### Author Topic: Measuring scope channel skew and cable velocity with Jim Williams pulser  (Read 1700 times)

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#### jpb

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##### Measuring scope channel skew and cable velocity with Jim Williams pulser
« on: June 15, 2013, 12:53:02 pm »
Sorry for the long title, but this is two experiments in one.

I've been having a lot of fun with my Jim Williams pulse generator (thanks Free_electron/Vince for the kit). One experiment that I've just done is to use it to measure both the velocity of signals in my coax cable and at the same time get the channel-to-channel skew in my scope at the 1ns/div scale. I hope this will be of interest to others who have got bored of just measuring the rise time.

Setup

Very simple. I have a bnc t-connector and three bnc cables whose end-to-end lengths are 1017mm, 535mm and 268mm (standard cables I bought from Conrad I think).

I connect the pulse via a sma-to-bnc adapter to one of the t-branches and a cable to the other. The t is then connected to either channel 1 of the scope or channel 3 and the other end
of the cable connected to channel 3 or channel 1.

The scope display then shows a pulse and delayed pulse and the delay can be easily measured. To allow for channel skew the delay is measured from 1 to 3 and then from 3 to 1 and the difference in delay is twice the skew (adds one way and subtracts the other).

The slope of the delay vs cable length line gives the velocity at 2.01005 E 8 m/sec or 0.67048 c (where c is the speed of light).

The skew shows a roughly linear relationship with delay varying from 50 psecs for no delay to potentially 130 psecs for 10 nsecs delay - by extrapolation, the actual skews I
measured were 60, 70 and 90 psecs.

I attach the screen shots and plots.
« Last Edit: January 12, 2019, 11:47:07 am by jpb »

Smf