You can't do better than a few percentage points with any measuring technique,
probe setup, the coax launch or your testpoint lands, board material variance,
etc.
Even with the best equipment and bench practices.
I think what our poster is asking for, is did I make a grave mistake.
Read our friends, Ritchey, Johnson, Bogatin.
The real TDR, is when you have the active signaling devices down, in operation
and looking at near and far ends on the scope.
Did you plan the stackup; (In co-operation with the PCB vendor, laminates come
in quantum thickness) take into account not only the Er of the dielectric but
also the prepreg;
Trace width and spacing, is this diff pairs or single ended?
Copper pours, stitching vias to plane around the signal path - the return
currents, they follow the signal path?
Do you have multiple signals, consider crosstalk and propagation delays.
Randy
From: jeff.loyer.si@xxxxxxxxxxx
To: dharanidhar.signalintegrity@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Impedance measurement
Date: Wed, 12 Aug 2015 15:09:51 -0700
A TDR is basically a pulse generator and an oscilloscope. If you have
these, you can build your own. You can "crudely" determine the
characteristic impedance of your PCB trace by terminating one end of it with
a variable resistor while you inject a pulse from the other end and observe
that injected waveform with your oscilloscope. When the resistor's value
matches that of the trace's impedance, there will be no reflection at the
termination. This is only going to be a crude approximation, but it used to
be good enough for many applications. For most of today's high speed
signaling, it's not accurate enough, but it's better than nothing.
Good luck,
Jeff Loyer
Signal Integrity Engineering
On Behalf Of dharanidhar chatrathy
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
Sent: Wednesday, August 12, 2015 8:53 AMsimplest way to do. I neither have TDR nor the network analyzer. I 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
looking for a crude method of measuring it. Please, suggest ways of doing
it.
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