[time-nuts] Re: Identifying GPSDO phase disturbers

[Bob kb8tq]

Hi

It’s a good bet that the TCXO has an AT cut crystal in it. That would 
give it a 1 to 2 ppb / G sensitivity for typical blanks mounted in a 
normal fashion. 

Quantifying the shock delivered by a “tap” on a part can be exciting. 
It can be done, but the gear to do it properly does cost more than most
of us would like to spend. You can get some pretty large G levels for
very short durations ( 100 G’s for 1 ms maybe ). 

Combine the two and you get a very measurable change in the part
from some pretty simple messing around. 

Bob

> On Jun 6, 2022, at 9:05 AM, Erik Kaashoek via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> During measurement  of a  GPSDO there was some concern about very short term phase stability. E.g. for tau between 0.001 and 1 second. It proved to be possible to measure the stability for tau larger than 0.1 s using a frequency counter but neither the counter (limited accuracy for very short tau) nor Timelab (shortest tau was 0.02 s) where able to reach a tau of 0.01 s.
> Looking at the old HP/Agilent application notes a phase detector approach was selected.
> The output of the GPSDO was send to the RF port of a mixer. The LO port was connected to the output of a VC-TCXO and the IF port output was low pass filtered (to remove the 10MHz and higher) and added to the Vtune to the VC-TCXO. Course tuning of the VC-TCXO was done using a 10 turn potmeter supplied from a very stable linear supply.
> It proved to be possible to set the V-tune with the potmeter such that the GPSDO and VC-TCXO frequencies where in phase and the loop locked.
> Using a dual input frequency counter the ratio of the GPSDO and VC_TXCO was measured to confirm they where in lock.
> An oscilloscope with FFT was also connected to the LPF output to monitor short term phase disturbances. No high frequency (above 10Hz)  components where observed in the FFT suggesting the initial concern was not justified
> Three main phase disturbances where observed.
> 1: The GPSDO was in phase lock with the GPS PPS and every time the tuning DAC was updated a change in frequency resulting in a change in angle on the scope of the mixer output was observed. These changes where also visible on the frequency counter
> 2: Any temperature changed caused strong phase fluctuations. Even tough the GPSDO uses a TCXO there is still a large temperature sensitivity. Thermal isolation (adding some towels) helped to remove fast temperature fluctuations.
> 3: Mechanical shock caused clearly visible phase variations. The VC-TCXO acted as a sensitive microphone and, to a lesser extend, also the TCXO of the GPSDO. Tapping on the workbench with one finger was visible. The net effect of the mechanical shock was about zero phase change which made it difficult to see on the frequency counter with 0.1 s gate time but the higher BW of the phase detector allowed to observe this. It is yet unclear how to isolate the TCXO in the GPSDO from mechanical shock
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