[time-nuts] Advice on using a surplus LPRO-101 oscillator

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Apr 21 18:18:08 EDT 2007


michael taylor wrote:
> I recently acquired a surplus Datum/Efratom LPRO-101 Rubidium
> oscillator, found a PDF manual from Datum and was wondering if anyone
> had any advice or warning on using these oscillators.
>
> I was planning on building a GPS disciplined oscillator using the LPRO
> and the 1 PPS out initally from a Trimble Ace III until I get a
> i-Lotus M12M Timing GPS.
>
> I've seen comments that Brooks Shera GPSDO design could be improved
> upon, and I was wondering if anyone has a concrete design for such an
> improved GPSDO.
>
> Thanks,
>  Michael, VE3TIX
>
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>   
Michael

You are limited by the PPS accuracy of the ACE III.
There is no mention on the data sheet of any sawtooth correction being 
available.
The specified 95ns PPS positioning accuracy (not stated if this is a 
1-sigma or 3 sigma limit or something else) will determine the maximum 
"sensible" phase resolution to around 1/2 the 1 sigma PPS error. If the 
95ns is the 3 sigma error then a phase error resolution of around 30ns 
or so is adequate .

So until you get a better timing receiver there is little point in using 
any of the more precise techniques.

Since applying the sawtooth correction available from the iLotus M12M 
requires software specific to that receiver (and other receivers that 
use the same commands etc), it cannot be tested with an ACE III.

A phase error resolution of 1ns or better is required to take advantage 
of the claimed accuracy of a sawtooth corrected M12M timing receiver.
Whilst several techniques (faster counter clock, Time to digital 
converter, TAC, ADC sampling a sinewave on the PPS edge) are available 
to achieve this they are neither simple nor inexpensive to implement. 
Perhaps the simplest is to use an ADC to sample a sinewave generated by 
dividing down your reference to say 500kHz or so, and  low pass 
filtering the divider output to produce a low distortion sinewave. 
Suitable ADCs are readily available. The ADC output is used as the phase 
error which is sawtooth corrected in software and used as the input to 
the phase lock loop system implemented in software.

However the effort required to build such a system seems hardly worth it 
when one can use a suitable GPS receiver itself as the phase detector 
with a resolution of a few picoseconds.

Bruce




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