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

[Bruce Griffiths]

Matt

Matt Ettus wrote:
> 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.
>
>   
That estimate should have been 280ps with a 500MHz LO, however that
still doesnt help very much.

This is more a function of the timing receiver noise than the
characteristics of the OCXOs themselves.
You cant expect too much better than that using code phase timing receivers.
However at 30MHz a sawtooth corrected M12+T or M12M will get close to
the 50 degrees of phase error ( 4.76ns).
Using carrier phase techniques you can easily achieve sub 100ps
resolution at least for integration times of up to a few minutes.
Longer term cycle slips can cause problems.


> 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
>
>
>   
For the carrier itself absolute phase measurements cant be done.
Such phase measurements only have meaning when modulation is present.

Bruce