Hi Lars. I own a G3RUH (Miller) GPSDO. >From what I can recall.... While testing mine with my collection of >HP5370B's and my assortment of references the performance of mine while locked >was generally comparable to the one in the link you provided. I never tested >it "un locked."
I couldn't measure phase noise and the performance of my test setup at taus of less than 100 seconds or so was probably a limitation as well. My references (BVA OCXO, PRS10 rb and a cherry picked Z3805 GPSDO) probably weren't really good enough either. It is a nice unit that I have currently loaned out to a local amateur radio operator. Acquiring it lead to me spending a sizeable amount of time and money on time nuts pursuits over the years. Sent from my iPhone > On Sep 16, 2016, at 1:13 PM, Lars Walenius <[email protected]> wrote: > > Hi Nick, > > Jim Millers design is very clever and as I see can give results as good as a > digital approach but it has the same limitations: > The GPS jitter and the oscillator jitter in combination with the loop > bandwidth. > > The only ADEV I have seen for the Miller GPSDO is this one > http://www.leapsecond.com/pages/gpsdo/ . > Can anyone point me to other tests? > > The ADEV I see on leapsecond.com indicates for me a time constant of around > 200seconds. That is what you get with a OCXO range of 0.25ppm. You don´t need > to have a large RC-filter. The original R1-C1 time constant with 16 seconds > time constant will work as long as the internal oscillator in the Jupiter-T > is at least some Hz away from a multiple of 10kHz. If that is true the > sawtooth out of the Jupiter will have enough high frequency to be easily > filtered by the RC. This is of course a risk. See for example: > https://www.febo.com/pipermail/time-nuts/2005-August/019106.html . If you get > a hanging bridge you have at least as much trouble as a non sawtooth > corrected GPSDO and probably much worse as the RC filter is only 16s. In the > digital approach the prefilter may filter away the hanging bridge (in best > case). > > As I understand the Miller GPSDO with a OCXO with a 0.25ppm range and 10kHz > into an XOR phase detector and 16s RC will be very similar to a digital > approach with a TIC (Time interval counter ) with +-25us range followed by a > prefilter with time constant 16s and a loop with just the P-term with a time > constant of 200secs. As it has no I-term it will have slightly less noise but > the output phase will drift as soon as the OCXO drifts (as a FLL). The > resolution of the XOR phase detector (TIC) will be limited by the noise but > as 0.1mV is 1ns the resolution can be seen as better than 1ns is my > conclusion. A problem might be that the output of the XOR drifts with the 5V > supply but this can be seen as the same problem as the DAC drift in a digital > approach. > > My conclusion (without testing) is that the Jupiter-T 10kHz is very good but > not better than the sawtooth corrected outputs from M12 or LEA6T receivers. > That is the ADEV can be approximated by 1E-9/Tau (1E-12 at 1000s) in good > conditions. > > My experience with the Venus838-T is only 2 weeks but disappointing. This can > also be guessed from the datasheet ADEV curve, that I guess is sawtooth > corrected values as it starts at 3E-9 at 1s, but is only 1E-11 at 1000s a > factor 10 worse than I get with the LEA-6T with the same antenna and setup. > If anyone have ADEV-MDEV curves to share I would be glad to see what can be > achieved with the venus838-T. My conclusion is also that sawtooth correction > is useless on my 838-T. > > Lars > > >> Nick wrote: > >> Jim Miller's 10 kHz GPSDO that’s been referenced here has either solved this >> problem, or the 10 kHz output of the >Jupiter is substantially better than >> the Venus’ 10 MHz output, or the design doesn’t give the results time-nuts >> expect >from a GPSDO. Which of those applies? > > > _______________________________________________ > 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. > _______________________________________________ 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.
