[time-nuts] Common sky pps errors for any GPSDOs?

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Jan 5 23:02:51 UTC 2009


Matt

Matt Ettus wrote:
> On Mon, Jan 5, 2009 at 1:54 PM, Bruce Griffiths
> <bruce.griffiths at xtra.co.nz> wrote:
>
>   
>> If the application is somewhat analogous to VLBI, then the maximum
>> (uncorrectable ie random) allowable carrier frequency phase errors
>> between receivers depends on the integration time.
>> Maximum integration times for VLBI are typically 10,000 sec or less,
>> with corresponding relative ADEV (after correction for drift etc) on the
>> order of 1E-14 (or better) @ 10,000 sec for a carrier frequency of a few
>> GHz.
>>
>> What is the integration time in your application?
>>     
>
> We are trying to receive data which is modulated at rates which vary,
> but are in the 100 Hz to 100 kHz range, so the integration times are
> very short, all below 10ms.
>
>   
>> What is the carrier frequency?
>>     
>
> 30 MHz and below for now, but we'd like to be able to go as high as
> 500 MHz or so.
>
>
>   
What's your tolerance for phase variations between a pair of LOs?
Its on the order of 50 degrees or so for VLBI.
At 500MHz this corresponds to about 140ps.
The corresponding relative ADEV is around 1E-8 @ tau = 10ms.
Almost any good crystal oscillator can achieve this.
>> For short integration times most rubidium standards are much noisier
>> than a good OCXO.
>> GPS carrier phase techniques will allow lower noise comparisons of LO
>> phase differences than a GPS derived PPS based system.
>>     
>
> Don't the Rb standards end up disciplining an OCXO so you get the
> benefit of the crystal at the shortest integration times?
>
>   
Yes, but compare their resultant short term ADEV with that of a good OCXO.
They can be 1-2 orders of magnitude worse than the better OCXOs.
Most of them use a relatively short time constant FLL to steer the the
crystal oscillator to the passive Rubidium cell.
A few (typically older more expensive ones) can be relatively quiet.
An external low noise OCXO phase locked to the rubidium standard with a
relatively long time constant can be used achieve lower ADEV at sort
averaging times if the operating environment allows this.

Whether any of this is necessary depends on your applications tolerance
to short term phase variations between LOs.
> Thanks,
> Matt
>
>   
Bruce



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