[time-nuts] Question about frequency counter testing

[Oleg Skydan]

Hi

I have got a pair of not so bad OCXOs (Morion GK85). I did some 
measurements, the results may be interested to others (sorry if not), so I 
decided to post them.

I ran a set of 5minutes long counter runs (two OCXOs were measured against 
each other), each point is 1sec gate frequency measurement with different 
number of timestamps used in LR calculation (from 10 till 5e6). The counter 
provides continuous counting. As you can see I reach the HW limitations at 
5..6e-12 ADEV (1s tau) with only 1e5 timestamps. The results looks 
reasonable, the theory predicts 27ps equivalent resolution with 1e5 
timestamps, also the sqrt(N) law is clearly seen on the plots. I do not know 
what is the limiting factor, if it is OCXOs or some counter HW.

I know there are HW problems, some of them were identified during this 
experiment. They were expectable, cause HW is still just an ugly 
construction made from the boards left in the "radio junk box" from the 
other projects/experiments. I am going to move to the well designed PCB with 
some improvements in HW (and more or less "normal" analog frontend with good 
comparator, ADCMP604 or something similar, for the "low frequency" input). 
But I want to finish my initial tests, it should help with the HW design.

Now I have some questions. As you know I am experimenting with the counter 
that uses LR calculations to improve its resolution. The LR data for each 
measurement is collected during the gate time only, also measurements are 
continuous. Will the ADEV be calculated correctly from such measurements? I 
understand that any averaging for the time window larger then single 
measurement time will spoil the ADEV plot. Also I understand that using LR 
can result in incorrect frequency estimate for the signal with large drift 
(should not be a problem for the discussed measurements, at least for the 
numbers we are talking about).

Does the ADEV plots I got looks reasonable for the used "mid range" OCXOs 
(see the second plot for the long run test)?

BTW, I see I can interface GPS module to my counter without additional HW 
(except the module itself, do not worry it will not be another DIY GPSDO, 
probably :-) ). I will try to do it. The initial idea is not try to lock the 
reference OCXO to GPS, instead I will just measure GPS against REF and will 
make corrections using pure math in SW. I see some advantages with such 
design - no hi resolution DAC, reference for DAC, no loop, no additional 
hardware at all - only the GPS module and software :) (it is in the spirit 
of this project)... Of cause I will not have reference signal that can be 
used outside the counter, I think I can live with it. It worth to do some 
experiments.

Best!
Oleg UR3IQO 
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