[SI-LIST] Re: Instruments / Methods for measuring a cable's chara cteristic impedance
- From: "john lipsius" <johnlipsius@xxxxxxxxx>
- To: <tom@xxxxxxxxxxxxx>, <James_R_Jones@xxxxxxxx>,<peter.baxter@xxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Fri, 7 Mar 2003 22:02:29 -0800
Tom,
If I'm not mistaken, the formula you gave is off a little and
should be:
Zcable = Zsource * Vcable / (Vsource - Vcable)
----- Original Message -----
From: Tom Dagostino
To: James_R_Jones@xxxxxxxx ; peter.baxter@xxxxxxxxxxxxx ;
si-list@xxxxxxxxxxxxx
Sent: Friday, March 07, 2003 8:58 AM
Subject: [SI-LIST] Re: Instruments / Methods for measuring a cable's chara
cteristic
impedance
An alternative is to take a good pulse generator with a known output
impedance of 50 Ohms. Attach it to the cable of interest. At the attachment
point probe the junction with a good FET probe (to minimize C loading). You
can then compute the voltage divider of the open circuit pulse output to the
measured initial level at the cable. For example, assume you have a 10 volt
pulse open circuit and the output impedance is 50 Ohms. If you observe a 6
volt level at the beginning of the edge when it is loaded by the test cable
then you would calculate the effective Zo of the cable is 75 Ohms. Zo =
V*Zo/(V-X)
Where X is the voltage at the cable
V is the input open circuit voltage
Zo is the impedance of the source
Zo is the impedance of the cable under test
in this case Zo = 6*50/(10-6) = 300/4 = 75.
Tom Dagostino
Teraspeed Consulting Group LLC Teraspeed Consulting Group LLC
2926 SE Yamhill St. Device Modeling Division
Portland, OR 97214 13610 SW Harness Lane
Beaverton, OR 97008
http://www.teraspeed.com 503-430-1065
tom@xxxxxxxxxxxxx
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of James_R_Jones@xxxxxxxx
Sent: Friday, March 07, 2003 7:58 AM
To: peter.baxter@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Instruments / Methods for measuring a cable's
chara cteristic impedance
Peter,
A simple, cheap trick that I have used before to measure Z0:
Hook up a pulse generator with known Rout on one end of unterminated cable.
Place a potentiometer in series with the source driving the cable. Also
attach a scope probe to the cable. Turn on the pulse generator. Now vary
the potentiometer and watch how the reflections change. You will get a 100%
reflection at the unterminated end of the cable, and it will bounce off the
source as well if the source is not matched correctly. This will be visible
on the scope. So tune the potentiometer until the source reflection goes
away, then measure the pot. Sum the pot resistance with the source
resistance, and you have Z0. (Beware of parasitic C)
If you are looking for something quick and dirty that you can do with some
standard lab equipment, this works. Check it out.
Of course if you want to take more accurate Z0 measurements, then I
recommend the Tek TDS8000 with 80E04 sampling head.
Hope this helps,
James R. Jones
Dell
-----Original Message-----
From: Peter Baxter [mailto:peter.baxter@xxxxxxxxxxxxx]
Sent: Friday, March 07, 2003 4:19 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Instruments / Methods for measuring a cable's
characteristic impedance
Hi,
I've been using the LC method to determine the characteristic impedance of a
1 metre long transmission line (flat) cable (around 60 ohms at 100kHz). This
method works fine for me. However getting access to the "beautiful /
expensive" HP LCR meter (on another companies premises) is getting harder
and harder. I'm now considering what alternatives I might consider.
For the work I do, purchasing gear costing US$10,000 up, isn't practical.
Might readers be able to suggest alternate methods, or products that I might
consider? (Recommendations too, for second hand test equipment outlets - in
USA or the rest of the world - others may also be interested). Could a
network cable tester be adapted to the application? Or is my cable length
too short?
Thanks
Peter Baxter
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