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