[time-nuts] Agilent appnote
cfmd at bredband.net
Mon Jul 25 07:16:33 EDT 2005
From: "Poul-Henning Kamp" <phk at phk.freebsd.dk>
Subject: Re: [time-nuts] Agilent appnote
Date: Mon, 25 Jul 2005 12:53:27 +0200
Message-ID: <71765.1122288807 at phk.freebsd.dk>
> In message <20050725.124305.26278789.cfmd at bredband.net>, Magnus Danielson writes:
> >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:
> Normally this is done with a network analyzer by measuring the phase difference
> at two different frequencies and calculating which delay would cause that.
That is two methods yes (you need to distinguish between phase-delay and
group-delay). However, considering that he was reading AP200-3 the text giving
the reference is:
+/- Systematic Error
In most measurement situations, the use of equal length cables cancels out
this error [the difference between start and stop channel cable delay];
however, cable delay varies with the dielectric constant of the insulation so
cables of the same kind should be used. Where this is not practical, the
electrical lengths of the cables can be measured by the counter to determine a
correct factor to apply to the counter reading. Application Note 174-10,
Measuring Electrical Length (Delay) of Cables, discusses this measurement in
So, if you read the second sentence carefully, you see that they are saying
that you can measure this delay with your counter and then in the following
sentence they point out that AP174-10 details on how to make such a
measurement. So, in this case I think the network analyser is out of the
picture. This is why I gave the counter based hint, because I do beleive this
is more or less what is happening in AP174-10. Also, when doing this measure
one is measuring more or less under the same conditions as the system is to be
used. If you use a network analyser you might need to spend some more time than
you anticipate to come up with the correct value, since that is really a
frequency-wise measure and the counter is doing time-wise measures, so you
should do an inverse Fourier transform if you want to do it properly, so it
gets a bit messy.
As a side-note, some of the systematic errors in the measurement methods I
gave will cancel (and some will partially cancel) when calculating the delay
difference which is to be used for start-stop correction.
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