[time-nuts] A new take on the all-hardware GPSDO concept
lars.walenius at hotmail.com
Fri Sep 16 16:13:44 EDT 2016
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.
>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?
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