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

[Matt Ettus]

On Mon, Jan 5, 2009 at 3:02 PM, Bruce Griffiths
<bruce.griffiths at xtra.co.nz> wrote:
> 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.

As you said, our ADEV requirements are pretty loose and could be met
by any decent crystal.

But Said just told us that he sees about 25ns difference between
units, which is about 180 times worse than 140 ps.

In VLBI I think that they can deal with an unknown phase error between
the two LOs as long as that phase error remains relatively constant
over the period of integration.  That is why they care about ADEV.
They just correlate the two signals, and subtract out that constant
phase error.  My problem is that we can't just do that in this
application.  We need to know that phase error a priori.

Matt