[time-nuts] HP 106B quartz frequency standard...the story so far

[Jim Palfreyman]

OK, I'm puzzled. Can someone with a good knowledge of OCXOs explain my
observation. This is my HP 106B double ovened quartz oscillator, but
I'm sure the theory applies generally.

It's easiest to show these observations as made-up but approximate
numbers an hour apart. Say the device is set to 5.000 000 000 MHz and
then measurements are done on a 5370B with a GPSDO as an external
reference. (Note that the 5370B has more jitter than shown below when
measuring frequency, but by following it for a minute or so you can
see what numbers it hovers around.)

Connect an HP rubidium (to prove it's not a measurement error):

Hour     0                   1                     2
   3                      4                      5
5.000 000 000    5.000 000 000   5.000 000 000   5.000 000 000   5.000
000 000   5.000 000 000

My 106B as it currently is running:

5.000 000 000    5.000 000 001   5.000 000 002   5.000 000 003   5.000
000 004   5.000 000 005

If I turn the inner oven control in one direction (presumably the
hotter way) a fraction of a turn and then sit back and watch:

5.000 000 000    5.000 000 003   5.000 000 006   5.000 000 009   5.000
000 012   5.000 000 015

If I turn the inner oven control the other way (a bit further past the
original point):

5.000 000 000    4.999 999 995   4.999 999 990   4.999 999 985   4.999
999 980   4.999 999 975


What I don't understand is why changing the oven temperature cause the
frequency to continually increase or decrease. If you look at Quartz
temperature curves (and I'm presuming this is an AC cut since SC
wasn't invented until 1976) they show a frequency offset dependent on
temperature. But I'm not getting an offset, I'm getting a steady
increase - all the time.

Sure I can find my sweet spot where it moves minimally (as it is now)
but that is hard to improve and seems to be sitting on a knife edge.

The only other thing to note is that my power supply rail is a few
volts too high (see previous post). Could this be the cause?

If what I'm seeing is normal I'd love to have it explained to me!

Regards,

Jim