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

WarrenS warrensjmail-one at yahoo.com
Mon Nov 5 21:16:48 UTC 2012



Maybe need to define "Correctly"

True, It sure helps if one has lots of nice test equipment around to start 
with, but
Then again if one already has the better test equipment needed to be able to 
quickly test a new GPS,
then there would not be much reason to be building a home built thing.
because none of test equipment stated is actually needed to make and test 
your own home built GPS disciplined oscillator.

With a little care and a lot of time, It can be done without anything very 
special by just Bootstrapping up with home built things.

ws

********
Azelio Boriani  wrote:

>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 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 running. 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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