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Need a low noise VXO for narrow sweep
Hi
need a low-noise xtal oscillator using 14.7 or 15.7MHz rubber xtals in series mode circuit to sweep xtal filters with good as possible noise distance. The available xtals are believed to pull +/-10kHz or more. Any suggestions. 73 Jan-Martin, LA8AK http://home.online.no/~la8ak/c.htm --- J. M. Noeding, LA8AK, N-4623 Kristiansand http://home.online.no/~la8ak/c.htm |
In article ,
says... Hi need a low-noise xtal oscillator using 14.7 or 15.7MHz rubber xtals in series mode circuit to sweep xtal filters with good as possible noise distance. The available xtals are believed to pull +/-10kHz or more. Any suggestions. Use a DDS hooked up to your PC's parallel port? -- jm ------------------------------------------------------ http://www.qsl.net/ke5fx Note: My E-mail address has been altered to avoid spam ------------------------------------------------------ |
In article ,
says... Hi need a low-noise xtal oscillator using 14.7 or 15.7MHz rubber xtals in series mode circuit to sweep xtal filters with good as possible noise distance. The available xtals are believed to pull +/-10kHz or more. Any suggestions. Use a DDS hooked up to your PC's parallel port? -- jm ------------------------------------------------------ http://www.qsl.net/ke5fx Note: My E-mail address has been altered to avoid spam ------------------------------------------------------ |
John Miles wrote:
In article , says... Hi need a low-noise xtal oscillator using 14.7 or 15.7MHz rubber xtals in series mode circuit to sweep xtal filters with good as possible noise distance. The available xtals are believed to pull +/-10kHz or more. Any suggestions. Use a DDS hooked up to your PC's parallel port? -- jm ------------------------------------------------------ http://www.qsl.net/ke5fx Note: My E-mail address has been altered to avoid spam ------------------------------------------------------ A DDS isn't going to have good enough phase noise. The OP is correct in using a pullable crystal oscillator. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com |
John Miles wrote:
In article , says... Hi need a low-noise xtal oscillator using 14.7 or 15.7MHz rubber xtals in series mode circuit to sweep xtal filters with good as possible noise distance. The available xtals are believed to pull +/-10kHz or more. Any suggestions. Use a DDS hooked up to your PC's parallel port? -- jm ------------------------------------------------------ http://www.qsl.net/ke5fx Note: My E-mail address has been altered to avoid spam ------------------------------------------------------ A DDS isn't going to have good enough phase noise. The OP is correct in using a pullable crystal oscillator. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com |
J M Noeding wrote:
Hi need a low-noise xtal oscillator using 14.7 or 15.7MHz rubber xtals in series mode circuit to sweep xtal filters with good as possible noise distance. The available xtals are believed to pull +/-10kHz or more. Any suggestions. 73 Jan-Martin, LA8AK http://home.online.no/~la8ak/c.htm --- J. M. Noeding, LA8AK, N-4623 Kristiansand http://home.online.no/~la8ak/c.htm This is a fairly standard circuit -- why not just look in an ARRL manual or one of the many QRP transmitter designs for an oscillator? You can check for phase noise by mixing it with a good (fixed) crystal oscillator, or you can just trust it. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com |
J M Noeding wrote:
Hi need a low-noise xtal oscillator using 14.7 or 15.7MHz rubber xtals in series mode circuit to sweep xtal filters with good as possible noise distance. The available xtals are believed to pull +/-10kHz or more. Any suggestions. 73 Jan-Martin, LA8AK http://home.online.no/~la8ak/c.htm --- J. M. Noeding, LA8AK, N-4623 Kristiansand http://home.online.no/~la8ak/c.htm This is a fairly standard circuit -- why not just look in an ARRL manual or one of the many QRP transmitter designs for an oscillator? You can check for phase noise by mixing it with a good (fixed) crystal oscillator, or you can just trust it. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com |
Why do you need such such good phase noise?
