[ntp:questions] backup PPS source

Hal Murray hal-usenet at ip-64-139-1-69.sjc.megapath.net
Mon Apr 12 05:36:16 UTC 2010

In article <82f3qcFg17U1 at mid.individual.net>,
 David Lord <snews at lordynet.org> writes:

>Latest experiment is to attempt to make a backup PPS source and
>I've now made a start with a watch xtal oscillator and divider
>that can be synchronised by a PPS source when present (except
>I've not yet wired up for synchronisation).

In general, watch crystals aren't very good.  If you want to
see how good/bad your setup is, just set it up as noselect and
let it run for a day or week.  Then plot the offset from peerstats.

You can compute the frequency error from the time drift over
a given time, and the reverse.

>Any hints/tips welcome, I'm also considering an oscillator using
>an AT cut xtal but design for that is considerably more complex,
>however it's then a possibility to convert the HF to be used as
>system clock.

The simple way to get a PPS out of a crystal is feed a small
micro (PIC, AVR, 8051) and do it all in software.

What you probably want is a GPSDO: GPS Disciplined Oscillator.
The basic idea is that you start with a good oscillator, use
GPS to keep it running at the right frequency and phase too
if you want a PPS.  If the GPS fades out, the oscillator will
coast for a while.  (Magic word is holdover.)  If the crystal
is good enough, it will stay close in time.  How close for how
long will be in the data sheet.

Google for HP Z3801A and Trimble Thunderbolt.
There was an article in the HP Journal on the Z3801A.

They are used in cell phone towers and 911 offices.
There are a lot of them on the recycle market.  Check

>I also keep googling for rubidium sources but not found any
>within my budget, cheapest I've found (new) starting at ukp 900.

Try EBay.  Again, there are a lot of them getting recycled
from telco offices and/or cell phone towers.

These are my opinions, not necessarily my employer's.  I hate spam.

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