[time-nuts] Question about frequency counter testing

[Oleg Skydan]

Dear Ladies and Gentlemen,

Let me tell a little story so you will be able to better understand what my 
question and what I am doing.

I needed to check frequency in several GHz range from time to time. I do not 
need high absolute precision (anyway this is a reference oscillator problem, 
not a counter), but I need fast high resolution instrument (at least 10 
digits in one second). I have only a very old slow unit so, I constructed a 
frequency counter (yes, yet another frequency counter project :-). I is a 
bit unusual - I decided not to use interpolators and maximally simplify 
hardware and provide the necessary resolution by very fast timestamping and 
heavy math processing. In the current configuration I should get 11+ digits 
in one second, for input frequencies more then 5MHz.

But this is theoretical number and it does not count for some factors. Now I 
have an ugly build prototype with insanely simple hardware running the 
counter core. And I need to check how well it performs.

I have already done some checks and even found and fixed some FW bugs :). 
Now it works pretty well and I enjoyed looking how one OCXO drifts against 
the other one in the mHz range. I would like to check how many significant 
digits I am getting in reality.

The test setup now comprises of two 5MHz OCXO (those are very old units and 
far from the perfect oscillators - the 1sec and 10sec stability is claimed 
to be 1e-10, but they are the best I have now). I measure the frequency of 
the first OCXO using the second one as counter reference. The frequency 
counter processes data in real time and sends the continuous one second 
frequency stamps to the PC. Here are experiment results - plots from the 
Timelab. The frequency difference (the oscillators are being on for more 
than 36hours now, but still drift against each other) and ADEV plots. There 
are three measurements and six traces - two for each measurement. One for 
the simple reciprocal frequency counting (with R letter in the title) and 
one with the math processing (LR in the title). As far as I understand I am 
getting 10+ significant digits of frequency in one second and it is 
questionable if I see counter noise or oscillators one.

I also calculated the usual standard deviation for the measurements results 
(and tried to remove the drift before the calculations), I got STD in the 
3e-4..4e-4Hz (or 6e-11..8e-11) range in many experiments.

Now the questions:
1. Are there any testing methods that will allow to determine if I see 
oscillators noise or counter does not perform in accordance with the theory 
(11+ digits)? I know this can be done with better OCXO, but currently I 
cannot get better ones.
2. Is my interpretation of the ADEV value at tau=1sec (that I have 10+ 
significant digits) right?

As far as I understand the situation I need better OCXO's to check if HW/SW 
really can do 11+ significant digits frequency measurement in one second.

Your comments are greatly appreciated!

P.S. If I feed the counter reference to its input I got 13 absolutely stable 
and correct digits and can get more, but this test method is not very useful 
for the used counter architecture.

Thanks!
Oleg
73 de UR3IQO 
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