RE: [SI-LIST] : Resistive probe and twisted pair revisited

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From: Dagostino, Tom (tom_dagostino@mentorg.com)
Date: Wed Jan 03 2001 - 08:53:37 PST


You might try Precision Interconnect for cable assemblies. I've seen some
very flexible and small coax they have used.
 

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 5:47 PM
To: si-list@silab.eng.sun.com
Subject: [SI-LIST] : Resistive probe and twisted pair revisited

It seems that the biggest down side to using twisted pair for the resistive
probe is the lack of the coaxial shield for noise reduction and impedance
stabilization.
 
The actual application is for a very low cost relatively high performance
logic analyzer (500 MHz sample rate), so there must be 16 probes. The
cables must be very flexible to allow easy probing without pulling off the
probe clips due to the cables stiffness. I like the idea of the resistive
probes because other low cost analyzers have probe capacitances in the 10pf
range, which negates their high input impedance at high frequencies.
 
The biggest reason that I like the higher impedance cable and resulting
higher signal level is that the cable directly drives a high speed
comparator. The biggest signal integrity problem is probably the on board
noise level that the comparator sees, relative to the incoming signal level.
With a 100 ohm cable, you can get a 10:1 attenuation with a ~1K probe
resistance, and that means that you are looking for your comparator to
resolve 10mv to be accurate to 0.1V logic threshold setting. This is
borderline. In reality, 20mv / 0.2V would probably be acceptable.
 
As far as cost, does anyone know of a good source of low cost very flexible
high impedance micro coax? The RG-180 (95 ohms) that I have seen is a
little too stiff. I have looked at the Gore Speedline micro-coax assemblies
that are normally used for routing critical signals on PC boards, but the
cost is prohibitive. I suppose there is no reason that the PC board can't
be designed to have either coax or twist and flat cable soldered to it, and
charge extra for the coax version.
 
Chris Johnson
 

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