[time-nuts] Question about frequency counter testing

[Oleg Skydan]

Hi

From: "Bob kb8tq" <kb8tq at n1k.org>
Sent: Friday, April 27, 2018 4:38 PM
> Consider a case where the clocks and signals are all clean and stable:
>
> Both are within 2.5 ppb of an integer relationship. ( let’s say one is 10
> MHz and the other is 400 MHz ). The amount of information in your
> data stream collapses. Over a 1 second period, you get a bit better than
> 9 digits per second.  Put another way, the data set is the same regardless
> of where you are in the 2.5 ppb “space”.

Thanks a lot for pointing me to this problem! It looks like that was the 
reason I lost a digit. The frequency in my experiment appear to be close to 
the exact subharmonic of the PLL multiplied reference. It was not less than 
2.5ppb off frequency (the difference was approx 0.3ppm), but it still was 
close enough to degrade the resolution.

Fortunately it can be fixed in firmware using various methods and I have 
made the necessary changes. Here are Allan deviation and frequency drift 
plots. The first one with the old firmware, the second one with the updated 
firmware that count for the lost of information you mention.

The frequency difference plot also shows the measurement "noise" now is much 
lower. It looks like I have got 11 significant digits now and my old OCXOs 
are better than manufacturer claims by almost 10 times.

Thanks!
Oleg UR3IQO
 
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