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

Dr. David Kirkby drkirkby at medphys.ucl.ac.uk
Tue Apr 26 10:46:21 EDT 2005

The SRS is actually looking more and more attractive I must say, mainly 
because of its size (the Z3801A is not small), and the long holdover 
period if I can't get a GPS signal - which is quite likely.

I'm tempted to take this in stages, using a reasonably large box, giving 
room for a number of items. My thoughts are:

1) Buy the SRS PRS10 and assume that is *right* for the purpose of the 
inital aim, which was to sync my pendulum clock. Errors in the rubidium 
source should be small.

I'll leave the GPS initially - I have a lot of expenses at the minute.

2) Put some sealed lead acid batteries for backup - I suffer a lot of 
power failures at home. A friend has offered me a charger for 24V 

3) Sync the pendulum clock to a crystal (either TCXO or perhaps an OCXO) 
- the latter being a bit over the top. This is going to be the hard 
part, especially as I will need a PIC to get 1 point something Hz, and I 
have not used PICs before.

This will go in its own box.

5) Add an HP 10811A + Brooks Shera board to get low phase noise, 
syncing *not* to GPS, but to the PRS10. I have all the bits for the 
Brooks Shera board (apart from a few cheap ICs), so the cost in doing 
this is small.

6) Finally add a GPS, which should give me the low phase noise of the 
10811A, with a decent holdover time from the PRS10 if there is no GPS 

I suspect I could fit that lot (apart from 3 which I want sepparte) in a 
3U rack.

I'd need to synchonise the power-up of the two ovens as the power supply 
I have (24V, 2.4A) would not be capable of starting both ovens at the 
same time. The PRS10 take 2.2A on startup, but only 0.6A when running. 
Hence the 2.4A power supply should be okay in running both the PRS10 and 
the HP 10811A, but *not* starting them together.

Any thoughts on that sort of idea above?

Any obvious flaws?? Apart from the fact it is getting more and more 
expensive, and less and less related to the initial aim of making a 
couple of hundred year old pendulum clock more accurate!!!

Dr. David Kirkby

Brooke Clarke wrote:
> Hi Tom:
> Don't forget that SRS has the free PC program Rbmon that talks to the 
> PRS10, although it's listed with a different Rb product.
> 73,
> Brooke
> Tom Van Baak wrote:
>>> Why would it be cheaper than building Brook Shera's unit? You still 
>>> need all the same electronics, but a more expensive oscillator (I 
>>> assume the PRS10 is going to cost more than an HP 18011A.)
>> The data sheet for the PRS10 is at:
>> http://www.thinksrs.com/products/PRS10.htm
>> http://www.thinksrs.com/downloads/PDFs/Manuals/PRS10m.pdf
>>> How does the short term phase noise of the PRS10's compare with the 
>>> HP 10811A? I guess in the long term, they will both be the same if 
>>> you link them to a GPS source, but short term you have said will be 
>>> dominated by the crystal oscillator.
>> The ADEV for a PRS10 at 1 to 10 seconds is on the
>> order of 1e-11 making it 10x worse than the 10811
>> inside a Z3801A. So the choice depends on what
>> you want to use the output of your GPSDO for.
>>> I guess a current unit is likely to be better than a 20-30 year old one.
>> Not necessarily true. Some of the best oscillators in
>> the world were made 30 to 40 years ago. On the other
>> hand, you can't beat a PRS10 for features. Check out
>> the manual for the list of commands.
>> /tvb
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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.

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