[time-nuts] Re: Identifying GPSDO phase disturbers

[Magnus Danielson]

Hi Erik,

On 2022-06-06 19:05, Erik Kaashoek via time-nuts 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.
This is where you transition over to phase-noise measurements rather 
than counter measurements.
> 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

This is the tight PLL phase noise measurement technique. Care should be 
taken to calibrate it's response, which can be done with an auxillary RF 
generator that inject offset signals with known amplitude-relation and 
offset to the carrier frequency. PLL bandwidth will filter responce, but 
you are recommended to look at the output of the mixer for the high 
frequency part, as this will be supressed by the lowpass filter.

If you use a PI-loop rather than a low-pass filter, it will always lock 
up. Trimming of oscillator EFC offset only controls how fast it locks. 
Considering that a PI-loop is an op-amp, two resistors and a capacitor, 
it can usually be motivated.

Choose your PLL bandwidth to not obstruct your frequency range.

> 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.

If you just use a low-pass filter and not have an integrator, the 
limited DC gain require there to be a phase offset to compensate for the 
frequency offset change. Using a PI-loop where you have a full 
integrator, the high gain of the integrator will work to nullify the DC 
offset of the phase detector and move the needed offset of frequency 
into the integrator state. This will also eat up any changes that 
occurs. Sure, there will be slight deviations, but long-term they will 
be compensated. The phase response will be a high-pass filter of the 
steered oscillator, and the higher frequency of the PLL, the better 
surpression of any local effects. Also, as mentioned before, the PI-loop 
also locks up on itself. You can aid it with a trimmer only to reduce 
the initial frequency offset which significantly reduces the lock-in 
time. A high bandwidth on the PLL also has a very good effect on lock-in 
time.

For other purposes, you want a more narrow PLL, which put more 
requirements on the locked oscillator. For the measurement it's mainly 
the widebandwidth noise which is a limiting factor.

> 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

Sure, it's the nature of the piezoelectric nature of quartz crystals. It 
converts mechanical stress to voltage and back, so it is expected that 
it would also be sensitive in it's oscillator setup, and the tension 
vector is important for it's acoustical properties. It will be sensitive 
to both gravitational forces as well as shock and vibration.

Providing shock-mount as well as vibration reduced mount may be needed 
for some environments.

For environmental effects, the IEEE Std 1193 is the relevant document, 
and it is going through it's balloting process after revision and I 
expect it to be published in the fall.

Cheers,
Magnus