Matt Matt Ettus wrote: > On Mon, Jan 5, 2009 at 3:02 PM, Bruce Griffiths > <bruce.griffi...@xtra.co.nz> wrote: > >> Matt >> >> Matt Ettus wrote: >> >>> On Mon, Jan 5, 2009 at 1:54 PM, Bruce Griffiths >>> <bruce.griffi...@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 _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.