An average oscillator will be good enough to measure to -70 dBc or so. You could use a DDS with a crystal clock - this will have good close in phase noise (neasrly as good as the clock), though coherent spurs might be a problem. Richard J M Noeding wrote: Hi need a low-noise xtal oscillator using 14.7 or 15.7MHz rubber xtals in series mode circuit to sweep xtal filters with good as possible noise distance. The available xtals are believed to pull +/-10kHz or more. Any suggestions. 73 Jan-Martin, LA8AK http://home.online.no/~la8ak/c.htm --- J. M. Noeding, LA8AK, N-4623 Kristiansand http://home.online.no/~la8ak/c.htm |
Why do you need such such good phase noise?
An average oscillator will be good enough to measure to -70 dBc or so. You could use a DDS with a crystal clock - this will have good close in phase noise (neasrly as good as the clock), though coherent spurs might be a problem. Richard J M Noeding wrote: Hi need a low-noise xtal oscillator using 14.7 or 15.7MHz rubber xtals in series mode circuit to sweep xtal filters with good as possible noise distance. The available xtals are believed to pull +/-10kHz or more. Any suggestions. 73 Jan-Martin, LA8AK http://home.online.no/~la8ak/c.htm --- J. M. Noeding, LA8AK, N-4623 Kristiansand http://home.online.no/~la8ak/c.htm |
On Wed, 16 Jun 2004 22:23:01 -0700, Tim Wescott
wrote: need a low-noise xtal oscillator using 14.7 or 15.7MHz rubber xtals in series mode circuit to sweep xtal filters with good as possible noise distance. The available xtals are believed to pull +/-10kHz or more. This is a fairly standard circuit -- why not just look in an ARRL manual or one of the many QRP transmitter designs for an oscillator? You can check for phase noise by mixing it with a good (fixed) crystal oscillator, or you can just trust it. many of those amateur constructions are not very good, just put together, and if they generate a sort of signal the constructors are happy without worrying what they sound like. Some of the constructor have of course well reputation, but ARRL handbook usually don't tell who have designed it. and in this case I want a definite low noise construction. Remember when I built my first of many VHF/UHF beacons made after an amateur concept I listened to the carrier with R-4C and converter, the sound was awful, so I decided to built a new 12MHz xtal oscillator using some practice described by Ulrich Rohde, DJ2LR. The improvement on 144MHz was increadible, and I later learned that the sound was very good compared with all sorts of rubbish somebody else used, even on 2320MHz Jan-Martin, LA8AK --- J. M. Noeding, LA8AK, N-4623 Kristiansand http://home.online.no/~la8ak/c.htm |
On Wed, 16 Jun 2004 22:23:01 -0700, Tim Wescott
wrote: need a low-noise xtal oscillator using 14.7 or 15.7MHz rubber xtals in series mode circuit to sweep xtal filters with good as possible noise distance. The available xtals are believed to pull +/-10kHz or more. This is a fairly standard circuit -- why not just look in an ARRL manual or one of the many QRP transmitter designs for an oscillator? You can check for phase noise by mixing it with a good (fixed) crystal oscillator, or you can just trust it. many of those amateur constructions are not very good, just put together, and if they generate a sort of signal the constructors are happy without worrying what they sound like. Some of the constructor have of course well reputation, but ARRL handbook usually don't tell who have designed it. and in this case I want a definite low noise construction. Remember when I built my first of many VHF/UHF beacons made after an amateur concept I listened to the carrier with R-4C and converter, the sound was awful, so I decided to built a new 12MHz xtal oscillator using some practice described by Ulrich Rohde, DJ2LR. The improvement on 144MHz was increadible, and I later learned that the sound was very good compared with all sorts of rubbish somebody else used, even on 2320MHz Jan-Martin, LA8AK --- J. M. Noeding, LA8AK, N-4623 Kristiansand http://home.online.no/~la8ak/c.htm |
J M Noeding wrote:
