[time-nuts] OCXO aging behavior

[John Green]

I had always thought that crystals changed frequency consistently in one
direction as they age. That is until recently. I was trying to get the
reference in my 1992 as close as possible to my Z3801 and noticed that as I
adjusted the fine pot, there didn't seem to be a place where the frequency
stayed put. It seemed to drift lower over a days time. So, I thought I would
set it a little high and over the weekend it would go lower and closer to
the Z3801. Much to my surprise, I found it had gone even higher over the
weekend. I gave up on trying to get it so close and settled for around 1E-9
or so. I am going to watch it over a few weeks to see what happens next. I
have also noticed something like hysteresis in frequency adjustment. A
little tweak in one direction can only be offset by a larger tweak in the
opposite direction. I assume all this is normal. I just had not been able to
see it until I started using a GPSDO to compare things to. After hearing so
much about how sensitive Tbolts are to temperature and antenna location, I
am going to feed both the Z3801 and a Tbolt off the same antenna and see if
I can see any differences. I don't know if I have the equipment for it, but
it should be interesting.