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

David C. Partridge david.partridge at perdrix.co.uk
Mon Oct 29 06:38:52 EDT 2018


If you have a Tektronix 7000 series 'scope, then a 7S12 equipped with a S52
pulse generator and an S6 sampler head will talk all the pain out of
measuring cable length and will also show you any impedance mismatches.
This way you don't need to suspect a bad cable, you can prove it's bad.

If you need a cable checked, I can do it as I have 7S12 plugins.
Dave

-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces at lists.febo.com] On Behalf Of Dr.
David Kirkby
Sent: 29 October 2018 00:50
To: time-nuts at lists.febo.com
Subject: [time-nuts] How can I measure time-delay of a cable with HP 5370B
time-interval counter?

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-g
enerator-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-postition
s.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
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