On Wed, 16 Jun 2004 22:23:01 -0700, Tim Wescott wrote: need a low-noise xtal oscillator using 14.7 or 15.7MHz rubber xtals in series mode circuit to sweep xtal filters with good as possible noise distance. The available xtals are believed to pull +/-10kHz or more. This is a fairly standard circuit -- why not just look in an ARRL manual or one of the many QRP transmitter designs for an oscillator? You can check for phase noise by mixing it with a good (fixed) crystal oscillator, or you can just trust it. many of those amateur constructions are not very good, just put together, and if they generate a sort of signal the constructors are happy without worrying what they sound like. Some of the constructor have of course well reputation, but ARRL handbook usually don't tell who have designed it. and in this case I want a definite low noise construction. Remember when I built my first of many VHF/UHF beacons made after an amateur concept I listened to the carrier with R-4C and converter, the sound was awful, so I decided to built a new 12MHz xtal oscillator using some practice described by Ulrich Rohde, DJ2LR. The improvement on 144MHz was increadible, and I later learned that the sound was very good compared with all sorts of rubbish somebody else used, even on 2320MHz Jan-Martin, LA8AK --- J. M. Noeding, LA8AK, N-4623 Kristiansand http://home.online.no/~la8ak/c.htm Well, that's why I suggested verifying it's phase noise with a good fixed crystal oscillator. I certainly wouldn't proceed without doing this check -- the nice thing is that unless the oscillators lock together too tightly you get an upper bound to your phase noise, so you know when you're doing well enough. "Solid State Design for the Radio Amateur" has some good (older) circuits. It's been superseded by "Experimental Radio Design" which should also have some good ones. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com |
J M Noeding wrote:
On Wed, 16 Jun 2004 22:23:01 -0700, Tim Wescott wrote: need a low-noise xtal oscillator using 14.7 or 15.7MHz rubber xtals in series mode circuit to sweep xtal filters with good as possible noise distance. The available xtals are believed to pull +/-10kHz or more. This is a fairly standard circuit -- why not just look in an ARRL manual or one of the many QRP transmitter designs for an oscillator? You can check for phase noise by mixing it with a good (fixed) crystal oscillator, or you can just trust it. many of those amateur constructions are not very good, just put together, and if they generate a sort of signal the constructors are happy without worrying what they sound like. Some of the constructor have of course well reputation, but ARRL handbook usually don't tell who have designed it. and in this case I want a definite low noise construction. Remember when I built my first of many VHF/UHF beacons made after an amateur concept I listened to the carrier with R-4C and converter, the sound was awful, so I decided to built a new 12MHz xtal oscillator using some practice described by Ulrich Rohde, DJ2LR. The improvement on 144MHz was increadible, and I later learned that the sound was very good compared with all sorts of rubbish somebody else used, even on 2320MHz Jan-Martin, LA8AK --- J. M. Noeding, LA8AK, N-4623 Kristiansand http://home.online.no/~la8ak/c.htm Well, that's why I suggested verifying it's phase noise with a good fixed crystal oscillator. I certainly wouldn't proceed without doing this check -- the nice thing is that unless the oscillators lock together too tightly you get an upper bound to your phase noise, so you know when you're doing well enough. "Solid State Design for the Radio Amateur" has some good (older) circuits. It's been superseded by "Experimental Radio Design" which should also have some good ones. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com |
In article ,
says... A DDS isn't going to have good enough phase noise. The OP is correct in using a pullable crystal oscillator. Eh? He wants to sweep a filter. You don't particularly care about phase noise when you do that. -- jm ------------------------------------------------------ http://www.qsl.net/ke5fx Note: My E-mail address has been altered to avoid spam ------------------------------------------------------ |
In article ,
says... A DDS isn't going to have good enough phase noise. The OP is correct in using a pullable crystal oscillator. Eh? He wants to sweep a filter. You don't particularly care about phase noise when you do that. -- jm ------------------------------------------------------ http://www.qsl.net/ke5fx Note: My E-mail address has been altered to avoid spam ------------------------------------------------------ |
John Miles wrote:
