appreciate all the harmonics produced in a square wave would it? I don't know how it would work at RF, but as a square wave is made up of all odd harmonics, if you use a low pass filter after it you should get a good sine wave. This was done years ago in a teletype tone generator to generate pure sine waves to modulate a SSB transmitter with a single tone. The filter was made of coils and capacitors. It may be made more compact now there are many op amps and other active devices in production. Just something to think about. On the other hand, the Huff-and-Puff approach can be improved by making the reference frequency adjustable. Tune the receiver in the normal way and when you press the button the MCU determines the appropriate frequency to use for the PLL reference. -- Rick --- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com |
Frequency accuracy in older RXs
On 06/10/2014 16:43, rickman wrote:
building this. But there is only one problem with the idea. To use the Si570 as the VFO for a receiver it would need to output a sine wave. However the Si570 outputs square waves for digital circuitry. I don't think the mixer would appreciate all the harmonics produced in a square wave would it? Then use a DDS. PA |
Frequency accuracy in older RXs
On 10/6/2014 12:33 PM, Ralph Mowery wrote:
"rickman" wrote in message ... Gareth really had me going on this one. I was actually thinking of building this. But there is only one problem with the idea. To use the Si570 as the VFO for a receiver it would need to output a sine wave. However the Si570 outputs square waves for digital circuitry. I don't think the mixer would appreciate all the harmonics produced in a square wave would it? I don't know how it would work at RF, but as a square wave is made up of all odd harmonics, if you use a low pass filter after it you should get a good sine wave. This was done years ago in a teletype tone generator to generate pure sine waves to modulate a SSB transmitter with a single tone. The filter was made of coils and capacitors. It may be made more compact now there are many op amps and other active devices in production. Just something to think about. That is fine if your frequency is fixed, but where would you put the corner frequency for a tunable VFO? Also, it is hard to get enough attenuation of the second harmonic since it is the closest to the fundamental and the one most likely to give you trouble in the mixer. -- Rick |
Frequency accuracy in older RXs
On 10/6/2014 12:38 PM, Peter Able wrote:
On 06/10/2014 16:43, rickman wrote: building this. But there is only one problem with the idea. To use the Si570 as the VFO for a receiver it would need to output a sine wave. However the Si570 outputs square waves for digital circuitry. I don't think the mixer would appreciate all the harmonics produced in a square wave would it? Then use a DDS. There are lots of things you could do. I think Gareth got the idea because the Si570 looked like it would be easy to use. But it just doesn't fit the bill. What frequency range would be of interest for tuning these older receivers? Once I read up on the Huff and Puff modification, I like the idea. It doesn't change the receiver really, it just keeps it on the correct frequency by using the VFO as a VCVFO. Is it common to have a voltage control on the VFO in these receivers? Or is that a mod that would have to be made? -- Rick |
Frequency accuracy in older RXs
On 06/10/2014 18:12, rickman wrote:
On 10/6/2014 12:38 PM, Peter Able wrote: On 06/10/2014 16:43, rickman wrote: building this. But there is only one problem with the idea. To use the Si570 as the VFO for a receiver it would need to output a sine wave. However the Si570 outputs square waves for digital circuitry. I don't think the mixer would appreciate all the harmonics produced in a square wave would it? Then use a DDS. There are lots of things you could do. I think Gareth got the idea because the Si570 looked like it would be easy to use. But it just doesn't fit the bill. You posed a problem; I gave you an answer. BTW there cannot be an easier item to lash to a microcontroller than a DDS What frequency range would be of interest for tuning these older receivers? Up to 30MHz? Once I read up on the Huff and Puff modification, I like the idea. It doesn't change the receiver really, it just keeps it on the correct frequency by using the VFO as a VCVFO. Huff-and-Puff is fine - but its weak point is that the oscillator being controlled must drift by less than the Huff-and-Puff step in the Huff-and-Puff update period. Is it common to have a voltage control on the VFO in these receivers? Or is that a mod that would have to be made? No, most applications of Huff-and-Puff have included the varicap diode as part of the mod. Not a big issue - unless you want to keep the receiver totally authentic. This is really quite an interesting idea. I've DDS'ed some classic sets. The change in performance was startling - and I had no problem getting used to driving the set via a 4 by 4 keypad - but I can readily appreciate that this proposed combination of the operational advantage of the newer technology with the older style of user interface has real charm. PA |
Frequency accuracy in older RXs
