[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.
> Michael, VE3TIX
> time-nuts mailing list
> time-nuts at febo.com
You are limited by the PPS accuracy of the ACE III.
There is no mention on the data sheet of any sawtooth correction being
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.
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