[time-nuts] 1 pps correction

Stijn Nestra stijn at pe1rks.nl
Mon Apr 2 09:35:50 UTC 2012


The PRS10 schematics are available on line from Didier.
See the following link:
http://www.ko4bb.com/manuals/index.php?dir=05%29_GPS_Timing/SRS

Sincerely,

Stijn

Op 02-04-12 09:48, Azelio Boriani schreef:
> OK, thank you. I'll collect the documentation you suggest to study it. Yes,
> the PRS10 manual available online has no schematic. The paper one does
> have. Does this means that Stanford Research want it not to be disseminated?
>
> On Mon, Apr 2, 2012 at 5:06 AM, Said Jackson<saidjack at aol.com>  wrote:
>
>> Hi Azelio,
>>
>> its a dual slope interpolator, much like the HP 5334A counter. In fact the
>> 5334A service manual is very nice to go through to get lectured on how this
>> works. The capture hardware is similar to the Linear Tech app note written
>> by Jim Williams (mentioned in the time nuts archives). Basically its a very
>> fast constant current source, and a high quality capacitor. Except Jim
>> charges the cap, then uses an analog to digital converter to capture the
>> time difference. We use a micro controller to capture the time difference
>> on the cap, then capture how long it takes to discharge the cap with about
>> ~1000x slower current than the charge current. Hence we get ~1000x to 1
>> time dilution, which means the underlying 16.66ns counter resolution
>> becomes a ~16.7ps resolution. While I have never seen the PRS-10 Rubidium
>> schematics (anyone have them in PDF format?) I gather from the description
>> in the service manual that they do something similar to this. The Wavecrest
>> DTS user manuals floating around on the internet also explain how this
>> works. So in short, all that is required to build a unit like this is a
>> bunch of fast analog charge hardware, and an analog comparator that can
>> trigger a counter capture event, and some software for calibration and
>> control...
>>
>> bye,
>> Said
>>
>> Sent from my iPad
>>
>> On Apr 1, 2012, at 4:07, Azelio Boriani<azelio.boriani at screen.it>  wrote:
>>
>>> Said,
>>> how complex is your 20pS time interval counter? Is it analog, FPGA,
>>> something else (if you can disclose some info, of course)?
>>>
>>> On Sat, Mar 31, 2012 at 11:44 PM, Hal Murray<hmurray at megapathdsl.net
>>> wrote:
>>>
>>>>
>>>> martyn at ptsyst.com said:
>>>>> I’ve seen that the peak to peak jitter is reduced from something like
>> 27
>>>> ns
>>>>> to<  10 ns.
>>>>
>>>>> Is this a reduction of just the jitter, or is the actual accuracy to
>> UTC
>>>>> also improved by this amount.
>>>>
>>>> Have you read the hanging-bridges paper?
>>>> Tom Clark and Rick Hambly: Timing for VLBI
>>>> http://gpstime.com/files/tow-time2009.pdf
>>>> I think that is the key to understanding this area.
>>>>
>>>> If you could average over many sawtooth cycles, you should get an
>> accurate
>>>> answer.
>>>>
>>>> The problem is that you don't get to pick how many cycles fit into your
>>>> averaging time.  The sawtooth pattern is the beat between two
>> frequencies.
>>>> One of them is drifting with time/temperature.  If you are unlucky, the
>>>> beat
>>>> frequency can be very very low.
>>>>
>>>> The sawtooth correction lets you correct on a cycle-by-cycle basis.  You
>>>> don't need to average over many samples.
>>>>
>>>>
>>>> --
>>>> These are my opinions, not necessarily my employer's.  I hate spam.
>>>>
>>>>
>>>>
>>>>
>>>>
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