"rickman" wrote in message ... I don't know how it would work at RF, but as a square wave is made up of all odd harmonics, if you use a low pass filter after it you should get a good sine wave. This was done years ago in a teletype tone generator to generate pure sine waves to modulate a SSB transmitter with a single tone. The filter was made of coils and capacitors. It may be made more compact now there are many op amps and other active devices in production. Just something to think about. That is fine if your frequency is fixed, but where would you put the corner frequency for a tunable VFO? Also, it is hard to get enough attenuation of the second harmonic since it is the closest to the fundamental and the one most likely to give you trouble in the mixer. There will not be a second harmonic for a square wave, just the odd number such as the 3,5,7 and so on. That was one of the reasons for generating the tones and passing them through a circuit that converted them into good square waves and then to the low pass filter. I don't recall if any frequency range of the VFO was mentioned. So if going from a range of 5 to 8 MHz the corner frequency could start around 9 MHZ and should be dropping off alot at 15 MHz where the 3 rd harmoinc of the starting point of 5 MHz would be. --- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com |
Frequency accuracy in older RXs
On 10/6/2014 1:31 PM, Ralph Mowery wrote:
"rickman" wrote in message ... I don't know how it would work at RF, but as a square wave is made up of all odd harmonics, if you use a low pass filter after it you should get a good sine wave. This was done years ago in a teletype tone generator to generate pure sine waves to modulate a SSB transmitter with a single tone. The filter was made of coils and capacitors. It may be made more compact now there are many op amps and other active devices in production. Just something to think about. That is fine if your frequency is fixed, but where would you put the corner frequency for a tunable VFO? Also, it is hard to get enough attenuation of the second harmonic since it is the closest to the fundamental and the one most likely to give you trouble in the mixer. There will not be a second harmonic for a square wave, just the odd number such as the 3,5,7 and so on. That was one of the reasons for generating the tones and passing them through a circuit that converted them into good square waves and then to the low pass filter. You are right, no even harmonics in a square wave. What circuit clips a tone into a square wave just so it could be run through a low pass filter? I don't recall if any frequency range of the VFO was mentioned. So if going from a range of 5 to 8 MHz the corner frequency could start around 9 MHZ and should be dropping off alot at 15 MHz where the 3 rd harmoinc of the starting point of 5 MHz would be. So is this a viable practice to shape a square wave with a filter to use in a mixer? I just find it hard to imagine that the harmonics would not create some real problems. I'd have to do the math, but I expect even a third harmonic has potential of creating a lot of spurs. I guess I'm used to dealing with people who want very selective receivers. But I have to admit I have forgotten a lot of the little bit of receiver design I did learn. -- Rick |
Frequency accuracy in older RXs
On 10/6/2014 1:26 PM, Peter Able wrote:
On 06/10/2014 18:12, rickman wrote: On 10/6/2014 12:38 PM, Peter Able wrote: On 06/10/2014 16:43, rickman wrote: building this. But there is only one problem with the idea. To use the Si570 as the VFO for a receiver it would need to output a sine wave. However the Si570 outputs square waves for digital circuitry. I don't think the mixer would appreciate all the harmonics produced in a square wave would it? Then use a DDS. There are lots of things you could do. I think Gareth got the idea because the Si570 looked like it would be easy to use. But it just doesn't fit the bill. You posed a problem; I gave you an answer. BTW there cannot be an easier item to lash to a microcontroller than a DDS Thank you for your suggestion. What frequency range would be of interest for tuning these older receivers? Up to 30MHz? Once I read up on the Huff and Puff modification, I like the idea. It doesn't change the receiver really, it just keeps it on the correct frequency by using the VFO as a VCVFO. Huff-and-Puff is fine - but its weak point is that the oscillator being controlled must drift by less than the Huff-and-Puff step in the Huff-and-Puff update period. I have to say I did not look at this circuit long enough and thought I understood it but in fact I was missing some of the crudeness to the operation. There are a number of things that can be done to address that particular limitation, the most obvious one is to use a longer counter for the frequency measurement. I still may not fully understand all the limitations but I'm not suggesting this circuit should be used. Is it common to have a voltage control on the VFO in these receivers? Or is that a mod that would have to be made? No, most applications of Huff-and-Puff have included the varicap diode as part of the mod. Not a big issue - unless you want to keep