[time-nuts] Re: Is this amount of measurement errors to be excepted when measuring with a small frequency difference?

[Bob kb8tq]

Hi

It is not at all uncommon to find that this or that measuring instrument
has various issues. On some of the more popular gear, they are pretty
well known. There might even be tech notes diving into just why this or
that issue pops up. With the more obscure gear, you may be the first
to spot the problem ….

There are a number of instruments that have a “dead zone” when the
input is very close to the reference. Just how close that is gets into the
details on that specific device. In some cases 0.1 Hz at 10 MHz is just
starting to get “close enough”. Is that the case here or is it something
different? Time to dig deeper :) :) :)

Bob

> On Jul 20, 2022, at 7:34 AM, Erik Kaashoek via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> During testing of some oscillators that where not exactly on the right frequency it appears that there is a measurement error in the frequency measurements every halve the inverse of the frequency difference between reference and measured signal when the gate time of the frequency meter is set to 20ms.
> An example plot can be found here: http://athome.kaashoek.com/time-nuts/Frequency_pulling.png
> It shows the measured frequency with 0.5Hz (blue) and 1Hz (pink) frequency difference.
> The normal frequency variations measured with a gate time of 20 ms are below 2e-9 but when (presumably) an edge of the reference and the input signal coincide(?) the frequency deviation can go up to almost 1e-8.
> As you can understand this effect is not visible with a larger gate time, such as above 50ms
> Is this normal behavior for a frequency meter?
> Does this imply that when you measure a signal that is phase locked to a reference with a 20ms gate time you may have substantial larger noise in the measurement because you have locked the system in the worst case phase relation?
> The frequency meter used is a Picotest U6200A with an external reference. The results are the same when using the internal reference but the internal reference has much more phase noise obscuring the effect a bit.
> Erik.
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