[SI-LIST] : HIgh-speed interconnect characterization

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From: Ravinder Ajmani (ajmani@us.ibm.com)
Date: Fri Dec 08 2000 - 11:48:58 PST


Hi SI Gurus,

I am characterizing a differential interconnect comprised of a flexible
cable attached to a PCB through a connector. I am using the following
setup:

Two 2" semi-rigid cable are soldered to one end of interconnects on the
flexible cable. Other ends of the semi-rigid cables are connected through
36" long 50 ohm cables to + and - outputs of a pulse generator. The other
end of the interconnects is connected to an oscilloscope through two 18"
long cables soldered to the PCB. The pulse generator output rise time is
less than 100 ps. The purpose of my testing is to ensure that the
interconnects (flex cable, connector, and PCB traces) can be used to
transfer data at 1 GHz.

The oscilloscope waveform for a 1 GHz input showed almost no reflection, or
attenuation or distortion of any kind. As I played with the pulse
generator frequency, I found that the waveform was getting severely
distorted at 700 MHz, although I was able to get a reasonably good pulse
shape upto 1.3 GHz.

Next, I tried to model this interconnect, and frequency-domain simulation
showed a dip at 1.3 GHz (0.5V signal drops to 0.36V). As I played with the
model, it became apparent that the dip was due to a 90 degrees bend in the
interconnect on flexible cable. When I changed the model to eliminate the
bend, I still observed the dip, but it was very small (about 0.05V).

This experiment has left me with more questions than answers. I will
really appreciate answers to the following questions:

1) How valid is my test setup for the type of measurements I am trying to
make. Am I seeing an actual response of the interconnects, or will there
be an impact of long 50 ohm cables, that I have used to connect my setup to
signal source and oscilloscope.

2) If my setup is correct then why do I see the dip at 1.3 GHz in my model.

3) Can a 90 degree bend cause the kind of dip I am observing.

Regards, Ravinder
PCB Development and Design Department
IBM Corporation
Email: ajmani@us.ibm.com
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