the receiver totally authentic. This is really quite an interesting idea. I've DDS'ed some classic sets. The change in performance was startling - and I had no problem getting used to driving the set via a 4 by 4 keypad - but I can readily appreciate that this proposed combination of the operational advantage of the newer technology with the older style of user interface has real charm. As was initially suggested, I think the original dial of the receiver can still be used. Either the knob can be connected to something other than the tuning capacitor or something like the Huff and Puff or better a PLL can be used to stabilize the frequency. I expect this has been done already by someone. Anyone seen a PLL used to stabilize a VCO in a similar way as the Huff and Puff? -- Rick |
Frequency accuracy in older RXs
"rickman" wrote in message ... On 10/6/2014 1:31 PM, Ralph Mowery wrote: .. You are right, no even harmonics in a square wave. What circuit clips a tone into a square wave just so it could be run through a low pass filter? I thought we were going to start with a square wave. Nothing shapes the square wave. You just run it through a low pass filter just above the fundimental or 1st harmonic if you want to call it that.. The circuit I am thinking about is from an old RTTY audio tone generator I built and was designed by Irv Hoff years ago, around 1970 or so. He used a unijunction to generate a tone of 2125 or 2295 Hz (rtty tones) then fed it into a 2 transistor multivibrator to generate good square waves. Then into a low pass filter made of two coils and I think 2 or 3 capacitors. This filtered out all the odd harmonics ( as there are no even harmonics in the square wave) and just leaves the fundimental frequency as a pure sine wave. That way you could have a VFO going from 5 to 6 MHz and set the filter for about 8 MHz and anything above that would be filtered out. No harmonics or anything but a pure sine wave. As I said, not sure if this would work at RF or not, just something to think about. --- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com |
Frequency accuracy in older RXs
On Tue, 7 Oct 2014, Ralph Mowery wrote:
That way you could have a VFO going from 5 to 6 MHz and set the filter for about 8 MHz and anything above that would be filtered out. No harmonics or anything but a pure sine wave. But the problem is, most rigs aren't using that kind of design now. They are single conversion (with a 9MHz or so IF) or an upconversion to above 30MHz, so the "VFO" covers a large territory, and since most rigs are now general coverage (since now it's almost as easy as a hamband only rig), you need a lot more low pass filters. You're right, if this was in the old days, with a fixed range VFO, a low pass filter would be fine, since only one would be needed. As I said, not sure if this would work at RF or not, just something to think about. One reason Irving Hoff divided that unijunction oscillator output was because the unijunction put out a tiny pulse, which has a lot more harmonnc content, dividing it by 2 gave the waveforme a 50% square wave, which has harmonics, but not at every harmonic, so it's simpler to filter. There are better ways now. With almost the same amount of division (especially in this case since the unijunction oscillator frequency doesn't matter, so long as it's low enough to be stable), you can generate "stepped sinewaves" with a ripple counter and some weighted resistors, giving you a cleaner waveform that requires less filtering. That sort of thing was around not long after that Irving Hoff article, just a few years. Michael |
Frequency accuracy in older RXs
On Mon, 6 Oct 2014, rickman wrote:
On 10/5/2014 2:30 PM, gareth wrote: Being somewhat of a polymath (just spent all day fence judging at a horse trial) I find that I have a string of ideas faster than I could ever implement them (rather unkindly described in one area as vapourware), but I think it to be useful to punt them for a wider discussion. Musing upon the Huff-and-Puff technique, I wondered if there was a better way to improve the frequency stability of older RXs, because the Huff-and-Puff necessarily brings about a punctuated frequency span (eg, multiples of 32 Hz), and this is what I came up with ... Using the ubiquitous timers to be found en masse in most micros that seem to sell for only a few pence / cents these days, implement a frequency counter to measure the local oscillator. Then, when the user presses a Lock button (yet to be provided) the same micro can program an si570 to generate the same frequency indefinitely and to switch the mixer stage from the original to this new oscillator. Gareth really had me going on this one. I was actually thinking of building this. But there is only one problem with the idea. To use the Si570 as the VFO for a receiver it would need to output a sine wave. However the Si570 outputs square waves for digital circuitry. I don't think the mixer would appreciate all the harmonics produced in a square wave would it? Since a lot of mixers wnat to be driven hard, the square wave may not matter. About 1974, there was an article in Ham Radio for a phasing type direct conversion SSB receiver, and he used ECL to generate the needed signal to the mixers that were 90degrees apart. He didn't seem to find the square wave an issue. A bit later, someone did an article in Ham Radio wondering about this sort of thing, and he tried square waves with a mosfet mixer, and a 1496 type mixer, and I thought he found the outcome wasn't a problem with a square wave. If the input to the mixer isnt' well filtered, then I think you're going to get all kinds of weird responses, since something can mix with a harmonic of the oscillator and be converted down to the IF. But you'd generally want good front end filtering, for other reasons. Michael |
