[time-nuts] Why would Keysight UK uncertainty measuring 1 MHz be as high as 7.6 Hz?

Dr. David Kirkby (Kirkby Microwave Ltd) drkirkby at kirkbymicrowave.co.uk
Fri Aug 28 16:48:57 EDT 2015


My LCR meter came back from Keysight  UK last week, where it was
calibrated. This instrument works at various frequencies from 20 Hz to 1
MHz, so obviously has some sort of oscillator in it. But I don't think the
absolute accuracy on frequency is important on this, as it does not even
have the ability to set to an arbitrary frequency. There are only 8000 or
so steps, and at the high end, some of those steps are more than 100 kHz
apart!!!  So clearly frequency accuracy on this instrument is not that
important.

Anyway, the cal certificate, a copy of which I put here

http://www.kirkbymicrowave.co.uk/cal_certificates/Keysight-standard-calibration-with-uncertainties-for-4284A-precison-LCR-meter-18-08-2015.pdf

shows on page 5 that it was checked at 1, 8, 20, 80, 400 kHz, and 1 MHz.
But the uncertainty reported (7.6 Hz) seems extremely high, given they used
a 53132A counter as a working standard, and a 5071A primary frequency
standard. Why should the uncertainty be so high? Am I missing something?

When they done my VNA last year

http://www.kirkbymicrowave.co.uk/cal_certificates/Keysight-standard-calibration-with-uncertainties-for-8720D-vector-network-analyzer-16-09-2014.pdf

the uncertainty on frequency was about 5 orders of magnitude better than
that. The 10 MHz timebase was measured with an uncertainty of 0.0010 Hz.

I checked the Keysight UK accreditation (by UKAS) for frequency

http://www.keysight.com/upload/cmc_upload/All/UKAS_S_2015-08-14_Eng.pdf

and see over the range 0.1 Hz to 500 MHz, which covers the LCR meter, their
accreditation is 6.0 in 10^11 + 0.020 nHz.

I can't believe they are unable to measure better than 7.6 ppm on
frequency, so are wondering why the uncertainty is so high, even though I
am sure such an uncertainly is very acceptable for this application.

It is either an error on the cal certificate, or I am missing something. I
expect it is the latter, and hoping someone here can fill me in.

Dave


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