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

[Dr. David Kirkby]

I'm trying to do something which would seem conceptually easy, but I'm
getting results I can't understand. I wish to measure the delay (in
seconds) of a bit of length of coaxial cable.

I'm feeding a sine wave from a Stanford Research DS345 30 MHz function
generator via a coax to the START input of the counter, then with a BNC
T-piece, of 480 mm of 50 ohm cable to the STOP input of the counter. Here's
a photo of the complete setup.

https://www.kirkbymicrowave.co.uk/Experiments/Delay-of-coax/Path-is-signal-generator-to-start-then-stop.jpg

I've set the 5370B's START impedance to be 1 M ohm, and the STOP to be 50
ohms, so the function generator should see a 50 ohm load, as 1 M ohm in
parallel with 50 ohms is virtually 50 ohms.

The switch position on the counter are as shown here

https://www.kirkbymicrowave.co.uk/Experiments/Delay-of-coax/switch-postitions.jpg

So the main settings are

* TI mode.
* +/- TI
* START. 1 M ohm, positive slope, level to preset position (0 V)
* STOP 50 ohm, positive slope, level to preset position (0 V)

With the cable 480 mm in length, the velocity factor of the cable being
approximately 0.7, I would have expected an electrical length of around 686
mm, and so a delay of

time =  distance / velocity = 0.686 / 3e8
= 2.29 ns.

I would not be surprised by small changes in delay with frequency, which is
what I wanted to investigate. But I'm getting the following readings, for
different frequencies of the function generator

1 kHz - unstable readings, around 100~300 us.
10 kHz  -> -21.3 us
50 kHz -> -4.27 us
100 kHz -> -1.90 us
250 kHz -> - 528 ns
500 kHz -> 1.837 us
1 MHz -> 956 ns
2 MHz -> 490 ns
3 MHz -> -2.6 ns
4 MHz -> -0.33 ns
5 MHz -> 0.90 ns
6 MHz -> 1.50 ns
7 MHz -> 1.93 ns
8 MHz -> 2.15 ns
9 MHz -> 2.38 ns
10 MHz -> 2.52 ns
11 MHz -> 2.60 ns
20 MHz -> 2.85 ns
30 MHz -> 2.80 ns

The numbers look believable  with a frequency input of 10 MHz or more. I
did not do the complete set again, but using a cable of 1.53 m in length,
where I would expect the delay to be around 7.29 ns, the results were

1 MHz  -> -26.51 ns
5 MHz -> 9.70 ns
10 MHz -> 9.70 ns
15 MHz -> -57.81 ns
20 MHz -> -41.64 ns
30 MHz -> 7.13 ns

Note, the function generator and counter do not share a common frequency
standard for this test. I have not tried it with them locked to the same 10
MHz reference, but I somewhat doubt that is the cause of these issues.

I must be missing something, but I'm not sure what it is.

-- 
Dr David Kirkby Ph.D C.Eng MIET
Kirkby Microwave Ltd
Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, CHELMSFORD,
Essex, CM3 6DT, United Kingdom.
Registered in England and Wales as company number 08914892
https://www.kirkbymicrowave.co.uk/
Tel 01621-680100 / +44 1621-680100