[time-nuts] Agilent appnote

Magnus Danielson cfmd at bredband.net
Mon Jul 25 06:43:05 EDT 2005

From: "Joseph Gray" <jgray at zianet.com>
Subject: [time-nuts] Agilent appnote
Date: Mon, 25 Jul 2005 01:46:35 -0600
Message-ID: <037401c590ec$fba12210$66fea8c0 at antecp4>


> 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.

Sadly, I don't have that App-note, however...

May I propose that you take a sufficiently low frequency (100 kHz may suffice)
squarewave, split it so that you trigger START (directly, put the splitter on
the start BNC) and then the other output of the splitter goes to the cable
which connects to the STOP channel. Measure and preferably integrate over some
100 measurements at least.

That is at least what I would assume that 174-10 says:

The 100 kHz squarewave has a cycle of 10 us, so it should suffice for a cable
length of about 10000 [ns] * 2/3 * 0.3 [m/ns] = 2000 [m] = 2 km. A 1 MHz would
handle cables up to 200 m, a 5 MHz up to 40 m and then naturally a 10 MHz up to
20 m, so it all depends on your cable length. If one run with a too high
frequency, one has the electrical length modulo cycletime, but if you handle
that ambiguity it will work just as well.

There is a number of error-sources in the method (different delays down the
legs of the power-splitter, different trigger positions, different waveshapes
to trigger due to slope differences etc.) so if you *really* care you need to
take compensation actions (measuring rise-times and compensate difference for

The resolution for this measurement is limited by the counters time resolution.
However, if you are using the same counter for measuring the cable as you do
for the actual measurements, the time-resolution is relatively on par with
each other (depending on the detailed measurement setup) so it should be fairly

If you have access to a high speed TDR, then measure first the time to the
adapter (just leave it open), then hook on the cable and leave end open and
measure the time to the now delayed reflection. Half the time difference is the
electrical length of the cable. Unfortunatly high speed TDRs isn't lying around
as they should do, it is a good tool.


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