In article , says... A DDS isn't going to have good enough phase noise. The OP is correct in using a pullable crystal oscillator. Eh? He wants to sweep a filter. You don't particularly care about phase noise when you do that. -- jm ------------------------------------------------------ http://www.qsl.net/ke5fx Note: My E-mail address has been altered to avoid spam ------------------------------------------------------ That depends on your filter. If you're trying to design a high pole count filter with really steep skirts and you want to verify it's final rejection then yes, you need a low phase noise oscillator. This is probably why he has "low noise" in his title. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com |
John Miles wrote:
In article , says... A DDS isn't going to have good enough phase noise. The OP is correct in using a pullable crystal oscillator. Eh? He wants to sweep a filter. You don't particularly care about phase noise when you do that. -- jm ------------------------------------------------------ http://www.qsl.net/ke5fx Note: My E-mail address has been altered to avoid spam ------------------------------------------------------ That depends on your filter. If you're trying to design a high pole count filter with really steep skirts and you want to verify it's final rejection then yes, you need a low phase noise oscillator. This is probably why he has "low noise" in his title. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com |
On Thu, 17 Jun 2004 11:03:44 -0700, Tim Wescott
wrote: ------------------------------------------------------ http://www.qsl.net/ke5fx Note: My E-mail address has been altered to avoid spam ------------------------------------------------------ That depends on your filter. If you're trying to design a high pole count filter with really steep skirts and you want to verify it's final rejection then yes, you need a low phase noise oscillator. This is probably why he has "low noise" in his title. can't really see I've got any replies in the wanted direction; If I wish to measure a receivers ultimate performance I am not interested in seing something like 60dB selectivity when I am expecting 90dB or more. Had a discussion with LA8OJ, and he suggested that 6MHz ceramic resonator might be easier to use than a 15.7MHz xtal, 100kHz tuning range could be achieved instead of 20kHz. But I suppose it was a trick to limit the amplitude to improve phase noise of an xtal oscillator 73 Jan-Martin, LA8AK http://home.online.no/~la8ak/c.htm --- J. M. Noeding, LA8AK, N-4623 Kristiansand http://home.online.no/~la8ak/c.htm |
On Thu, 17 Jun 2004 11:03:44 -0700, Tim Wescott
wrote: ------------------------------------------------------ http://www.qsl.net/ke5fx Note: My E-mail address has been altered to avoid spam ------------------------------------------------------ That depends on your filter. If you're trying to design a high pole count filter with really steep skirts and you want to verify it's final rejection then yes, you need a low phase noise oscillator. This is probably why he has "low noise" in his title. can't really see I've got any replies in the wanted direction; If I wish to measure a receivers ultimate performance I am not interested in seing something like 60dB selectivity when I am expecting 90dB or more. Had a discussion with LA8OJ, and he suggested that 6MHz ceramic resonator might be easier to use than a 15.7MHz xtal, 100kHz tuning range could be achieved instead of 20kHz. But I suppose it was a trick to limit the amplitude to improve phase noise of an xtal oscillator 73 Jan-Martin, LA8AK http://home.online.no/~la8ak/c.htm --- J. M. Noeding, LA8AK, N-4623 Kristiansand http://home.online.no/~la8ak/c.htm |
In article ,
says... That depends on your filter. If you're trying to design a high pole count filter with really steep skirts and you want to verify it's final rejection then yes, you need a low phase noise oscillator. This is probably why he has "low noise" in his title. Perhaps, but if you want to verify final rejection, you want much wider tuning range than you're going to get from a pulled crystal. A 100 Hz- wide filter might be -100 dB down at 5 kHz but -50 dB down at 50 kHz, and if you care about that, you need to be able to look farther away from the carrier. His desire for a narrow-range source made me think he was more interested in shape-factor and ripple alignment than ultimate rejection. Maybe he can use two different oscillators to get the best of both worlds. A high-quality LC oscillator is actually pretty darned quiet at large offsets from the carrier. I think it was Tom Bruhns, or one of the other HP guys at least, who once pointed out that the old HP 608- series boatanchors were quieter at 100 kHz offsets than the flashy, high-dollar synthesizers that followed them. -- jm ------------------------------------------------------ http://www.qsl.net/ke5fx Note: My E-mail address has been altered to avoid spam ------------------------------------------------------ |
