[time-nuts] How can I measure time-delay of a cable with HP 5370B time-interval counter?

Chris Caudle chris at chriscaudle.org
Mon Oct 29 11:01:32 EDT 2018


On Mon, October 29, 2018 6:57 am, Artek Manuals wrote:
> The next flag for thought is your comment "assuming a velocity factor of
> .7" What if the velocity factor is really .66 ? This would account for
> almost half of the error.

Propagation velocity has an inverse dependence on permittivity, and
permittivity changes with frequency.  Electrical delay time will not be
constant with frequency because of that.

In addition to the fundamental physics at play, there are instrumentation
difficulties.  The rise time at low frequencies is long enough that any
50Hz/60Hz interference from power line related current flow can modify the
trigger point and influence the measurement.  The fast rise time signals
proposed for evaluating the measurement setup get around that, but then of
course you are measuring a wideband signal, which rather misses the point
of the original goal of measuring vs. frequency, so at some point after
verifying the setup basics you will have to go back to narrow band
signals.

A 5370 is a somewhat coarse instrument for this type of measurement, a VNA
which has a suitable lower measurement frequency would probably be more
suitable.

-- 
Chris Caudle







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