[time-nuts] GPSDO won't lock: OCXO aged out?

[ed breya]

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