[time-nuts] OCXO retrace, how long is reasonable?

[Bob kb8tq]

Hi

….. ummm …. errrr …..

> On Sep 4, 2020, at 2:04 PM, rfnuts <rfnuts at arcor.de> wrote:
> 
> Hi,
> 
> given it is a 10MHz OCXO, your drift is just 4E-11/hour or 9.6E-10/day, which isn't terribly bad, is it?
> 
> A good modern OCXO is specified at +/- 0.2ppb/day (2E-10) after 30 days of continuous operation. Some ultra high performance stuff does <1E-10/day, and may improve down into the E-11's after years of undisturbed operation.


The “Trimble” OCXO is not anything super duper. ( = it’s not a BVA that costs many thousands of dollars).
The proceeding post shows it well below 1x10^-11 / day on an “average” basis. It’s not doing that every 
single day, but the net is indeed well into the parts in 10^-12/ day. If you look at the tuning voltage on 10 
or 15 year old TBolt’s, this sort of performance is not at all unusual for that specific OCXO. 

They sell on eBay for $20 or less (delivered) so not up in the stratospheric heights charged for many
other 10 MHz OCXO’s on that site. 

> 
> Keep in mind that your GPS reference frequency may vary by up to about +/- 5E-11 during the day.

These days, GPS its self likely moves 10 to 20 ns over a 6 to 12 hour period. 20 ns / 3600 sec = 5x10^-12. 
What your GPSDO does or does not do on top of this is *highly* dependent on the exact unit you happen 
to have. 

If you look at the ADEV plot of *any* frequency source, there will always be some short tau that 
takes the frequency outside any stated bound. That’s a very different issue …..

> 
> And, your OCXO is sensitive to temperature, power supply and air pressure changes.

Based on the internal construction of the MV-89, air pressure is probably not an issue. Unless the supply
being used is heavily loaded by something else, most modern regulated supply setups will not contribute much
to the instability of a good OCXO ….

Bob

> 
> Regards,
> Adrian
> 
> 
> Am 04.09.2020 um 08:50 schrieb Matthias Welwarsky:
>> Hi,
>> I mentioned in a previous email that I seem to have picked a particularly bad
>> (broken?) OCXO for GPSDO testing. It drifts at a very high rate, from the DAC
>> graph I calculate it is around -4e-4 Hz/hour. It's been on for about 1 week
>> now and the drift doesn't seem to come down, or maybe just marginally.
>> Of course, this being a surplus OCXO with unknown history, odd startup
>> behavior is somewhat expected but so far I haven't seen anything this drastic.
>> Could this be OCXO retrace? How long can that phase last? I'm trying to decide
>> if I should give it another week or just scrap it and move on.
>> Regards,
>> Matthias
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