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3:1 range VCO and varactor RF voltage swing
Hi!. I need to cover the HF and VHF ranges with as few VCOs as
possible. Think of this as the varactor version of the old general purpose RF bench generators or grid-dip-meters. I know the penalties: high phase noise, high drift, high harmonic content. I don't care, this won't be made into a high dynamic range receiver. What I do need is a reliable means to keep the peak-to-peak RF voltage across the varactors (varicaps) below 1 or 0.5Vp-p. Otherwise, even having back-to-back diodes they rectify the RF, create a DC component into the 100k resistor that feeds the tuning voltage, and this dramatically raises the bottom frequency. If I reduce the R, the f goes lower, but the RF is nastily clipped by the diodes. The oscillator is a FET Hartley (no Colpitts capacitive tap in order to maximize C swing). All attempts I made on Spice and in real life to AGC the amplitude always created a low frequency relaxation oscillation. Can anybody tell me about a proven way to accomplish this?. I have already googled a little with no success. Many thanks! |
3:1 range VCO and varactor RF voltage swing
lw1ecp wrote:
Hi!. I need to cover the HF and VHF ranges with as few VCOs as possible. Think of this as the varactor version of the old general purpose RF bench generators or grid-dip-meters. I know the penalties: high phase noise, high drift, high harmonic content. I don't care, this won't be made into a high dynamic range receiver. What I do need is a reliable means to keep the peak-to-peak RF voltage across the varactors (varicaps) below 1 or 0.5Vp-p. Otherwise, even having back-to-back diodes they rectify the RF, create a DC component into the 100k resistor that feeds the tuning voltage, and this dramatically raises the bottom frequency. If I reduce the R, the f goes lower, but the RF is nastily clipped by the diodes. The oscillator is a FET Hartley (no Colpitts capacitive tap in order to maximize C swing). All attempts I made on Spice and in real life to AGC the amplitude always created a low frequency relaxation oscillation. Can anybody tell me about a proven way to accomplish this?. I have already googled a little with no success. Many thanks! See if you can find any of the old (now) op-amp cook books or general applications manuals. I saw one with a modified Wien-bridge that had an AGC, but that was in 1975 for a breadboard I was doing. Good luck. Bill Baka |
3:1 range VCO and varactor RF voltage swing
I saw one with a modified Wien-bridge that had an AGC (...)
Thanks Bill for your immediate answer!. Yes, RC oscillators have impressive f spans: you change R or C by 10:1 and f changes 10:1, not its square root. But... my needs go into the many hundreds MHz, sorry, I have to stick to LC oscillators. |
3:1 range VCO and varactor RF voltage swing
On Wed, 25 Nov 2009 17:10:07 -0800, lw1ecp wrote:
Hi!. I need to cover the HF and VHF ranges with as few VCOs as possible. Think of this as the varactor version of the old general purpose RF bench generators or grid-dip-meters. I know the penalties: high phase noise, high drift, high harmonic content. I don't care, this won't be made into a high dynamic range receiver. What I do need is a reliable means to keep the peak-to-peak RF voltage across the varactors (varicaps) below 1 or 0.5Vp-p. Otherwise, even having back-to-back diodes they rectify the RF, create a DC component into the 100k resistor that feeds the tuning voltage, and this dramatically raises the bottom frequency. If I reduce the R, the f goes lower, but the RF is nastily clipped by the diodes. The oscillator is a FET Hartley (no Colpitts capacitive tap in order to maximize