RE: [SI-LIST] : Resistive probe question

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From: Dagostino, Tom (tom_dagostino@mentorg.com)
Date: Tue Jan 02 2001 - 13:32:24 PST


I've looked at some twisted pair wire with a TDR. I could get any Zo I
wanted by moving the wires around, (twisting or untwisting the wires,
touching the wires, placing them near other pieces of metal). The response
you would see on the scope would be a function both the signal under test
and how you have handled the wire. Any time your hand moves on the wire you
would get a different response on the scope. Any time you place the twisted
wire near some other piece of metal you will get a different response on the
scope. Coax is cheap and you can buy it with good SMA or BNC connectors on
it. If you are using twisted cable you will have to terminate the wire into
the connector causing another location for a good reflection.
 

Tom Dagostino
IBIS and Tau Modeling Manager
SDD
Mentor Graphics Corp.
503-685-1613
tom_dagostino@mentor.com

-----Original Message-----
From: Christopher R. Johnson [mailto:crjohnson11@home.com]
Sent: Tuesday, January 02, 2001 9:08 AM
To: si-list@silab.eng.sun.com
Subject: [SI-LIST] : Resistive probe question

I have a question about the "Make your own resistive probe" article at
signalintegrity.com. Unfortunately Dr. Johnson is out of action for a while
due to an injury. His staff suggested I post my question to this list.
 
The article discusses making a resistive probe by just putting a series
resistor at the end of a piece of coax to make a cheap low capacitance
probe. The signal amplitude is reduced by the resistive divider formed by
the series resistor and the cable impedance. The probe capacitance s
essentially the capacitance across the series resistor.
 
Is there any reason that you can't use twisted pair flat cable instead of
coax? The required length is about 18 inches. Twisted pair cable flat is
available with 100 ohm single ended impedance. The higher impedance allows
you to have less of a resistive load on the circuit for the same output
signal level. The pairs can be separated such that they are only close
together in the untwisted mass termination portion of the cable, thus
reducing cross talk.
 
Any thoughts would be appreciated.
 
 
Chris Johnson
 
 

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