[SI-LIST] : Resistive probe question

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From: Christopher R. Johnson (crjohnson11@home.com)
Date: Tue Jan 02 2001 - 09:08:01 PST


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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