C swing). All attempts I made on Spice and in real life to AGC the amplitude always created a low frequency relaxation oscillation. Can anybody tell me about a proven way to accomplish this?. I have already googled a little with no success. Many thanks! How elaborate an AGC loop are you trying? The _really pedantic_ way to do it would be to use a separate amplitude detector, an op-amp controller (which would let your controller be if not arbitrarily then at least really really slow), and whatever bias point on the oscillator you're changing to effect the gain. This should let you get a pretty fancy transfer function on your controller, to relieve the motor-boating. -- www.wescottdesign.com |
3:1 range VCO and varactor RF voltage swing
lw1ecp wrote:
I saw one with a modified Wien-bridge that had an AGC (...) Thanks Bill for your immediate answer!. Yes, RC oscillators have impressive f spans: you change R or C by 10:1 and f changes 10:1, not its square root. But... my needs go into the many hundreds MHz, sorry, I have to stick to LC oscillators. Many hundreds of MHz? Are you trying for D.C. to daylight? I could poke around a bit and see if I could trip over something while I look for parts for my own projects. Are you looking to build or buy? It seems you want a circuit to build. Am I on the right track? Some of the older ARRL handbooks may have a circuit or two. Lately I have seen them migrating towards VHF and UHF and higher frequency stuff. Good luck. Bill Baka |
3:1 range VCO and varactor RF voltage swing
In article , Bill Baka
wrote: lw1ecp wrote: I saw one with a modified Wien-bridge that had an AGC (...) Thanks Bill for your immediate answer!. Yes, RC oscillators have impressive f spans: you change R or C by 10:1 and f changes 10:1, not its square root. But... my needs go into the many hundreds MHz, sorry, I have to stick to LC oscillators. Many hundreds of MHz? Are you trying for D.C. to daylight? I could poke around a bit and see if I could trip over something while I look for parts for my own projects. Are you looking to build or buy? It seems you want a circuit to build. Am I on the right track? Some of the older ARRL handbooks may have a circuit or two. Lately I have seen them migrating towards VHF and UHF and higher frequency stuff. Good luck. Bill Baka A typical way of getting this kind of range is to use a pair of oscillators, at least one voltage controlled, a mixer, and a low pass filter -- with the amplitude stabilizing loop described previously. Use a pair of YIG tuned oscillators and a wideband mixer, and you can get as wide a range as you want -- a lot of sweep generators work this way. You could use a pair of Mini Circuits VCOs in the 2 to 4 GHz range and build a vco with a few hundred MHz range. |
3:1 range VCO and varactor RF voltage swing
"lw1ecp" wrote in message
... I saw one with a modified Wien-bridge that had an AGC (...) Thanks Bill for your immediate answer!. Yes, RC oscillators have impressive f spans: you change R or C by 10:1 and f changes 10:1, not its square root. But... my needs go into the many hundreds MHz, sorry, I have to stick to LC oscillators. I remember what was known as a "seven league oscillator" which derived its name from the folklore of "seven league boots". The design was discussed in one of my old 1930's vintage vacuum tube books. If I remember correctly, one such oscillator covered from a few Hertz to several MHz. Unfortunately I am in SC and my library is back in Tennessee. My memory may be wrong, but I think the book was "Functional Circuits and Oscillators" by Herbert Reich. 73, Dr. Barry L. Ornitz WA4VZQ |
3:1 range VCO and varactor RF voltage swing
I'm astonished at the tsunami of answers this arose!.
