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

[Dr. David Kirkby (Kirkby Microwave Ltd)]

On 28 August 2015 at 21:48, Dr. David Kirkby (Kirkby Microwave Ltd) <
drkirkby at kirkbymicrowave.co.uk> 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?
>

I don't know if this thread is closed, but I did get a telephone call from
someone at Keysight today, in response to my query on this matter.
Essentially

1) At the particular calibration centre in the UK, the frequency  counter
used would have been locked to an external reference, as are all the
counters they use at that particular calibration centre in the UK. The
reference is an old HP  5071 cesium, which is regularly compared with GPS.
Their best uncertainty is 5 x 10^-13.

2) The LCR meter tolerance on the 1 MHz is +/- 100 Hz. This means it does
not need a particularly good counter.

3) To enable Keysight to use one calibration procedure world-wide, that
calibration procedure is written to allow the uncertainty to be as high as
the worst case with a counter running on its own internal oven.

So in essence, the meter when calibrated in the UK would have been measured
to far better than the +/- 7.6 Hz indicated on the calibration certificate.
but by specifying +/- 7.6 Hz, it allows the same procedure to be used
anywhere in the world.

Dave