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

John Ackermann. N8UR jra at febo.com
Sun Oct 28 21:32:21 EDT 2018


I suspect it's triggering or aliasing issues if you're using high frequency sine waves.  The canonical way to do that measurement is with a fast-rise-time edge at PPS sorts of rates.  And you'd normally use a common reference to reduce the number of variables.

John

On Oct 28, 2018, 8:51 PM, at 8:51 PM, "Dr. David Kirkby" <drkirkby at kirkbymicrowave.co.uk> wrote:
>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
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