[time-nuts] Re: Assessing performance of a frequency counter

[John Ackermann N8UR]

Hi Erik --

Unfortunately, your attachment did not make it through the list.  That 
sometimes happens when you put images "in line" or as HTML content. 
Best thing is to send them as a "real" attachment.

Your question breaks into two pieces: (1) what's the resolution or noise 
floor of the counter, and (2) how good is its internal oscillator?  I'm 
not sure which piece you are most interested in.

1.  To characterize the counter itself, the easiest test is to feed the 
same signal into both the counter timebase and its input (e.g., if 
there's a "10 MHz Out" on the rear panel, connect that to the input). 
That will show you what the counter is capable of -- what's its 
resolution, how stable is the gating, etc.

2.  To characterize the internal oscillator, there are two things to 
consider:

(a) is your reference oscillator stability/accuracy better than the 
counter's over the ADEV "tau" range you're considering?  Ideally you'd 
like to *know* that the reference is at least 10x more stable than the 
device under test. Otherwise you may find yourself measuring the 
reference rather then the DUT.

(b) what's the resolution of your measurement system (see point 1)?  For 
short measurement periods, the counter is likely to be the limiting 
factor because it has a fixed resolution and internal jitter that impact 
short measurements more than long ones.  If you think about it in the 
time domain, a "perfect" time interval counter with 2ns resolution will 
have a measurement floor of 2e-9 at 1 second (2ns / 1 second). At 10 
seconds, it's 2e-10 (2ns / 10 seconds), and this slope continues until 
you hit some ultimate noise floor.   In the frequency domain, the number 
of digits per second implies the same thing.  Most test-bench counters 
probably have a resolution of around 1e-8 to 1e-9 in 1 second.  Some of 
the best are in the 11s, but to get better than that at 1 second 
averaging you are getting into esoteric gear.

Based on all that, ADEV measurements of two good oscillators are 
probably going to be counter noise limited at tau (measurement interval) 
of less than 100 seconds.  Your 0.1 and 0.05 seconds measurements are 
almost certainly limited by the resolution of the counter.

I've ignored some subtleties and gotcha's in the above, but hopefully it 
gives you an idea of how to take and interpret measurements.

John
----


On 11/8/21 10:53 AM, Erik Kaashoek wrote:
> Hi all time-nuts.
> Hopefully one of you can provide some advice on how to proceed.
> The ambition is to assess the short term (less than 1 hour) performance 
> of a frequency counter.
> As I understood this can be done by plotting the Allan Deviation of a 
> repeated measurement of a very stable source.
> Below plot shows the Allan deviation of a 1 hour measurement of the 
> 10MHz output of an OCXO versus the internal reference of the counter 
> with a 1 and 0.1 second gate time.
> To be able to better see the gate time impact, short measurements with a 
> gate time of 0.05s and 5s have been added.
> Temperature variations during the measurement have not been recorded but 
> there where some more variations during the 1s gate time measurement.
> 
> 
> Are there any conclusions to draw from this plot?
> Or should I do a different measurement, or use different representation?
> A pointer to a web page or document describing how to do this type of 
> assessment would be most welcome.
> Erik.
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