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

Artek Manuals Manuals at ArtekManuals.com
Mon Oct 29 07:57:00 EDT 2018


David

First let me say that I have never used a 5370B

Ignoring the lower frequency stuff for the moment, Can you measure ( 
accurately) the trigger levels of both the start and stop gates? Slight 
differences in the trigger points at each end will will obviously add 
error in the measurement.

The next flag for thought is your comment "assuming a velocity factor of 
.7" What if the velocity factor is really .66 ? This would account for 
almost half of the error.

Rerun you data with larger and smaller output levels from the function 
generator. Increasing error" with lower signal levels (or vice versa 
lower erros with increased voltage ) would implicate a trigger 
differences as the source of error as well

Dave
manuals at artekmanuals.com

David on another note I tried replying to you directly and the email 
bounced ?

On 10/29/2018 6:38 AM, David C. Partridge wrote:

<SNIP>
> If
>
> 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.
>

-- 
Dave
Manuals at ArtekManuals.com
www.ArtekManuals.com


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