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

[Dr Bruce Griffiths]

Brooks Shera wrote:
> ----- Original Message ----- 
> From: "Ulrich Bangert" <df6jb at ulrich-bangert.de>
> To: "'Discussion of precise time and frequency measurement'" 
> <time-nuts at febo.com>
> Sent: Friday, December 15, 2006 05:47
> Subject: Re: [time-nuts] LPRO-101 with Brooks Shera's GPS locking circuit
>
>
> .......
>   
>> I second Bruces's opinion about what is an overshot or not. When ps
>> reolution is ready available then why not use it? I attach a online
>> output from my DIY GPSDO from a few minutes ago that shows the M12+'s
>> signal properties when measured with abt. 110 ps resolution against a
>> FTS1200. The yellow line reperesents a prefiltered version of the
>> sawtooth corrected values (blue). The filter time constant is 1/3 of the
>> loop time constant as in a PRS-10. The yellow values are the ones to
>> feed the regulation loop.
>>     
>
>   
>> What I wanted to explain is the Shera concept noise floor is a factor 20
>> above what a modern receiver can deliver (again inc. the sawtoth
>> correction). And yes, you are right: There were different numbers when
>> this concept was thought out! And exactly because different number were
>> there when this concept was thougt out I am going to ask why people
>> still built it today.
>>     
>
>   
>> Best regards
>> Ulrich Bangert, DF6JB
>>     
>
>
> I believe the sawtooth correction is of little or no value for a GPSDO, 
> which typically requires a low pass filter between the GPS 1pps and the 
> disciplined oscillator.  This filter is quite effective in removing the 
> sawtooth quantization introduced by the GPS rcvr clock, just as it removes 
> the similiar quantization caused by my phase detector.
>
> For example, reading from your graph I averaged the raw data (as best I 
> could by reading the blue line).  The running average of the raw data over 
> 40 sec (starting at 12:31:30) was -4.5 nsec, after 60 sec it was -4.2 nsec. 
> These values appear to be indistinguishable from the values you get by 
> averaging the "sawtooth corrected" data (the yellow line).
>
> It appears from your plot that the sawtooth correction has contributed very 
> little or nothing that averaging does not already provide.   Have I 
> misunderstand something?
>
> I believe that your "noise floor is a factor 20 above what a modern receiver 
> can deliver" statement is incorrect.  With an HP 5720B (about 100 psec 
> resolution), I have measured the phase difference between the GPS 1pps and 
> the phase of a 5 MHz oscillator controlled by my controller. This data has 
> been compared with simultaneous phase serial output from the controller as 
> determined its maligned 24 MHz asynchronous internal phase measurement 
> circuitry.
>
> ADEV Stable 32 plots of both data sets are essentially identical.  From this 
> I conclude that nothing would be gained, for the purpose of discipling an 
> oscillator, by using a more elaborate and expensive phase detector  (the 
> phase detector in my controller costs $6.61, including $5.35 for the dual 24 
> MHz osc that is shared as the PIC clock).  It was my goal when I designed 
> the controller was to make the design transparent to the builder and to use 
> as few parts as necessary consistant with performance limited only by 
> available GPS receivers and VCXOs.  When I wrote the QST article I was 
> totally ignorant of the fact that I could buy the HP58503 with similiar 
> performance for $5400.
>
> Your earlier comment about the capture range of the phase detector is well 
> taken.  For the past several years the PIC software I provide has included 
> an option designed for use with inexpensive TCVCXOs.  It requires only an 
> external 128 divider chip and produces EFC voltages suitable for inexpensive 
> oscillators.  It works very well and provides sufficient performance for 
> many applications.
>
> Regards,  Brooks
>
>
>
>
>
> --------------------------------------------------------------------------------
>
>
>   
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>   
Brooks

Your comparison of your circuit with measurements taken with the "5270" 
(is this a typo? did you mean 5370? which is known to have  differential 
non linearities well in excess  of  100 picoseconds,  at least according 
to the designers - some later modifications  to the circuitry reduced 
this effect somewhat) demonstrates very little unless the measurements 
were corrected for the sawtooth error.

The only true test is to compare a sawtooth corrected GPSDOCXO alongside 
a sawtooth corrected GPSDOXO. Both should of course use equivalent 
performance oscillators and GPS timing receivers.

The short plot that Ulrich furnished doesn't include any hanging bridges 
which occur when the GPS oscillator drifts through a harmonic of 1Hz.
Most M12+ GPS timing receivers produce sawtooth correction errors in 
which such "hanging bridges" are not infrequent.

No one is disputing that with an low performance oscillator its not 
worth going to too much trouble in improving the phase detector performance.
However some of us have oscillators with much better performance than 
such cheap oscillators. We also have a need to achieve an oscillator 
instability of less than a few parts in 1E12 which your circuit cannot 
reliably provide in the presence of hanging bridges and aberrant PPS 
pulses which do occur from time to time.

The existence of a commercial GPSDOCXO that achieves an Allan variance 
of 2E-13 or better from tau = 1 sec to 1 year, indicates that it is 
possible to do much better than your circuit is capable of. All we are 
doing is exploring cheaper ways of approaching this performance within a 
factor of 10 or so.

Bruce