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

Dr. David Kirkby drkirkby at kirkbymicrowave.co.uk
Mon Oct 29 11:23:49 EDT 2018


On Mon, 29 Oct 2018 at 09:43, Tom Van Baak <tvb at leapsecond.com> wrote:

> David,
>
> Just to see if your setup is working:
>
> 1) Set the pulse generator to as fast a risetime as possible; ns or less.
> Use a low pulse rate (100 Hz is fine).
>

Unfortunately, I don't have such a pulse generator, so I can't run that
test. But it is clear the system is sensitive to the trigger levels, so I
guess is the problem. I have done all the confidence checks in the manual
on this TI counter before and it was fine.

Also, I am interested in the delay of the cable at low frequencies, as I
suspect that might depart significantly from the usual figure based on the
"velocity factor". Certainly the impedance of coax  rise at low frequencies
because the normal formula

Zo = sqrt(L/C)
is not valid before a few MHz. A more accurate formula is

Zo = sqrt ( (R + j w L )/ ) / (G + j w C))

where R = Resistance per unit length
L = inductance per unit length
G = Conductance per unit length
C = Capacitance per unit length.

So feeding in short pulses brings the validity of such a test into
question.


> 2) Use a BNC tee at the generator, into two equal 2 meter cables, each one
> into a 5370B input.
> 3) Set manual trigger, 50R, 1.0 V, DC
> 4) Now collect time interval data in block/stats mode. You should see a
> mean of under +/-1 ns and a stdev in low ps.
>
> /tvb
>

Thank you. I will look for a pulse generator. I would use the 1 pps from
the GPS receiver, but my HP 58503A GPS receiver has decided to pack up. I
need to have a look at that, but it is not the highest priority task just
now.

This possible trigger issue metioned by Hal Murray is probably a result of
the knobs not exactly lining up with the positions they are in. It is
fairly clear that the marker on one of them is vertical at 0  V, but the
other is not. I need to try to get the knob back on the shaft in a slightly
different position. But they were both set to preset, but it is clear that
the reading is sensitive to the trigger points.

I have a 100 MHz scope, and can borrow a 300 MHz scope, but I don't have
anything really fast.

I have a VNA which can make measurements of phase difference down to 300
kHz, but don't trust those because of the fact one calibrates with a 50 ohm
load, 50 ohm calibration standards, yet I know the impedance will rise well
above 50 ohms at low frequencies.

I was looking for a different approach than a VNA, to make comparisons with
a VNA.

Dave


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