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.
