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.
_______________________________________________
time-nuts mailing list -- [email protected] -- To unsubscribe
send an email to [email protected]
To unsubscribe, go to and follow the instructions there.
_______________________________________________
time-nuts mailing list -- [email protected] -- To unsubscribe send an
email to [email protected]
To unsubscribe, go to and follow the instructions there.
