[time-nuts] Identifying GPSDO phase disturbers

[Erik Kaashoek]

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)Â  
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