- Tim: yes: detector + error amplifier (minimum gain, even a single NPN) + FET bias shifting. - Bill: I love old ARRL publications, 80's QSTs are my favorites, but 3:1 (continuous) coverage is not common on ham bands. I am looking to build, and design (oh, well, guesstimate). - Artie and Barry: I knew the heterodyne way of getting broad coverage thanks to the old Wavetek 2000 Sweep/Signal Generator. This is also used in tracking generators for spectrum analyzers. But keeping unwanted mixing products low is an art. Maybe later. But, the first answer from Tim made me think: what's the difference in amplitude stabilizing methods on a Wien bridge and my Hartley?. In a RC oscillator you control amplitude thru some voltage dependent resistance (a lamp, a FET channel, etc.). But in my oscillator I vary it thru gate bias, which varies the average transconductance of a device working in a (nonlinear) class C. So, I Spice'd a FET damping the coil (source) tap with its Rds being controlled by oscillations amplitude, an voilą!, I got ~1Vp-p at the gate (and the varicaps), the gate's 1N4148 almost does not conduct. I will try a more decent of controlling the feedback (without ruining Q) and let you know. |
3:1 range VCO and varactor RF voltage swing
Bill Baka wrote:
lw1ecp wrote: Hi!. I need to cover the HF and VHF ranges with as few VCOs as possible. Think of this as the varactor version of the old general purpose RF bench generators or grid-dip-meters. I know the penalties: high phase noise, high drift, high harmonic content. I don't care, this won't be made into a high dynamic range receiver. What I do need is a reliable means to keep the peak-to-peak RF voltage across the varactors (varicaps) below 1 or 0.5Vp-p. Otherwise, even having back-to-back diodes they rectify the RF, create a DC component into the 100k resistor that feeds the tuning voltage, and this dramatically raises the bottom frequency. If I reduce the R, the f goes lower, but the RF is nastily clipped by the diodes. The oscillator is a FET Hartley (no Colpitts capacitive tap in order to maximize C swing). All attempts I made on Spice and in real life to AGC the amplitude always created a low frequency relaxation oscillation. Can anybody tell me about a proven way to accomplish this?. I have already googled a little with no success. Many thanks! See if you can find any of the old (now) op-amp cook books or general applications manuals. I saw one with a modified Wien-bridge that had an AGC, but that was in 1975 for a breadboard I was doing. Good luck. Bill Baka The old school method - for a wien bridge for example - was to place a small filament bulb in the feedback path. This had a non linear response to amplitude - and a low natural bandwidth. It's that slow response that you have been lacking, apparently. Putting together an op amp with variable amplitude output that is smoothed on a five+ second time constant is what you need. Brian W |
3:1 range VCO and varactor RF voltage swing
Hey OM:
Maybe you should looky at an old NTSC tv tuner. The brothers at RCA, made sum tuners for the cable tv bands that went from 5Mhz all the way up to 900Mhz And they did it with quad matched varicaps, dual gate FET's, and a PLL. Not only did they control the VCO, but the TRF front end also. Me thinks your OSC is way to efficient and your Q is way to high. Roll the Q down with the right LC ratio or swamp it with sum resistance. If you series varicaps you would increase the the amount of RF voltage they could handle before starting to rectifry the RF. 73 OM de n8zu |
3:1 range VCO and varactor RF voltage swing
Raypsi:
- Thank you for the hint, the CATV tuners cover 54 to 900 MHz (5 to ~40 is for the upstream), an impressive 16:1 range, but the L.O. is much narrower in relative values. Suppose 1st IF at 1200MHz, the L.O. would sweep 1254 to 2100, 1.67:1, not enough for me. - "The TRF front end also". Are you sure?. I dismantled a Jerrold and the incoming CATV hit directly the balanced modulator thru a fixed high-pass. But even if it really tuned 54-900, it would be easier than my needs because the CATV amplitude is much lower than in an oscillator. - "Roll the Q down"?. It wouldn't help, I just need to vary 9:1 the LC product. - "Series varicaps"?. Sorry... they are back-to-back in series already!. Doug: Thank you for encouraging me about the heterodyne way, and the links. Yes, I think I will go this way for my gen-purp lab generator, but for the dipper I still need the coil to oscillate right at the measured frequency. Brian: I'm afraid this is more complicated than choosing a long time constant, I have already tried in the simulation. The problem seems to stem from the fact that the effective FET gm is dependent not only on applied bias but also on RF amplitude. The error amplifier senses the amplitude is high, applies more -Vgs, gm decreases, amplitude decreases, but this lower amplitude makes effective gm to fall even more, and as a result oscillation ceases and then starts again, and again, and again. I think I will have to prevent the FET from working in class C and forget about Vgs as a control means. Again, many thanks to all of you. As soon as I can come back to this subject I will let you know the results. |
