Assuming your trace is long enough and you have a fast pulse generator and a
low capacitance scope probe with high input R you should be able to get a
good approximation of the trace impedance. I'll assume your pulse generator
has a 50 Ohm output impedance and the rise or fall times is shorter than the
electrical length of the trace. Attach the pulse generator to the end of
the test trace, hopefully you made provisions to do this in your layout with
an RF connector. If not solder a piece of coax to the board, center
conductor to the trace and ground as near as possible to the trace and keep
the leads as short as possible, you don't want a lot of inductance at the
transition from coax to trace. Probe the trace close to the place you
soldered the coax with the high Z scope probe. Let's say your pulse
generator is set to 1V into a 50 Ohm load. The output voltage will then be
2 volts into an open circuit. The output impedance of the generator and the
Zo of the trace form a voltage divider. Measure the voltage on the trace
after the ringing has settled but before the reflection off the open end of
the trace returns to the probe point. You now have all you need to solve
for the trace impedance using the voltage divider equation. Vmeas =
(2V*Zo)/(50 + Zo).
Tom Dagostino
Teraspeed Labs
9999 SW Wilshire Street
Suite 102
Portland, OR 97225
971-279-5325
tom@xxxxxxxxxxxxxxxxx
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of dharanidhar chatrathy
Sent: Wednesday, August 12, 2015 8:53 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Impedance measurement
I am looking to measure impedance of a trace on a PCB. What is the simplest
way to do. I neither have TDR nor the network analyzer. I am looking for a
crude method of measuring it. Please, suggest ways of doing it.
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