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

[Azelio Boriani]

Guys,
be aware, first of all, that to correctly test your new GPSDO you need an
already running GPSDO as a reference (and a 10 digits-per-second
interpolating counter). Don't forget/overlook the reference: start always
with a known, good reference.

On Mon, Nov 5, 2012 at 9:12 PM, WarrenS <warrensjmail-one at yahoo.com> wrote:

> 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
> ************************
>
>
> [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
>
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