[time-nuts] 10MHz Rubidium reference source for frequency counter

Angus not.again at btinternet.com
Sat Nov 1 17:51:54 EDT 2014

On Fri, 31 Oct 2014 23:40:04 -0400, you wrote:

>John wrote:
>>It seems to me that it's just a case of expecting too much from a counter.
>Possibly, but a well-tuned 5370B can get to the low e-12's at 1-10 
>seconds.  It has better one-shot resolution than the Pendulum, but 
>not by a factor of 10.  So, I'd have thought the Pendulum would at 
>least get to the mid e-11's if the time base was up to it.

The 'Smart Freq' feature on the CNT-91 appears to do multiple
measurements over the gate time to get an improved resolution, kind of
like the 53131/2. 
In practice, when 11 digits or whatever are claimed, I usually find
that there is just a little bit of information in the last digit, not
that it's a solid reading to that number of digits.

In the Measurement Uncertainties section of the manual, would the
formula for frequency suggest that Smart Freq makes the result up from
multiple measurements of 0.4 times the nominal gate time?

>>To get the best performance from a counter, you need to set up the 
>>trigger thresholds well away from ground as Charles says.  Also 
>>consider taking TI readings rather than frequency 
>>readings.  Frequency readings are OK for initial setup and basic 
>>ADEV comparisons but they won't generally yield the best ADEV 
>>fidelity or the lowest possible measurement floor.
>Yes, using TI mode is essential for getting down to the counter's 
>limits.  Was Karen using frequency mode?  I wasn't paying attention 
>to that end of things.

I don't know exactly how the CNT-91 does it, but the older CNT-81 can
do a lot of measurements in Time Interval A to B mode. With a static
delay, my 6681 gives a SD of about 0.22ps for a set of 100 1-second
measurements (a 10MHz square wave was driving the inputs). One little
thing I noticed there was that the reference freq needed to be at
least 3ppm off the input frequency to get quite that low. 
The newer series seem to need a bit of an offset too:


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