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

Dr. David Kirkby drkirkby at medphys.ucl.ac.uk
Tue Apr 26 11:30:36 EDT 2005


The pendulum clock was what got me started on this, but I do have other 
test equipment, including a microwave counter, which could do with a 
decent reference
Brooke Clarke wrote:
> Hi David:
> 
> How accurate is the pendulum clock now?  How accurate do you want it to be?
> 
> Have Fun,
> 
> Brooke
> 
> Dr. David Kirkby wrote:
> 
>> 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 
>> batteries.
>>
>> 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 
>> signal.
>>
>> 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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