[time-nuts] Agilent appnote

Dr. David Kirkby drkirkby at medphys.ucl.ac.uk
Mon Jul 25 06:48:00 EDT 2005

Joseph Gray wrote:
> Does anyone have a copy of Agilent appnote 174-10 "Measuring the 
> electrical length (delay) of cables"? A search of the Agilent site 
> doesn't find it.
> Thanks.

Sorry I can't help directly but do have some suggestions

1) I have set up a web page where people can request application notes, manuals, help etc they want on 
test equipment, hoping others will provide a copy (legally of course). In the case of an Agilent 
application note like that, I would stick it on my web site, as I do have permission from Agilent to 
do so.

I only 'finished' this web page at 2am local time this morning, so there are going to be buge. I would 
appreciate any comments. It ia also my first attempt at a semi large perl program. I've always seemed 
to avoid learning perl, but realised it was almost essential in the end. #!/bin/sh does have its limits.

I would appreciate some feedback -  if people think this is useful, what catagoies of test equipment 
should be added, any spelling mistakes etc etc. See it at


Feel free to put in any silly entries (to test it), or a serious entry if you want to use it properly. 
Once I get some feedback, I'll remove the silly entries and leave the serious requests. Then I will 
announce the page a bit more widely, with the hope others will add their resources.

Here are a couple of suggestions of how you might measure cable lenght.

1) Buy a Vector Network Analyser with S-parameter test set - an expensive option.

2) If you have a time interval counter, measuring absolute phase delay at a frequency within the TI 
counter's range, would not be hard. Just measure time, and compute length knowing frequency.

Unless you do it with sexless connectors (APC7 or similar), or your cable has opposite sex connectors 
on its ends, you are going to need an adapter that you remove to replace with the cable.

So you are really measuring cable_length-adapter_length. The electrical length of the adapter could be 
estimated with a high degree of accuracy with a ruler amd knowing the diectric constant (its usually 
PTFE). I suspect (but have not thought about it too much) you could measure the length of the adapter 
by making multiple measurements at different

Comparing the length of two cables is even easier, as you don't need to know the length of an adapter.

A vector network analyser would be the best way of course, but you are more likely to have a TI counter.

3) Another option would be a directional coupler and TI counter. Feed the ouput of a sig gen into the 
start input of a TI counter, and put it in the input port of a directional coupler. Put the reflected 
power from the coupler into the stop input. There will be a high reflection, as the output of the 
coupler is unterminated. Now add your cable, and again there will be a large reflection, but this time 
it would be delayed by twice the cable length.

This method, is basically what is used in the calibration procedure of vector network analysers. 
Application notes on their theory should be on the Agilent web site.

Dr. David Kirkby PhD CEng MIEE,
Senior Research Fellow,
Department of Medical Physics,
Mallet Place Engineering Building,
Gower St,
University College London,
London WC1E 6BT.

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