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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Jan 5 18:03:00 EST 2009

Matt Ettus wrote:
> On Mon, Jan 5, 2009 at 12:13 PM, Lux, James P <james.p.lux at jpl.nasa.gov> wrote:
>>> -----Original Message-----
>>> From: time-nuts-bounces at febo.com
>>> [mailto:time-nuts-bounces at febo.com] On Behalf Of Matt Ettus
>>> Sent: Monday, January 05, 2009 11:31 AM
>>> To: Discussion of precise time and frequency measurement
>>> Subject: [time-nuts] Common sky pps errors for any GPSDOs?
>>> I am working with someone who needs to have time synchronized
>>> reception of signals in various locations which are separated
>>> by less than 100 km.  This is a situation similar to VLBI,
>>> but since the distances are shorter, the center frequencies
>>> are lower, and the integration times are much shorter, we
>>> probably don't need a Hydrogen Maser, and the application
>>> can't afford one.
>>> The real question is whether we can get away with a GPS
>>> disciplined OCXO or whether we would need to use a Rubidium.
>>> Does anyone have any data on the relative frequency and/or
>>> phase errors of the 10 MHz reference out, and relative PPS
>>> time errors of any commonly available GPSDOs?
>> Isn't that just the Allan Deviation data? Symmetricom has datasheets on their website for their various modules. They have a GPS discplined quartz oscillator in several flavors.
>> http://www.symmetricom.com/media/files/downloads/product-datasheets/ds%20XLi%20Options%202.pdf
> I don't think Allan Deviation is the right measure.  First, standard
> Allan dev numbers won't take environmental differences into account.
> Also, isn't Allan Dev measured vs. a better reference?
Not so, ADEV actually includes environmental effects during the measurement.
However, data sheet ADEV specs may have been obtained in a more closely
controlled environment that your intended use.
ADEV is measured between oscillators one of which may be better than the
The corresponding ADEV of each oscillator may then be derived from the
relative ADEV data if some assumptions are made.
One can use the datasheet ADEV to estimate worst case ADEV for pairs of
such oscillators.
To gauge the relative ADEV between oscillator pairs in your environment
you will need to make some measurements as the various unspecified
thermal time constants of the OCXO will be significant.

>> Something else to consider is doing post processing.. Use a nice quiet 10MHz oscillator for your source/sampling clock, and record the 1PPS from the GPS receiver as well as your unknown, then figure out after the fact what the oscillator was doing.
> Unfortunately, post processing isn't possible in this app, since it is
> a real-time communications application.
> Thanks,
> Matt

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