[time-nuts] LPRO-101 with Brooks Shera's GPS locking circuit

[Dr Bruce Griffiths]

Tom Van Baak wrote:
>> But is it really an improvement that you get out of it? The answer is
>> NO! He, why not? The answer is: Because you have to PAY the increase in
>> precision with the increase in observation time. For every increase of
>> 10 in precision you need to increase the observation time by 10! 
>>     
>
> Ulrich,
>
> Thanks for the long contribution. One minor correction:
> you imply that increase in observation time is a bad or
> undesirable thing. This is usually true. But not really in
> the case of a GPSDO. Due to GPS receiver 1PPS noise
> you must average over many minutes anyway so this
> greatly relaxes the requirements on the TIC.
>
> What you say later about the sigma-tau lines is all
> correct. I just wanted to point out, for example, that
> a picosecond accurate TIC is a complete waste for
> a GPSDO when the 1PPS jitter is on the order of
> several nanoseconds.
>
>   
>> Would the Shera design make use of a Agilent 51151
>> as a phase comparator its noise floor would start at
>> 5E-10 @ 1s which is a REAL improvement by a factor
>> of almost 100!!! For any given precision the Shera TIC
>> will need 100 X the time that the 51131 needs. 
>>     
>
> Don't mislead yourself. At 1 s you are limited by GPS
> 1PPS noise. Having a better TIC doesn't fix this. If your
> GPS noise is 2e-9 at 1 s you don't really need a TIC
> that is good to 5e-10 at 1 s. So the gain isn't as useful
> as you might think.
>
> I should remind readers that the Shera design came
> from an era of S/A and Oncore VP receivers where the
> modern numbers you throw around do not apply.
>
> /tvb
>
>
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>   
Tom

A TIC with 0.5ns jitter at 1 second isn't actually too much in the way 
of overkill when the PPS signal has 2ns of jitter.
If the TIC has 1ns of jitter and the TIC and PPS jitters are 
statistically independent then the system jitter will be around 2.36ns 
which is about 12% greater than the jitter of the PPS signal itself. 
Conservative engineering practice usually dictates a jitter degradation 
of no more than 5% corresponding to a TIC measurement jitter of about 
1/3 the PPS jitter, that is a TIC jitter of 666ps  of less is advisable 
when the  PPS jitter is 2ns.

If one's GPS timing receiver has a jitter of 2ns then the phase 
detector/TIC used in the phase lock loop to discipline the OCXO/Rubidium 
standard should have  subnanosecond jitter and resolution to avoid 
significantly degrading the phase measurement jitter. This requirement 
will (if one wishes to improve the performance of one's GPSDXO) become 
even more stringent as the GPS system is upgraded and the Galileo system 
becomes operational.

Thus devising inexpensive phase detectors/TICs with subnanosecond 
performance allows one to take advantage of improvements in GPS timing 
receiver performance when they occur.

The possibility of utilising GPS carrier phase tracking techniques in a 
timing receiver offers a potential timing resolution and jitter in the 
picosecond range which would allow enhanced  GPSDO performance. 
Alternatively one could then achieve much better performance with less 
expensive oscillators. Currently dual frequency GPS geodetic receivers 
achieve subnanosecond resolution and stability when the data is 
processed, albeit not in real time.

Bruce