In article , Tim Wescott
writes: John Miles wrote: In article , says... A DDS isn't going to have good enough phase noise. The OP is correct in using a pullable crystal oscillator. Eh? He wants to sweep a filter. You don't particularly care about phase noise when you do that. -- jm ------------------------------------------------------ http://www.qsl.net/ke5fx Note: My E-mail address has been altered to avoid spam ------------------------------------------------------ That depends on your filter. If you're trying to design a high pole count filter with really steep skirts and you want to verify it's final rejection then yes, you need a low phase noise oscillator. This is probably why he has "low noise" in his title. The "need" for low-noise RF sources was prompted by the electronics industry going hot and heavy on cellular telephony which uses partly phase demodulation and clock recovery circuits in digital electronics. Because of those particular markets, "low noise" has become a Big Buzzword. Whether you have one pole or twelve or whatever, you will NOT need a specific "low noise oscillator!" The very ordinary sweep oscillators of ten, twenty, or thirty years ago are quite fine. retired (from regular hours) electronic engineer person |
In article , Tim Wescott
writes: John Miles wrote: In article , says... A DDS isn't going to have good enough phase noise. The OP is correct in using a pullable crystal oscillator. Eh? He wants to sweep a filter. You don't particularly care about phase noise when you do that. -- jm ------------------------------------------------------ http://www.qsl.net/ke5fx Note: My E-mail address has been altered to avoid spam ------------------------------------------------------ That depends on your filter. If you're trying to design a high pole count filter with really steep skirts and you want to verify it's final rejection then yes, you need a low phase noise oscillator. This is probably why he has "low noise" in his title. The "need" for low-noise RF sources was prompted by the electronics industry going hot and heavy on cellular telephony which uses partly phase demodulation and clock recovery circuits in digital electronics. Because of those particular markets, "low noise" has become a Big Buzzword. Whether you have one pole or twelve or whatever, you will NOT need a specific "low noise oscillator!" The very ordinary sweep oscillators of ten, twenty, or thirty years ago are quite fine. retired (from regular hours) electronic engineer person |
The "need" for low-noise RF sources was prompted by the
electronics industry going hot and heavy on cellular telephony which uses partly phase demodulation and clock recovery circuits in digital electronics. Because of those particular markets, "low noise" has become a Big Buzzword. Whether you have one pole or twelve or whatever, you will NOT need a specific "low noise oscillator!" The very ordinary sweep oscillators of ten, twenty, or thirty years ago are quite fine. Len, I'm sorry, but I disagree. Concern with phase noise in *all* kinds of communications systems really got hot in the 1970s. Adjacent channel rejection is limited by phase noise performance. It is presently the limiting factor in HF receiver performance. Ham transmitter phase noise can easily be heard during CW DX contests as a keyed increase in noise floor. Measurement of a crystal filter with steep sides could be compromised by PM to AM conversion on the slopes. And in response to earlier posts, DDS synths *can* give very good phase noise performance, but their usefulness is usually limited by quantization spurs unless a cleanup PLL is added. 73, John - K6QQ retired RF circuit design engineer |
The "need" for low-noise RF sources was prompted by the