Frequency accuracy in older RXs
"Michael Black" wrote in message news:alpine.LNX.2.02.1410071258330.14980@darkstar. example.org... But the problem is, most rigs aren't using that kind of design now. They are single conversion (with a 9MHz or so IF) or an upconversion to above 30MHz, so the "VFO" covers a large territory, and since most rigs are now general coverage (since now it's almost as easy as a hamband only rig), you need a lot more low pass filters. You're right, if this was in the old days, with a fixed range VFO, a low pass filter would be fine, since only one would be needed. For new designs I am sure there are beter ways of doing things. I was just throwing out the low pass filter sort of thing as someone was going to start with an oscillator that put out square waves. I was just thinking of an easy way to get from the square wave output to a sine wave and eliminate the harmonics that may or may not cause problems. As I said, I don't even know if it is practical to do that at RF. I don't have a way to generate square waves at RF, but did play around with a couple of audio filters I had with a scope and function generator. I would thing it would work the same at RF as it does at AF if the stray components can be managed. Very interisting to me. I decided to play for a while in my shop. The filter with coils in it would take out the harmonics and give a very good sine wave output. I had a low pass filter designed to use on a frequency counter or scope with a cut off of 5 KHz. It did a poor job of rounding off the square wave, but of all things when I switched the function generator to a triangle wave it did a good job of making a sine wave out of the signal. I thought the triangle wave had even harmonics in it, but found out it actually has odd harmonics but they decrease with the square of the order instead of just a simple 1/N. That ment a lot less harmonic content to filter out. It was a long time ago that I played with the equations for the various waveforms that can be made out of a bunch of sine waves added together. A good memory refresher. --- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com |
Frequency accuracy in older RXs
"Ralph Mowery" wrote in message
... I thought the triangle wave had even harmonics in it, but found out it actually has odd harmonics but they decrease with the square of the order instead of just a simple 1/N. It is the sawtooth (equal slopes at rise and decay) that is made up of even harmonics. (I cannot speak with any authority in the triangle wave) |
Frequency accuracy in older RXs
On 10/7/2014 10:53 AM, Ralph Mowery wrote:
"rickman" wrote in message ... On 10/6/2014 1:31 PM, Ralph Mowery wrote: . You are right, no even harmonics in a square wave. What circuit clips a tone into a square wave just so it could be run through a low pass filter? I thought we were going to start with a square wave. Nothing shapes the square wave. You just run it through a low pass filter just above the fundimental or 1st harmonic if you want to call it that.. The circuit I am thinking about is from an old RTTY audio tone generator I built and was designed by Irv Hoff years ago, around 1970 or so. He used a unijunction to generate a tone of 2125 or 2295 Hz (rtty tones) then fed it into a 2 transistor multivibrator to generate good square waves. Then into a low pass filter made of two coils and I think 2 or 3 capacitors. This filtered out all the odd harmonics ( as there are no even harmonics in the square wave) and just leaves the fundimental frequency as a pure sine wave. That way you could have a VFO going from 5 to 6 MHz and set the filter for about 8 MHz and anything above that would be filtered out. No harmonics or anything but a pure sine wave. As I said, not sure if this would work at RF or not, just something to think about. You say the filter removes "all" of the harmonics... that is obviously not correct. The filter may reduce them, but it does not and can not completely remove them. The nearest tones (which are also the largest amplitude tones) will only be reduced a small amount really. Or maybe you are planning to use a brick wall filter? My question intended to ask *WHY* would anyone design a circuit to produce a square wave and then spend the time and trouble to filter it? Earlier you mention that filters are easier now, but in reality analog filters are still much more difficult than just generating a tone in the first place. I can use a single chip and a DAC to produce tones up to many MHz with very high accuracy and purity. I don't get it... -- Rick |
Frequency accuracy in older RXs
