Have you tried with just a single pulse?
> On Oct 28, 2018, at 20:49, Dr. David Kirkby <[email protected]> > 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 > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
