[time-nuts] This may be my new favorite old oscillator

Skip Withrow skip.withrow at gmail.com
Tue Nov 8 00:16:48 EST 2016

Hello Time-Nuts,

I recently acquired an HP 5065A rubidium oscillator (with 10811 10MHz
OCXO). I think I pretty much have it running now and have been letting it
cook for the last couple of weeks.  I offset the C-field + and - and
measured the frequency to calculate the C-field sensitivity. My unit came
out to 1.96x10E-12 per dial unit, which agrees with the manual stated
2x10E-12. So, calculated the on frequency C-field value and dialed it in.

Attached is a Lady Heather plot of the frequency over the last 3 days.  The
purple line is the 1pps plot with the vertical scale being 20ns per
division.  So, the unit is off about 125ns over the last 72 hours (running
about 4.92x10E-13 slow).  So my C-field setting is off about 1/4 of a
division, but I think I'm going to leave well enough alone.

The yellow line is the NTBW50AA temperature sensor, and you can clearly see
when the furnace cycles.  I was away for the weekend, and you can also
clearly see when I came home this evening and turned up the heat.  At the
very end of the plot is the spike when I turned on the lights in the shop.

I love using a Thunderbolt/NTBW50AA for making frequency measurements this
way.  I remove the OCXO, and insert the 10MHz from the DUT.  Then disable
disciplining so the DAC voltage doesn't try to chase the open loop
oscillator.  Of course the short-term performance looks worse than it
actually is because of measuring against GPS, but the long-term
measurements are very good.

I want to log this unit at regular intervals to see what the aging looks
like.  Also need to do some measurements against the cesium to see what the
short-term performance might be.  But, I think this oscillator will be a
good reference in many cases in lieu of using the cesium.

Skip Withrow
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