On 10/7/2014 2:07 PM, Ralph Mowery wrote:
"Michael Black" wrote in message news:alpine.LNX.2.02.1410071258330.14980@darkstar. example.org... But the problem is, most rigs aren't using that kind of design now. They are single conversion (with a 9MHz or so IF) or an upconversion to above 30MHz, so the "VFO" covers a large territory, and since most rigs are now general coverage (since now it's almost as easy as a hamband only rig), you need a lot more low pass filters. You're right, if this was in the old days, with a fixed range VFO, a low pass filter would be fine, since only one would be needed. For new designs I am sure there are beter ways of doing things. I was just throwing out the low pass filter sort of thing as someone was going to start with an oscillator that put out square waves. I was just thinking of an easy way to get from the square wave output to a sine wave and eliminate the harmonics that may or may not cause problems. As I said, I don't even know if it is practical to do that at RF. I don't have a way to generate square waves at RF, but did play around with a couple of audio filters I had with a scope and function generator. I would thing it would work the same at RF as it does at AF if the stray components can be managed. Very interisting to me. I decided to play for a while in my shop. The filter with coils in it would take out the harmonics and give a very good sine wave output. I had a low pass filter designed to use on a frequency counter or scope with a cut off of 5 KHz. It did a poor job of rounding off the square wave, but of all things when I switched the function generator to a triangle wave it did a good job of making a sine wave out of the signal. I thought the triangle wave had even harmonics in it, but found out it actually has odd harmonics but they decrease with the square of the order instead of just a simple 1/N. That ment a lot less harmonic content to filter out. It was a long time ago that I played with the equations for the various waveforms that can be made out of a bunch of sine waves added together. A good memory refresher. Yes, it is good to start with a signal that has less harmonic content. Can your function generator put out a sine wave? I bet that has pretty low harmonic content... ;) -- Rick |
Frequency accuracy in older RXs
On 10/7/2014 2:07 PM, gareth wrote:
"Ralph Mowery" wrote in message ... I thought the triangle wave had even harmonics in it, but found out it actually has odd harmonics but they decrease with the square of the order instead of just a simple 1/N. It is the sawtooth (equal slopes at rise and decay) that is made up of even harmonics. You seem to have them backwards... http://en.wikipedia.org/wiki/Triangle_wave http://en.wikipedia.org/wiki/Sawtooth_wave (I cannot speak with any authority in the triangle wave) Or much authority in the sawtooth wave as it turns out... ;) -- Rick |
Frequency accuracy in older RXs
On 10/7/2014 11:29 PM, Ralph Mowery wrote:
"rickman" wrote in message ... You say the filter removes "all" of the harmonics... that is obviously not correct. The filter may reduce them, but it does not and can not completely remove them. The nearest tones (which are also the largest amplitude tones) will only be reduced a small amount really. Or maybe you are planning to use a brick wall filter? My question intended to ask *WHY* would anyone design a circuit to produce a square wave and then spend the time and trouble to filter it? Earlier you mention that filters are easier now, but in reality analog filters are still much more difficult than just generating a tone in the first place. I can use a single chip and a DAC to produce tones up to many MHz with very high accuracy and purity. I don't get it... When I said all harmonics, that should have been taken as to reduce them to a very low value as not to cause any problems. If the third harmonic is 40 or 50 db down or more , it probably won't cause a problem. Big "IF"... That's my point. Knocking the third harmonic down to where it is tolerable can be hard. My whole thoughts was that someone already had a device that was on the corrrect frequency but it put out square waves. I just thought that if it could be ran through a low pass filter it would knock the harmonics down enough not to cause problems in a mixer circuit. If starting from the start, then there are beter ways to do it just as you are stating. No one *has* a device. Someone suggested using a device to make a circuit and I didn't notice it had a square wave output at first. So we are starting from the start... :) -- Rick |
Frequency accuracy in older RXs