electronics industry going hot and heavy on cellular telephony which uses partly phase demodulation and clock recovery circuits in digital electronics. Because of those particular markets, "low noise" has become a Big Buzzword. Whether you have one pole or twelve or whatever, you will NOT need a specific "low noise oscillator!" The very ordinary sweep oscillators of ten, twenty, or thirty years ago are quite fine. Len, I'm sorry, but I disagree. Concern with phase noise in *all* kinds of communications systems really got hot in the 1970s. Adjacent channel rejection is limited by phase noise performance. It is presently the limiting factor in HF receiver performance. Ham transmitter phase noise can easily be heard during CW DX contests as a keyed increase in noise floor. Measurement of a crystal filter with steep sides could be compromised by PM to AM conversion on the slopes. And in response to earlier posts, DDS synths *can* give very good phase noise performance, but their usefulness is usually limited by quantization spurs unless a cleanup PLL is added. 73, John - K6QQ retired RF circuit design engineer |
In article , "John Moriarity"
writes: The "need" for low-noise RF sources was prompted by the electronics industry going hot and heavy on cellular telephony which uses partly phase demodulation and clock recovery circuits in digital electronics. Because of those particular markets, "low noise" has become a Big Buzzword. Whether you have one pole or twelve or whatever, you will NOT need a specific "low noise oscillator!" The very ordinary sweep oscillators of ten, twenty, or thirty years ago are quite fine. Len, I'm sorry, but I disagree. Concern with phase noise in *all* kinds of communications systems really got hot in the 1970s. Adjacent channel rejection is limited by phase noise performance. It is presently the limiting factor in HF receiver performance. The limiting factor was, and continues to be, the receiver front-end noise (usually in the first RF stage or 1st Mixer, often described by NF or Noise Factor), the ultimate selectivity (usually done by a quartz crystal filter). Some things exacerbate the front-end noise such as cascaded low-gain stages, the second stage contributing some slight amount of extra noise. Ham transmitter phase noise can easily be heard during CW DX contests as a keyed increase in noise floor. Whether that is done in contests or not, the "phase noise" heard is MUCH more likely to be generated by the Xmtr RF section. That noise is more than likely to be just AM noise from that transmitter or the AM-type noise from the local receiver front-end. I'm at a loss to see where an AM-type demodulator is going to see a small amount of phase noise or even frequency noise...a diode or "product detector" (multiplier of signal input times local IF oscillator or "beat" oscillator) is going to pick up the amplitude-modulated noise of the distant Tx or the local Rx front-end. That demod isn't going to be able to "hear" much in the way of phase or frequency noise. Measurement of a crystal filter with steep sides could be compromised by PM to AM conversion on the slopes. Perhaps...if the PM or FM was quite large. The actual PM or FM from even moderately-stable L-C oscillators is still quite small even with the large shape-factor values of very good quartz crystal bandpass filters. I did a bunch of measurements of quartz-substrate SAW filters back in 1974 at RCA EASD which had Very steep skirts, steeper than 9-pole quartz crystal lattice filters. The only reason for concern was the _amplitude_ noise out of the RF generator, solved by getting an HP 608F. [60 to 70 MHz center frequency of the SAWs...608F has a nice amplitude stabilizer] That was better than trying for some averaging of multiple sweeps via a minicomputer data logger-averager which would have done the same thing with a controlled stepping RF synthesizer. Anyone who cares can find out what the PM or FM "noise" is by simply modifying an FM receiver to tune to an oscillator's output and calibrating the FM receiver in terms of deviation done by a signal generator and frequency counter combination. Since PM and FM are - in theory and practice - nearly identical in terms of sideband content versus deviation. That same setup can also determine what the modified FM receiver's own LO PM-FM is as well as the signal generator's stuff. In actual practice, that PM- FM is so low that it doesn't matter one whit insofar as a basically- AM demodulated HF signal is as far as "phase noise." Oscillator phase noise IS important with various levels of QAM where BOTH AM and PM are going on. The BER there is a very good indicator of whether or not phase or amplitude "noise" is a culprit and the Eye diagram can show which one is worst. There's lots more already been done on various QAM systems but the subject thread wasn't about digital modulations. The subject thread was about voltage-controlled crystal oscillators used for filter measurement. With good control to suppress garbage on the (VCO or VCXO in more common terms) control line and some ordinary good (and old) practices in making stable oscillators, plus a wideband limiter stage to damp out the AM as the oscillator sweeps, ain't going to be much of a problem except calibrating the setup. There are all sorts of frequency-variable RF sources that could be made up for this kind of measurement, nicely described in literature all over the industry, but "doing the numbers" on the theoretical "phase noise" of a particular source is what counts. The way I see it, after doing a bunch of that kind of thing (besides in the 1974 time frame mentioned) is that the now-touted "phase noise" figures just don't matter for this application. Phase noise can be a factor in obtaining very precise stats on a local frequency standard. "Allen Variance" and all that stuff (which is described in detail on the NTIS website in one section) isn't going to be needed for rather simple PPM tolerance measurements. This is not PPB tolerances. Lots of late-WW2 era FM-PM systems were used by the military, notably the TRC-1, -3, -4 VHF radio relay sets using PM through a reactance modulator from an ~500 KHz (?) crystal and then doing something like 96 times (? forget the details, long chain of doublers and triplers) frequency multiplication to reach 70 to 90 MHz output. After all that multiplying, the output modulation was so close to FM that ordinary FM demodulators had no trouble receiving it. Bandwidth went a bit beyond 12 KHz. Nobody talked about any "phase noise" then even though the CF-1 and CF-2 carrier bays were also requiring phase and frequency stability of the radio part. Oscillator "phase noise" specs got in there in more modern times as terms like Bit Error Rate (BER) and Eye Diagrams entered the scene along with the explosion of various cellular telephony and digital modulations. By that time, roughly the beginning of the 1970s, the ultimate in tuneable receiver sensitivity had already been achieved for HF bands and those receivers were on the market. Sensitivity hasn't really changed since that time for most signals involving AM-type modulations. For a setup to test filters with very precise frequency control as well as stability of that frequency (in all three AM, FM, PM), a DDS is probably the most versatile RF source. But, some DDSs have unique problems of their own, involving the generation of spurious sidebands. That's an entirely different matter than "low phase noise" oscillators. retired (from regular hours) electronic engineer person. |
In article , "John Moriarity"
writes: The "need" for low-noise RF sources was prompted by the electronics industry going hot and heavy on cellular telephony which uses partly phase demodulation and clock recovery circuits in digital electronics. Because of those particular markets, "low noise" has become a Big Buzzword. Whether you have one pole or twelve or whatever, you will NOT need a specific "low noise oscillator!" The very ordinary sweep oscillators of ten, twenty, or thirty years ago are quite fine. Len, I'm sorry, but I disagree. Concern with phase noise in *all* kinds of communications systems really got hot in the 1970s. Adjacent channel rejection is limited by phase noise performance. It is presently the limiting factor in HF receiver performance. The limiting factor was, and continues to be, the receiver front-end noise (usually in the first RF stage or 1st Mixer, often described by NF or Noise Factor), the ultimate selectivity (usually done by a quartz crystal filter). Some things exacerbate the front-end noise such as cascaded low-gain stages, the second stage contributing some slight amount of extra noise. Ham transmitter phase noise can easily be heard during CW DX contests as a keyed increase in noise floor. Whether that is done in contests or not, the "phase noise" heard is MUCH more likely to be generated by the Xmtr RF section. That noise is more than likely to be just AM noise from that transmitter or the AM-type noise from the local receiver front-end. I'm at a loss to see where an AM-type demodulator is going to see a small amount of phase noise or even frequency noise...a diode or "product detector" (multiplier of signal input times local IF oscillator or "beat" oscillator) is going to pick up the amplitude-modulated noise of the distant Tx or the local Rx front-end. That demod isn't going to be able to "hear" much in the way of phase or frequency noise. Measurement of a crystal filter with steep sides could be compromised