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" <[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.