"rickman" wrote in message ... You say the filter removes "all" of the harmonics... that is obviously not correct. The filter may reduce them, but it does not and can not completely remove them. The nearest tones (which are also the largest amplitude tones) will only be reduced a small amount really. Or maybe you are planning to use a brick wall filter? My question intended to ask *WHY* would anyone design a circuit to produce a square wave and then spend the time and trouble to filter it? Earlier you mention that filters are easier now, but in reality analog filters are still much more difficult than just generating a tone in the first place. I can use a single chip and a DAC to produce tones up to many MHz with very high accuracy and purity. I don't get it... When I said all harmonics, that should have been taken as to reduce them to a very low value as not to cause any problems. If the third harmonic is 40 or 50 db down or more , it probably won't cause a problem. My whole thoughts was that someone already had a device that was on the corrrect frequency but it put out square waves. I just thought that if it could be ran through a low pass filter it would knock the harmonics down enough not to cause problems in a mixer circuit. If starting from the start, then there are beter ways to do it just as you are stating. --- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com |
Frequency accuracy in older RXs
"rickman" wrote in message Yes, it is good to start with a signal that has less harmonic content. Can your function generator put out a sine wave? I bet that has pretty low harmonic content... ;) Yes my function generator can put out square, triangle and sine waves. I was just using the square waves to refresh my memory on what hapens when they go through a low pass filter. I have not played with that in many years. --- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com |
Frequency accuracy in older RXs
rickman wrote in :
What circuit clips a tone into a square wave just so it could be run through a low pass filter? Sound synthesis pitch following tone generator. Ramp generator based on natural pitch input (to get triangle wave). Preconditioner for faster capture in 4046 PLL to use the input that would otherwise be too sensitive to errors caused by irregular pulse width ratios... I suspect there are several cases for this. |
Frequency accuracy in older RXs
rickman wrote in :
On 10/7/2014 2:07 PM, gareth wrote: "Ralph Mowery" wrote in message ... I thought the triangle wave had even harmonics in it, but found out it actually has odd harmonics but they decrease with the square of the order instead of just a simple 1/N. It is the sawtooth (equal slopes at rise and decay) that is made up of even harmonics. You seem to have them backwards... http://en.wikipedia.org/wiki/Triangle_wave http://en.wikipedia.org/wiki/Sawtooth_wave (I cannot speak with any authority in the triangle wave) Or much authority in the sawtooth wave as it turns out... ;) Well, he's not far wrong as it happens... I'm new to RF and radio but have spent a lot more time with audio synthesisers. A triangle is a special case of a sawtooth (properly, ramp) wave. Like the square, it has no even harmonics, but the moment you change the speeds of the two parts of the ramp in one cycle, even harmonics occur with increasing strength the more it gets skewed. The really awkward bit with sawtooth waves is that they are a VERY general case, and include ramps but also nonlinear slopes. This is something I don't get into with maths, but it has strong implications for timbres especially when emulating natural instruments like brass or strings or even old synths that used relaxation oscillators. In radio techniques I imagine the clipping and filtering (or other means) to precondition a rough signal for the XOR phase comparator input of a PLL, taking advantage of its high noise immunity even with the raw clipped signal (it still requires a 50% width ratio), though a bit of filtering after clipping can help there. |
Frequency accuracy in older RXs
rickman wrote in :
You say the filter removes "all" of the harmonics... that is obviously not correct. The filter may reduce them, but it does not and can not completely remove them. The nearest tones (which are also the largest amplitude tones) will only be reduced a small amount really. Or maybe you are planning to use a brick wall filter? One thing I tried briefly, before getting into computers are doing things with code still a bit shy of getting into digital filtering just yet, was a switched capacitor filter, the attenuation there is excellent, and you can easily filter out the clock signal if it's 100 times the pass frequency. My question intended to ask *WHY* would anyone design a circuit to produce a square wave and then spend the time and trouble to filter it? I had a go at answerign that in my other posts just now.. Basically, conditioning a rough wave by Schmitt trigger into somethign much easier to process later, often for PLL input. Controlling hysteresis (using positive feedback) can be especially useful to elimnate some problems with complex waves that cross the centre more than twice per cycle. |
Frequency accuracy in older RXs
rickman wrote in :
You posed a problem; I gave you an answer. BTW there cannot be an easier item to lash to a microcontroller than a DDS Thank you for your suggestion. Assuming that DDS is Direct Digital Synthesis, I'm not sure anything needs to be lashed to anything. :) Just use a phase accumulator in a single DWORD or whatever native data size gives adequate resolution. Then just feed a DAC, assuming the processor has one. I think very many of them do. |
Frequency accuracy in older RXs
Michael Black wrote in