by PM to AM conversion on the slopes. Perhaps...if the PM or FM was quite large. The actual PM or FM from even moderately-stable L-C oscillators is still quite small even with the large shape-factor values of very good quartz crystal bandpass filters. I did a bunch of measurements of quartz-substrate SAW filters back in 1974 at RCA EASD which had Very steep skirts, steeper than 9-pole quartz crystal lattice filters. The only reason for concern was the _amplitude_ noise out of the RF generator, solved by getting an HP 608F. [60 to 70 MHz center frequency of the SAWs...608F has a nice amplitude stabilizer] That was better than trying for some averaging of multiple sweeps via a minicomputer data logger-averager which would have done the same thing with a controlled stepping RF synthesizer. Anyone who cares can find out what the PM or FM "noise" is by simply modifying an FM receiver to tune to an oscillator's output and calibrating the FM receiver in terms of deviation done by a signal generator and frequency counter combination. Since PM and FM are - in theory and practice - nearly identical in terms of sideband content versus deviation. That same setup can also determine what the modified FM receiver's own LO PM-FM is as well as the signal generator's stuff. In actual practice, that PM- FM is so low that it doesn't matter one whit insofar as a basically- AM demodulated HF signal is as far as "phase noise." Oscillator phase noise IS important with various levels of QAM where BOTH AM and PM are going on. The BER there is a very good indicator of whether or not phase or amplitude "noise" is a culprit and the Eye diagram can show which one is worst. There's lots more already been done on various QAM systems but the subject thread wasn't about digital modulations. The subject thread was about voltage-controlled crystal oscillators used for filter measurement. With good control to suppress garbage on the (VCO or VCXO in more common terms) control line and some ordinary good (and old) practices in making stable oscillators, plus a wideband limiter stage to damp out the AM as the oscillator sweeps, ain't going to be much of a problem except calibrating the setup. There are all sorts of frequency-variable RF sources that could be made up for this kind of measurement, nicely described in literature all over the industry, but "doing the numbers" on the theoretical "phase noise" of a particular source is what counts. The way I see it, after doing a bunch of that kind of thing (besides in the 1974 time frame mentioned) is that the now-touted "phase noise" figures just don't matter for this application. Phase noise can be a factor in obtaining very precise stats on a local frequency standard. "Allen Variance" and all that stuff (which is described in detail on the NTIS website in one section) isn't going to be needed for rather simple PPM tolerance measurements. This is not PPB tolerances. Lots of late-WW2 era FM-PM systems were used by the military, notably the TRC-1, -3, -4 VHF radio relay sets using PM through a reactance modulator from an ~500 KHz (?) crystal and then doing something like 96 times (? forget the details, long chain of doublers and triplers) frequency multiplication to reach 70 to 90 MHz output. After all that multiplying, the output modulation was so close to FM that ordinary FM demodulators had no trouble receiving it. Bandwidth went a bit beyond 12 KHz. Nobody talked about any "phase noise" then even though the CF-1 and CF-2 carrier bays were also requiring phase and frequency stability of the radio part. Oscillator "phase noise" specs got in there in more modern times as terms like Bit Error Rate (BER) and Eye Diagrams entered the scene along with the explosion of various cellular telephony and digital modulations. By that time, roughly the beginning of the 1970s, the ultimate in tuneable receiver sensitivity had already been achieved for HF bands and those receivers were on the market. Sensitivity hasn't really changed since that time for most signals involving AM-type modulations. For a setup to test filters with very precise frequency control as well as stability of that frequency (in all three AM, FM, PM), a DDS is probably the most versatile RF source. But, some DDSs have unique problems of their own, involving the generation of spurious sidebands. That's an entirely different matter than "low phase noise" oscillators. retired (from regular hours) electronic engineer person. |
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