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

[Dr Bruce Griffiths]

Jack Hudler wrote:
> If I'm totally missing something here please correct and enlighten me. 
>
> On the subject of Brooks Shera's design, the one thing that troubles me is the
> use of a 24 MHz oscillator to count the width of the 1PPS signal. 
> This yields a precision of 4.16e-8, but does it really? 
> This oscillator is uncontrolled and any drift would exist as noise that would
> have to be filtered (He uses a software low pass filter).
>
> Question: Why not multiply the VCXO or OCXO output by 5 or 10 and run that into
> 24 or 32 bit counter? OR just sample the counter on every 10th PPS?
>
> Thank,
> Jack
>
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On Behalf
> Of Richard H McCorkle
> Sent: Wednesday, December 13, 2006 4:44 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] LPRO-101 with Brooks Shera's GPS locking circuit
>
> Hi Brendan,
> I would contact Brooks Shera directly before you go any further as the span
> of his controller is set at 4.5e-8 and the maximum span on an LPRO is about
> 5e-9. He can reprogram the controller by changing the filter gain and loop
> time so it is about 8X more sensitive so it will work with a direct
> connection to the LPRO. I have a controller modified with the filter gain is
> scaled up 1 step and the loop time set to 120 seconds and this works well
> with an LPRO attached directly to the DAC.
> Enjoy!
> Richard
>
> ----- Original Message ----- 
> From: "Brendan Minish" <ei6iz.brendan at gmail.com>
> To: "Discussion of precise time and frequency measurement"
> <time-nuts at febo.com>
> Sent: Wednesday, December 13, 2006 9:13 AM
> Subject: [time-nuts] LPRO-101 with Brooks Shera's GPS locking circuit
>
>
>   
>> Hi I am looking for info on using the Brooks Shera GPS-VCXO Controller
>> with an EFRATOM LPRO-101?
>>
>> I currently have it locking an old and unknown single oven Xtal
>> oscillator this is working as well but I hope to replace this with the
>> LPRO-101
>>
>> Has anyone any suggestions as to how best to choose the correct values
>> of R5 and R6 for use with the LPRO-101 C field control input.
>>
>>
>> 73
>> Brendan EI6IZ
>>
>>
>>
>>
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>   
The Brooks Shera circuit relies on the 24 MHz oscillator not being 
locked to the OCXO or the PPS signal so that (in the absence of 
injection locking) averaging the measured time interval gives an 
unbiased estimate of the true value. Using a higher speed oscillator to 
measure the time interval would be advantageous with the more accurate 
GPS timing receivers currently available. If you have an M12+ or MTM 
timing receiver and a rubidium oscillator the Brooks Shera technique is 
not optimum you can easily achieve a performance that is 1-2 orders of 
magnitude better with a less complex phase detector. The Brooks Shera 
circuit has a single shot resolution of 41.6 nanosec which may be up to 
10 times worse than the jitter in the PPS output of a good timing 
receiver (after correcting for any sawtooth error - in software of 
course, to avoid any additional noise and errors caused by hardware 
correction).

The phase detector method employed is the digital equivalent of a 
classical sampling phase detector with a linear phase detection 
characteristic. A phase lock loop employing a sampling phase detector 
will lock to a harmonic of the sampling frequency, which in this case is 
1Hz. It is therefore necessary to divide down the OCXO (or other high 
stability oscillator) frequency to a value such that the efc tuning 
range of the oscillator precludes locking to the undesired harmonic. 
Unfortunately a phase detector with a linear characteristic can 
sometimes allow the phase lock loop to lock to a frequency which is not 
a harmonic of the sampling frequency (in this case frequencies like 
17/13, 12/11 Hz etc.).

A phase detector with a sinusoidal characteristic (such as an ADC 
sampling a sinewave produced by filtering the output of a counter 
clocked by the OCXO) avoids the problem of non harmonic locking and 
eliminates most of the digital circuitry.

A loop time constant of around 120 seconds is far too short for 
obtaining the optimum performance from a good rubidium standard, as the 
loop will degrade the rubidium standard to an accuracy of around 35E-10 
(with a 41.6ns single shot measurement resolution) when a good rubidium 
standard is capable of a considerably higher stability than this.

Bruce