Chris:
I doubt that you have test equipment that can directly measure
the hourly "aging" frequency drift of the HP10811.
To get a handle on what you are dealing with, you need
to use the spec sheet on the HP10811 to calculate
the tuning sensitivity of the HP10811, in Hz per Volts.
(more likely scaled to something like micro-Hz per micro-Volt)
It is not linear, but this calculation will be close enough
to get you in the ballpark.
Then convert the count change you are seeing every
three hours to a microVolt change at the output
of your D->A converter and Op amp.
Convert that to Hertz using the above tuning sensitivity.
That will give you an "estimate" of what the drift rate
is. Compare it to the spec on the HP10811 and see if
you are "in the ball park."
Be aware that HP10811 oscillators that have been
turned off for a year or more, will typically take
about three weeks of continuous operation before they
rejoin their spec sheet drift specification.
--- Graham / KE9H
==
On 11/5/2012 8:49 AM, Chris Howard wrote:
I built a GPSDO using my own power supply,
a VE2ZAZ board, a Trimble Resolution T GPS
and a surplus HP 10811 oscillator.
I'm having a bit of trouble with it. I have it set up and
it locks ok and stays in lock so far. But the recommended
long-term integration setting is not working for me.
I think it is about 3 hours. At the end of every cycle
it does a control voltage adjustment, always in one direction.
If I understand it right, the oscillator is slowing and needs
an incremental bump downward of control voltage every time.
That seems like it is more than just long term drift. But
I don't have my head around the quantities I'm looking at.
I can measure the control voltage change over time. Can I convert that
into a frequency drift? Or do I need to stop the voltage
adjustments and allow the drift to occur then do a measurement
of that directly somehow?
Is this type of behavior an indication of dire problems
with my 10811 oscillator?
Chris Howard
w0ep
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