news:alpine.LNX.2.02.1410071314370.14980@darkstar. example.org: If the input to the mixer isnt' well filtered, then I think you're going to get all kinds of weird responses, since something can mix with a harmonic of the oscillator and be converted down to the IF. The best wheeze I came up with (never built it though), is a tracking filter: a Schmitt trigger with a bit of positive feedback hysteresis to condition the signal for a PLL and divide-by-100 based on two decade counters, to create a stable clock signal for a 100X switched capacitor filter that takes a copy of the original, direct signal. The output would be damn close to a sine wave no matter what the input was like, but there is a clock signal that may need removing, but possibly a simple one-pole filter will do that well enough. |
Frequency accuracy in older RXs
On 09/10/2014 20:21, Lostgallifreyan wrote:
rickman wrote in : You posed a problem; I gave you an answer. BTW there cannot be an easier item to lash to a microcontroller than a DDS Thank you for your suggestion. Assuming that DDS is Direct Digital Synthesis, I'm not sure anything needs to be lashed to anything. :) Just use a phase accumulator in a single DWORD or whatever native data size gives adequate resolution. Then just feed a DAC, assuming the processor has one. I think very many of them do. Fine, but first find a microcontroller with such a fast (minimum 60MHz) DAC. Even if you do you'll spend many times the cost of a simple microcontroller plus DDS chip system - and you won't rival the cheaper system's performance until your DAC can work at several times 60MHz. PA |
Frequency accuracy in older RXs
Peter Able stuck@home wrote in news:ks2dncC5fJrIAqrJnZ2dnUVZ7s-
: Fine, but first find a microcontroller with such a fast (minimum 60MHz) DAC. Even if you do you'll spend many times the cost of a simple microcontroller plus DDS chip system - and you won't rival the cheaper system's performance until your DAC can work at several times 60MHz. Good point. Almost certainly easier to just send step size from CPU to the accumulator in fast hardware. The step size needs changing far less often than it needs stepping. |
Frequency accuracy in older RXs
Lostgallifreyan wrote:
Peter Able stuck@home wrote in news:ks2dncC5fJrIAqrJnZ2dnUVZ7s- : Fine, but first find a microcontroller with such a fast (minimum 60MHz) DAC. Even if you do you'll spend many times the cost of a simple microcontroller plus DDS chip system - and you won't rival the cheaper system's performance until your DAC can work at several times 60MHz. Good point. Almost certainly easier to just send step size from CPU to the accumulator in fast hardware. The step size needs changing far less often than it needs stepping. That is what the external DDS chip is doing. When you require only FM-modulated signals and no sinewave, existing microcontrollers can do it using their onboard timers and programmable clock dividers. For example, the Raspberry Pi has been turned into FM-broadcast and amateur radio shortwave FSK/ASK transmitter, the RF signal appears (as a square wave) directly on a GPIO pin. Just filter and amplify, or when you don't ca just connect a random wire as an antenna to transmit a couple of mW (and harmonics). |
Frequency accuracy in older RXs
Rob wrote in
: Lostgallifreyan wrote: Peter Able stuck@home wrote in news:ks2dncC5fJrIAqrJnZ2dnUVZ7s- : Fine, but first find a microcontroller with such a fast (minimum 60MHz) DAC. Even if you do you'll spend many times the cost of a simple microcontroller plus DDS chip system - and you won't rival the cheaper system's performance until your DAC can work at several times 60MHz. Good point. Almost certainly easier to just send step size from CPU to the accumulator in fast hardware. The step size needs changing far less often than it needs stepping. That is what the external DDS chip is doing. When you require only FM-modulated signals and no sinewave, existing microcontrollers can do it using their onboard timers and programmable clock dividers. For example, the Raspberry Pi has been turned into FM-broadcast and amateur radio shortwave FSK/ASK transmitter, the RF signal appears (as a square wave) directly on a GPIO pin. Just filter and amplify, or when you don't ca just connect a random wire as an antenna to transmit a couple of mW (and harmonics). An off-topic question, but very interesting at least to me... Do DDS chips exist with 128 or even 256 phase accumulators onboard with the step size adjustment being capable of matching the speed of the stepping itself (though taking external control of local phase modulations between accumulators), and allowing mixing of all outputs, perhaps in user-selected groups based on binary fractions of the total accumulator count? I ask because if they do it might be possible for me to convert my phase mod synth code to dedicated hardware without resorting to very fast CPU's... While the rates are audio only, the huge parallel array gets demanding of CPU time as it is. I suspect the answer to all that may be 'no' without custom VLSI chips because of the relatively complex paths between accumulators needed for a phase mod synth of N operators per algorithm, but maybe DDS chips come in enough varieties to surprise me. :) |
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