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

[Dr Bruce Griffiths]

Brooke Clarke wrote:
> Hi Ulrich:
>
> I think the answer is what other low cost options are available?  I 
> would like to have a more modern TIC capability to add to the clock I'm 
> working on.  But although there's been a lot of discussion about 
> different ways of making TIC measurements, it's not clear to me how to 
> do it on a budget.
>
> For example the TIC232 circuit by Richard H McCorkle is easy to 
> implement, but how good is it's noise floor.  See:
> http://www.piclist.com/techref/member/RHM-SSS-SC4/TIC232.htm
>
> Then there's the low cost frequency counting TIC that appeared in QEX 
> that we know trades performance for low cost so it's not a candidate.
>
> Any ideas on what circuits have a noise floor that's compatible with the 
> M12+T or it's newer equivalents and at the same time are in the low cost 
> category? 
>
> Have Fun,
>
> Brooke Clarke
>
> w/Java http://www.PRC68.com
> w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml
> http://www.precisionclock.com
>
> Ulrich Bangert wrote:
>
>   
>> <>Tom,
>> .....
>>
>> 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.
>>     
>
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>   
Brooke

I agree that most will tend to use an available circuit particularly if 
they are not too experienced/adventurous.

The noise/resolution of the TIC232 will be a little worse than that of 
the Brooks Shera circuit.
It would appear to use the internal counter timer which is clocked at 16MHz.
Also this timer has no hardware for latching its count on the leading 
edge of an external signal so there must be some software component used 
to do this.
This will almost inevitably add extra noise/uncertainty due to 
variations in the delay in reading the timer.
The quoted resolution of 1.04ns for a 1 minute average is probably 
derived from a 62.5ns resolution for each individual measurement.

One can only achieve the subnanosecond resolution required to avoid 
degrading the performance of an M12+ by using a clock frequency of 1GHz 
or more.
Thus this method is probably too expensive and difficult to implement.

Perhaps there would be some demand for a higher resolution replacement 
for the Brooks Shera system for those who have M12+ or equivalent 
performance timing receivers and high performance OCXOs or Rubidium 
standards who wish to achieve the best performance they can without 
breaking the bank.

If so then perhaps we can collectively design such a system.
Breaking the task down into more manageable parts will help ensure that 
the design is more quickly implemented

As I see it the following methods can achieve the desired phase 
measurement resolution

1) Use a commercial TDC chip as the phase detector.
Range 4millisec ( can be extended almost indefinitely by using a 
synchroniser and counter implemented in a gate array or its functional 
equivalent)
Noise 65ps rms
Cost ~ 100 euro
Advantages someone has already designed and debugged the chip as long as 
the circuit layout recommendations are adhered to there should be no 
unforeseen problems.

2) Use an ADC to sample a sinewave formed by dividing down the OCXO 
frequency and filtering the resultant square wave
Range half period of the sinewave frequency
Noise (rms) < 0.0005 of the sinwave period (500ps with a 1MHz sinewave)
Cost ~ $US20 ??

3) Use dual simultaneous sampling ADCs to sample quadrature phased 
sinewaves derived by dividing down the OCXO frequency filtering the 
resulting square wave and using a quadrature hybrid to produce the 
quadrature phase sinwave pair. Extend range to as much as 1 second or 
more using dual synchroniser to sample a continuously running digital 
counter/timer.
Range to several days or centuries if required, depending on counter lenght
Noise (rms) < 0.0005 of the sinewave period (500ps with a 1MHz sinewave)
Cost ~ $US40 ??

Bruce