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