[time-nuts] Next upgrade

Bob kb8tq kb8tq at n1k.org
Wed Nov 22 18:01:05 EST 2017


> On Nov 22, 2017, at 5:16 PM, Jerry Hancock <jerry at hanler.com> wrote:
> Three questions:
> 1) Now that I’ve split my Lucent RFTG-U into a REF0 and REF1 unit with both supplying 10Mhz and 1PPS, is there a way to combine the outputs or some other technique to improve the short and/or long term performance?

You can monitor one against the other. Ideally you would want three GPSDO’s and a 
monitoring setup. That way you can figure out which of the three has gone bad. 

> 2) I’ve become interested in Rubidium Disciplined Oscillators recently and was now thinking of purchasing one of the PRS-10 that I see on Ebay. If I did that and replaced one of the DOCXOs from one of the Lucent boxes, what impact would this have on the overall performance both with and without (when in hold-over)?  

If you go into holdover, you are the exception. Most setups rarely go into holdover. When they 
do, it’s because a hurricane just went over the house. Generally that’s not when the focus is going 
to be on timing experiments. 

> Basically, is it worth the money to upgrade one of the boxes to a Rubidium disciplined oscillator assuming the GPS signal is rarely lost?

Not worth the money if you are only looking at holdover and have a typical setup. 

> 3) Figuring the PRS-10 will cost around $250 when all is said and done, is there a better option to improve my GPSDO system? 

Disciplining implies continuously correcting. Rb standards age much less than a typical 
OCXO. Oddly enough their temperature stability may not be as good as a high end 
DOCXO. It is fairly common to try to stabilize the environment your standards operate in.
To the extent you are successful this reduces the need to deal with temperature. 

The net effect is that disciplining an Rb at a rate (filter / control loop / manual tweak) of less than 
a few days actually makes the Rb worse. Coming up with software to “back off” on the tuning is
not as simple as it might seem. 

This comes back to the fact that the GPS signal (or any of the sat signals) are quite noisy. You 
need to average them over a *long* time to get good performance. Rb’s are enough better than 
a GPSDO OCXO that the time ranges really stretch out ….

> I basically use the GPSDO as a reference for any equipment that takes an input. I have no monetary need for a reference, just an interest.

For most normal test equipment, a GPSDO output is “plenty good enough”. As a source for fancy
timing experiments … maybe not so much. As a phase noise reference or a spur free source for 
microwave games … also not the best way to go.


> Thanks.
> Jerry
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