3:1 range VCO and varactor RF voltage swing
lw1ecp wrote:
/snip/ - "Series varicaps"?. Sorry... they are back-to-back in series already!. I understood the suggestion was to place two varicaps in series NOT back to back in order to extend the voltage range. But that implies two series strings, in order to maintain the capacitance range over a longer voltage swing?? Brian W |
3:1 range VCO and varactor RF voltage swing
Hey OM:
Well the dual gate FET's RCA was using were sub 1Ghz range so not only were they controlling the bias, also they controled the miller effect with an isolated supply for the second gate. I should has ment 50Mhz to 900Mhz it was the splitters we used were rated 5Mhz to 900Mhz. So 8 varicaps in back to back series would be to much? Giving you double the voltage and the same C. You may be right 54Vdc would be way too much to have floating around to tune them. I remember the ones RCA used were so small if you dropped one on the floor you would never ever find it. They diode switched in 2 sets of coils in both the OSC and TRF, to get LO VHF, HI VHF, and UHF. So like they had 3 coils in series, all 3 for the LO, 2 for the HI, and 1 for the UHF 73 OM de n8zu On Nov 29, 11:32*pm, brian whatcott wrote: lw1ecp wrote: /snip/ - "Series varicaps"?. Sorry... they are back-to-back in series already!. I understood the suggestion was to place two varicaps in series NOT back to back in order to extend the voltage range. But that implies two series strings, in order to maintain the capacitance range over a longer voltage swing?? Brian W |
3:1 range VCO and varactor RF voltage swing
On Nov 29, 9:07*pm, lw1ecp wrote:
Raypsi: - Thank you for the hint, the CATV tuners cover 54 to 900 MHz (5 to ~40 is for the upstream), an impressive 16:1 range, but the L.O. is much narrower in relative values. Suppose 1st IF at 1200MHz, the L.O. would sweep 1254 to 2100, 1.67:1, not enough for me. I think maybe you're looking at the wrong generation of CATV tuners. I think Raypsi's suggestion was for the little tin-can tuners common in say 80's and 90's VCR's, these certainly did NOT have a 1200MHz IF, closer to 45MHz. Tim. |
3:1 range VCO and varactor RF voltage swing
On Nov 25, 8:10*pm, lw1ecp wrote:
Hi!. I need to cover the HF and VHF ranges with as few VCOs as possible. Think of this as the varactor version of the old general purpose RF bench generators or grid-dip-meters. I know the penalties: high phase noise, high drift, high harmonic content. I don't care, this won't be made into a high dynamic range receiver. What I do need is a reliable means to keep the peak-to-peak RF voltage across the varactors (varicaps) below 1 or 0.5Vp-p. Look at the Elecraft K2's VCO. A single VCO covers all the HF bands (some with up and others with downconversion), with many narrow ranges all cleverly switched using small relays to move the varactors/fixed capacitors in and out. I can't promise that this is exactly what you want but there's much to be learned by studying it if you want to build wide-range VCO's with commodity parts. The Elecraft K2 VCO also uses a clever scheme to do AGC such that the VCO output remains constant through its wide range, this probably also keeps the RF voltage across the varicap in check. There's a lot of cleverness in the K2. Schematics are online, www.elecraft.com. Tim N3QE |
3:1 range VCO and varactor RF voltage swing
In message
, Tim Shoppa writes On Nov 29, 9:07*pm, lw1ecp wrote: Raypsi: - Thank you for the hint, the CATV tuners cover 54 to 900 MHz (5 to ~40 is for the upstream), an impressive 16:1 range, but the L.O. is much narrower in relative values. Suppose 1st IF at 1200MHz, the L.O. would sweep 1254 to 2100, 1.67:1, not enough for me. I think maybe you're looking at the wrong generation of CATV tuners. I think Raypsi's suggestion was for the little tin-can tuners common in say 80's and 90's VCR's, these certainly did NOT have a 1200MHz IF, closer to 45MHz. In the 1980s and 90s, I was intimately involved in many types of CATV set-top boxes from one manufacturer. I would have thought that, even after 10 years, I would remember to the nearest Hz what the oscillator and IF frequencies were - but I'm afraid I can't! However, the general plan was essentially that entire input spectrum was bandpass filtered (via a cascaded highpass and lowpass filter), and presented to a 4-diode ring double-balanced mixer. I'm pretty certain that one model (50 to 550MHz) had a first IF at around 650MHz, with the LO running from 700 to 1200MHz. This was then down-converted to a second IF - the usual 45.75MHz (for NTSC). It was fairly obvious that you could make a wide-range variable oscillator (say 0 to 500MHz) by 'reversing things', using a variable oscillator (a readily achievable 650MHz to 1150MHz), and mixing it with a fixed oscillator on 650MHz. The output of the double balanced mixer is taken from the IF port, which (of course) goes down to DC, and is lowpass filtered to 500MHz. As has been stated, this is essentially the principle used by many sweep oscillators. For most 'amateur' purposes, there is no need to have any ALC applied to the output signal. This is achieved by ensuring that the level of the variable oscillator is relatively high (say 66dBmV or 2V), and that of the fixed oscillator is relatively low (say 0.3V or 50dBmV). A normal feature of a ring mixer is that the 'signal' loss through the mixer (typically 6dB) is more-or-less independent of level of the local oscillator - provided it is high enough. This means that, even if the level of the variable oscillator varies by a few dB across its tuning range (which it is bound to do), the mixer output level stays fairly constant 44dBmV or around 0.15V) over the entire frequency range. But things are not perfect. Unfortunately, in the mixer, you inevitably get mixing of the harmonics of two oscillators. The effect is to produce unexpected signals within the wanted band 0 to 500MHz (and greatly dependant on the actual frequency of the variable oscillator). I know of one commercial sweep oscillator which had, at the LF end of the band, several unwanted mixer products only 25dB down. But it was still quite usable for most lab purposes, provided you knew of its limitations. The cure for this inherent problem is to have the frequencies of both the fixed and variable oscillators as high as possible (not as stated above), but this obviously makes amateur construction more difficult. Once you have made a satisfactory wide range oscillator, it is very easy to convert it to a sweep generator by driving the varicap of the variable oscillator from a sawtooth. -- Ian |
3:1 range VCO and varactor RF voltage swing
On Dec 3, 4:58*am, Ian Jackson
wrote: In message , writes In the 1980s and 90s, I was intimately involved in many types of CATV set-top boxes from one manufacturer. I would have thought that, even after 10 years, I would remember to the nearest Hz what the oscillator and IF frequencies were - but I'm afraid I can't! However, the general plan was essentially that entire input spectrum was bandpass filtered (via a cascaded highpass and lowpass filter), and presented to a 4-diode ring double-balanced mixer. I'm pretty certain that one model (50 to 550MHz) had a first IF at around 650MHz, with the LO running from 700 to 1200MHz. This was then down-converted to a second IF - the usual 45.75MHz (for NTSC). All I know about the little tin-can tuners made by the billions in the 80's and 90's, is that antenna comes in one end, circa 45MHz IF came out the other end :-). Maybe the ones I saw were not the DC-to- daylight ones used for CATV. The generation I'm most familiar with took a tuning voltage that went up to 30V or 40VDC for the varactors, maybe that high voltage gave them a wider range with simpler circuitry. Of course the line-operated ones had that high voltage around for easy use in the chassis. For most 'amateur' purposes, there is no need to have any ALC applied to the output signal. Look at the Elecraft K2's LO system. It uses a single VCO to cover all bands, and one relevant factor that lets them use the same oscillator for all bands is the ALC. It's a very very clever design, one that minimizes not only parts count but the low parts count also means low power consumption (less than 200mA for the whole rig in receive.) Contrast that with, say, modern Japanese HF transceivers which draw ten times as much power in receive and have these ginormously complicated multi-loop synthesizers that in the end have worse phase noise than the K2's simple scheme. My gut feeling is that the Japanese rig philosophy of making sure their receivers do DC to daylight drives up their parts count enormously with no benefit (perhaps a negative effect) on ham band performance. Maybe that's what the Japanese hams want. Heck, it's probably what most US hams think they want, if for no other reason than because the YaeKenCom ads have been pushing it as a feature (not a bug) for decades, at least as long as they've been doing upconversion ham receivers. Of course the K2 does so well in comparison because it applies the ham- band performance simplicity philosophy not just to the LO chain but throughout. Tim N3QE |
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