[time-nuts] Oscillator used in HP 8920 service monitor?

[John Ackermann N8UR]

Does anyone know anything about the high-stability oscillator (option
01) in the HP 8920A/B service monitor?

I am pretty sure that it's not a 10811A as there isn't enough room in
the card cage for something that size.

I'm asking because I was very pleasantly surprised today when I did a
calibration for the first time in at least 2 years (maybe 5 -- I had it
in for some service but I don't know if they cal'd it then) and it was
off by well under 100 mHz at 10MHz.  The cal process is interesting --
you don't tweak the oscillator at all, rather adjust coarse and fine DAC
fields in the setup screen.  I only had to move about 15 fine steps (out
of a range, I think, of 255) to bring it to within about a milliHz.

I'm curious to know what sort of rock they're using in there.
Unfortunately, the 8920 is of the newer vintage HP gear where they don't
provide any real service information in the manuals, so there's no
indication of what it might be.

Thanks,

John