[time-nuts] Brooks Shera's GPS standard or HP Z3801A??

Tom Van Baak tvb at leapsecond.com
Mon Apr 25 13:08:31 EDT 2005

This issue comes up a lot. If weren't for the glut of
surplus Z3801A I'm sure the situation would be very
different. Brooks Shera did a fine job with his project
and reached a wide audience with its QST publication.

If your goal is a turn-key 10 MHz frequency standard
with short-term stability under 1e-11 then a Z3801A
is a good solution.

If your goal is to build something yourself, to use parts
you have already, to learn as you go, and achieve
similar performance then use the Brooks Shera design.

The performace of either one is limited by the quality
of the particular oscillator inside which is why there is
always a drvie to find the best one.

The issue of GPS jitter is also related to the resolution
of the embedded TI counter you are using. There's little
point trying to lower the GPS receiver jitter, for example,
if your TI counter is the limiting factor.

If you're a time-nut you'll end up with both anyway so
that you can compare them; the only question is which
to get first. ;-)


----- Original Message -----
From: "Dr. David Kirkby" <drkirkby at medphys.ucl.ac.uk>
To: "Discussion of precise time and frequency measurement"
<time-nuts at febo.com>
Sent: Monday, April 25, 2005 02:35
Subject: [time-nuts] Brooks Shera's GPS standard or HP Z3801A??

> I was going to build the GPS reference by Brooks Shera
> http://www.rt66.com/~shera/index_fs.htm
> and have bought the PCB and started populating it (not much yet).
> But now I am wondering if buying a used Z3801A might not be a better idea.
> My thoughts are:
> 1) Cost - the Z3801A will probably cost less than buying all the parts
> for Brook's Standard. This is especially so, with me in the UK, where
> shipping charges on lots of small items from the USA soon add up.
> 2) Medium term stability - the double oven in the Z3801A might be more
> stable than an HP 18011A.
> 3) Flexability - Brook's standard can be customised to whatever you want.
> 4) Long term servicability - if one builds the standard oneself, one is
> far more likely to be able to debug it if things go wrong. One can buy a
> few of the chips, for spares, just in case one dies and becomes hard to
> get. The chips on the board are not that expensive.
> 5) Jitter on GPS.
> 6) If one needs to buy a manual on the Z3801A, that will add to its cost.
> Any other thoughts ????
> --
> Dr. David Kirkby PhD CEng MIEE,
> Senior Research Fellow,
> Department of Medical Physics,
> Mallet Place Engineering Building,
> Gower St,
> University College London,
> London WC1E 6BT.
> _______________________________________________
> time-nuts mailing list
> time-nuts at febo.com
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