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

Bob Camp kb8tq at n1k.org
Sat Aug 29 09:14:00 EDT 2015


> On Aug 29, 2015, at 7:09 AM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
> Hi,
> On 08/29/2015 10:01 AM, Dr. David Kirkby (Kirkby Microwave Ltd) wrote:
>> On 28 Aug 2015 23:05, "Oz-in-DFW" <lists at ozindfw.net> wrote:
>>> The uncertainly listed seems to be 7.6 mHz (milliHertz, or .0076 Hz.  A
>>> bit better that you mention..
>> No, please look again.
>> The first line does show an uncertainty of 7.6 mHz, but that is when the
>> LCR meter was set to 1 kHz. The last line shows an uncertainty of 7.6 Hz
>> (1000x higher) when measuring 1 MHz, which is obviously 1000x higher in
>> frequency than 1 kHz. So the original uncertainty I quoted was correct.
>> The uncertainty rises proportionately with frequency.
> I agree. That's how I read it too, and it is very obvious as you look at the table.
> The calibration is in line with Chapter 10 of the manual:
> http://wiki.epfl.ch/carplat/documents/hp4284a_lcr_manual.pdf
> It might be that the 7.6 ppm number was a practical uncertainty measure found in deeper analysis along the lines of GUM and was added to the calibration chart afterwards. For the peak-error of 100 ppm, this provide a good margin anyway.

If it’s a +/- 100 ppm error and that’s all due to temperature, you get a linear “best guess” of 2 ppm / C. 

That would equate to needing to know the temperature to about 3.5 C. That’s doing pretty well in an open
room. If you are guessing a temperature inside a piece of gear, that’s doing pretty well running inside a chamber. 


> So, it's the instrument and the way it behaves, nothing in their test-setup which is way better than needed for this particular instrument.
> In short, don't worry about it, it's OK.
> Cheers,
> Magnus
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