Message-ID: <65AECDF1F89AD411900400508BFC869F0D75B4@pdw-mail-1.dera.gov.uk>
From: "Talbot Andrew" <ACTALBOT@dera.gov.uk>
To: rsgb_lf_group@blacksheep.org
Subject: LF: Re: GPS-Disciplined BPSK
Date: Wed, 14 Feb 2001 14:53:18 -0000
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> > in view. The 1PPS output is specified to be within 50ns of  "true
time" (GPS time
> > or UTC, selectable) with SA on. Now that SA is gone, the accuracy is
even better
> > (there is always a ~30ns sawtooth error due to finite  resolution in
the software/hardware).
> > 

Surely, if the 1us jitter is a function of clock timing synchronisation
then by averaging over many 1s periods, as the Brooks Shera design does,
errors will be averaged out and high accuracy frequency sources may be
properly locked to this 1PPS signal.    No GPS disciplined frequency
source worth its name would, or should, ever try to correct output
frequency absolutely on a pulse by pulse basis.

>From memory, I think that design only claims about 10^-9 accuracy anyway
locking with a time constant of many tens of minutes using a digital
phase locked loop, still far less than GPS 'ought' to be capable of,
although this was in the days when Selective Availability was still on.
But accurate enough for most LF experiments - at the moment.    For bit
timing purposes, I would have thought 1ms was good enough for 100ms or
slower BPSK.

RF carriers such as BBC Radio 4 on 198kHz which have a specified
accuracy are not actually themselves generated from a locked source, due
to the requirement to keep phase noise as low as possible.  Instead,
they run from very high stability oscillators which are manually
corrected in frequency and phase against a national standard from "time
to time" - presumably when they have drifted enough degrees to see.
Therefore the short term stability is excellent, better probably than
the Caesium standard  on site for the regular checks, and certainly
better than an off air MSF or GPS one.  Overall accuracy due to the
constant corrections in the long term equals that of the primary
standard.   Oscillators used will be of the modern Hewlett Packard
design (Agilent now) which claim +/- 5 *10^-10 / day   short term drift
and 1*10^-10 aging for their oscillators.  The oscillator in my Cs
standard, left free running, maintains better than 10^-9 over months and
that is for a thirty year old unit (well aged !)
Even the cheaper ovenned oscillators, often to be found on the surplus
market and in surplus test equipment can maintain a few parts in 10^-9
over days or even months.   

At a recent rally I saw a box of 5MHz ovenned standards selling for Ł10
each - and no one was buying them ! 
These were of the type that if NEVER turned off, can give 10^-9 accuracy
once adjusted.   They were used originally in co-channel UHF PMR
transmitters where 450MHz carriers in overlapping service areas are
maintained at just a Hz or so apart in frequency.  This ensures that any
receiver in an overlapping service with equal signal from both
transmitters carrying the same FM traffic does not suffer horrible beat
notes, just a fading signal that does not detract from reception too
much.

Andy  G4JNT


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