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

[Javier Herrero]

Hello,

The calibration certificate does not indicate that the measurements were 
done with the frequency counters referenced to the 5071A at the time of 
calbiration (if so, it would be listed under the Calibration Equipment 
Used table). It says that the 53132A were calibrated against the 5071A.

If for your calibration they have used 53132A witout the oven oscillator 
option it is very probable that its uncertainity is 7.6ppm as indicated 
in the certificate. Since the maximum error tolerable for the LCR meter 
is 100ppm (+/-100Hz @ 1MHz), it makes sense to perform the measurement 
with an instrument with an uncertainity of 7.6ppm, and not to use the 
better counter in the lab for that purpose.

Regards,

Javier

On 28/08/2015 22:48, Dr. David Kirkby (Kirkby Microwave Ltd) wrote:
> 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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