[time-nuts] Question about frequency counter testing
kb8tq at n1k.org
Fri Apr 27 15:46:50 EDT 2018
As you have noticed already, it is amazingly easy to get data plots with more than the
real number and less than the real number of digits. Only careful analysis of the underlying
hardware and firmware will lead to an accurate estimate of resolution.
This is by no means unique to what you are doing. Commercial counters are
very often falling into this trap. If you hook up a SR-620 to it’s internal standard,
you will see a *lot* of very perfect looking digits …. they aren’t real. The HP 5313x
counters have issues with integer related inputs / reference. This isn’t easy.
> On Apr 27, 2018, at 2:47 PM, Oleg Skydan <olegskydan at gmail.com> wrote:
> 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.
> Oleg UR3IQO
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