[time-nuts] Cable length calibration

Michael Wouters michaeljwouters at gmail.com
Thu Jun 30 18:13:35 EDT 2016


As a practical matter, in the lab we seldom need a cable delay
measured to better than +/- 0.5 ns, which we usually do as a time
interval measurement, with a 1 pps into a tee on channel A of a TIC
and then the cable from the tee to channel B.

For cables up to 40 m or so, just measuring the physical length is  as
accurate (experimentally determined!).

A while back, EURAMET ran a pilot where a few lengths of cable were
circulated amongst a number of NMIs who then had to measure the
delays. The reported scatter was about +/- 1 ns, as I recall. This
mainly came down to differences in test signals, trigger levels etc.

Cheers
Michael


On Fri, Jul 1, 2016 at 7:03 AM, Dr. David Kirkby (Kirkby Microwave
Ltd) <drkirkby at kirkbymicrowave.co.uk> wrote:
> On 30 June 2016 at 09:19, Scott McGrath <scmcgrath at gmail.com> wrote:
>
>> If the nominal velocity of propagation is known and length is known delay
>> is easily determined mathematically
>>
>
> Except that coax does not have a uniform impedance or velocity factor. Both
> will vary as a function of position and frequency. How relevant this is
> depends on the accuracy you require, but since it is time-nuts, it is
> reasonable to assume that such a simplistic method is not of the standard
> expected on time-nuts.
>
> Dave
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