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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 |
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