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

[Bob Camp]

Hi

If it is an un-compensated crystal oscillator (I’d bet it is) there will be a maximum temperature slope to the part.
In order to have absolute calibration, you would need to know both the frequency read *and* the temperature 
of the inside of the device. I’d bet somebody did the math, looked at the “rated” temperature control in the cal 
lab area and came up with a proper number for the ambiguity of the calibration. If the slope is 2 ppm / C then the 
number you have implies a 3.5C temperature control in the area. That seems pretty tight. A 1 ppm / C number gets
you to 7.6 C. That seems about right for the room plus the ambiguity in the internal temperature rise inside the box. 

Bob

> On Aug 28, 2015, at 4:48 PM, 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?
> 
> 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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