John has some good methods. Here is another method that requires an RF source
and an oscilloscope or a spectrum analyzer. Or, if your have a network
analyzer or even an MFJ-259B you can do it. Basically you want to find at what
frequency a certain length of cable is 45 degrees and 90 degrees. Roughly, a 1
meter length will be 90 degrees at 60 MHz. If CAT5 cable is balanced you will
want to drive it though a current balun.
Using a network analyzer, tune for the lowest frequency at which an open-ended
length appears as a short. This is 90 degrees. Now tune to exactly half this
frequency. You will measure a capacitive reactance. This is the line
characteristic impedance Zo.
Now divide the physical length by 90 degrees. This gives the physical length
per degree. Find the delay per degree (td) by td = 1/360f, where f is
frequency in Hz.
Now to find the L and C per degree:
L = (td)Zo
C = (td)/Zo
Since you know the physical length per degree you now know the L and C per
degree and the L and C per unit length.
Example: You find that an open 1 meter length presents a short at 60 MHz. At
30 MHz you measure a reactance of -j100 ohms. The Zo of the line is 100 ohms.
td = 1/(360)(60e6) = 46.3 ps/degree.
L = (46.3 ps/)(100) = 4.63 nH/degree = 4.63 nH/11.1 mm
C = (46.3 ps)/100 = 0.463 pF/degree = 0.463 pF/11.1 mm
Check with Zo = sq rt (L/C) = 100 ohms
The loss can be calculated from the return loss measured at the 90 degree
frequency. In fact, I would use a transmission line model rather than an RLC
model. Say you measure 1 dB return loss at 60 MHz for the 1 meter length. The
one-way loss is 1/2 dB per meter at 60 MHz. Below 1 GHz the loss is mostly
conductor loss which is proportional to the square root of the change in
frequency. I.E. at 120 MHz the loss is 0.71 dB. Your T-line model is a 100 ohm
line with 0.5 dB/meter at 60 MHz. This can be converted to a loss resistance
for your RLC model. I prefer to model line as periodic T or PI attenuators
rather than simply a series R. I won't go into the math for this now but I'm
sure you can figure it out.
Dave Cuthbert
Micron Technology
From: John Woodgate [mailto:[email protected]]
Sent: Friday, September 26, 2003 8:07 PM
To: [email protected]
Subject: Re: Model for CAT 5 cable
I read in !emc-pstc that Joe Randolph <[email protected]> wrote
(in <[email protected]>) about
'Model for CAT 5 cable' on Fri, 26 Sep 2003:
>I'm interested in obtaining a lumped model, consisting of R, L, and C
>elements, for a single pair of wires in a representative section of
>Category 5 UTP cable.
You could measure a length of cable yourself, if you don't need an
officially-certified model. R is fairly easy - measure at DC and
calculate any skin effect. C is also fairly easy, but you might have
difficulty with L. I have a very good transformer ratio-arm bridge that
will do this, but they are not common. One way that may work is to
measure the loop impedance, by comparing voltage-drops with that over a
known resistance and calculating the inductance from knowing the
resistance. You could also resonate the loop with a known added shunt
capacitance and measure the resonance frequency, but the Q may be low.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
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