On Mon, Jan 5, 2009 at 2:56 PM, Lux, James P <[email protected]> wrote: > >> -----Original Message----- >> From: [email protected] >> [mailto:[email protected]] On Behalf Of Matt Ettus >> Sent: Monday, January 05, 2009 2:28 PM >> To: Discussion of precise time and frequency measurement >> Subject: Re: [time-nuts] Common sky pps errors for any GPSDOs? >> >> On Mon, Jan 5, 2009 at 12:13 PM, Lux, James P >> <[email protected]> wrote: >> > >> > >> >> -----Original Message----- >> >> From: [email protected] >> >> [mailto:[email protected]] 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? > > Environmental differences, as in the environment of the box? Or the > propagation path? > For the box, the short term is going to be dominated by the OCXO, isn't it? > So it's relatively environment immune. > > Allan Dev is the box against itself,in the adjacent time slice, but wouldn't > that be pretty similar to a box against an identical box, because the noise > is assumed to be uncorrelated from time slice to time slice. > > > >> >> >> > 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. > > OH.. This is like doing a distributed phased array for EME (or, as N5BF wants > to do, EVE)
Yes, a very similar application. > > On the basis of your other mail a few minutes later, you're looking at very > short integration time (e.g. 10ms), so wouldn't phase noise be your more > appropriate thing to look at, and for that, you're almost certainly looking > at the basic properties of the quartz oscillator. Well, basically, I need to be able to coherently add signals from multiple locations without first looking at those signals to determine what the phase error is. > > OTOH, if you're trying to a phased uplink sort of thing, then you are > concerned about longer time intervals (e.g. you want the relative phases of > Tx1 and Tx2 to be constant over intervals of seconds) Yes, we would need the relative phase to be constant over time scales up through about a second or two. Matt _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
