[time-nuts] getting a grip on 10811 drift (beginner-ish question)

[WarrenS]

Chris

Q>I can measure the control voltage change over time and convert that into a 
frequency drift?

Yes, no problem as long as the discipline loop is working OK.
It is very easy to plot the oscillator's long term drift per day, by just 
plotting the "filtered" analog EFC control voltage.
Typical 10811 EFC sensitivity I've seen is 0.25Hz / volt

Q>Is this type of behavior an indication of dire problems with my 10811 
oscillator?

Depends how long it has been runing. A high ageing rate per day is typical 
for a Oscillator that has not been used for a while.

Depending on the size of the step, If you still have the same problem after 
running continuously a few weeks,
The "dire problems" may have more to do with your control loop tuning method 
than the 10811.

If the 3 hr integrating time is also the update cycle time, then no wonder 
there are 'large' steps at each update.
10K sec is too long of an update rate to get the best performance from a 
HP10811.
As an example, good settings for a TBolt when disciplining an external 
HP10811 is about  1000 sec integration time with an update rate of once per 
second.

Generally it is better to use small steps and update the EFC voltage more 
often, maybe more like one per minute,
and then reduce the overall "feedback gain" to increase the control loop 
time constant until it is around ~1000 seconds

One of the easy ways to improve most everything, is to limit how much EFC 
voltage change you allow.
After the oscillator has been running a few weeks continuously, a total 
change of well under 1 volt at the EFC input is plenty,
when there is a manual course frequency adjustment, like in the HP108111, 
that can be used to set the EFC control voltage to the center of it's range 
.

ws
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[time-nuts] getting a grip on 10811 drift (beginner-ish question)
Chris Howard chris at elfpen.com

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