One thing I forgot to mention, is that the EFC sensitivity isn't
necessarily the same in this new range as it was before. In other words,
having more gain may not be a big deal, or even beneficial. You're way
past the steeper dC/dV slope anyway, so the actual effect of the varicap
on the net frequency may be much less in the 5-20 Vt region than in the
original 0-5V range. So, it may be OK or even better with more gain. You
can estimate the new tuning sensitivity from your experiments, and see
how it shapes up compared to the likely original characteristic.
Restoring it to "original" would then involve proper gain and offset
adjustments to land close enough.
In my experiments with Si junctions (not just varicaps), I've seen that
once you get past a few volts reverse bias, the slope changes
dramatically (flatter), so passing the 5 V range as are doing could make
a big difference, and more gain may be OK. But, don't overdo it - most
varicap diodes I've seen will avalanche break down (not destructive if
current limited) around 30-50 V.
Ed
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