"lw1ecp" wrote in message
...
Hi!. I want to tie a ~40MHz VCO to a frequency stabiliser. A PIC measures
initial f and then makes corrections. Resolution is 10Hz. The PIC count is
shown on a LCD module.

Hi
I would suspect le reverse voltage on the varicap diode getting too close
to zero.
Try adding a small battery in serial to the control loop.