Randy
I think I have to disagree with you. Full disclosure, I support the
IConnect TDR tool that Tektronix sells.
With proper calibration you can get very good accuracy and resolution. When
I use/demo/lecture about TDR I always use a 50 Ohm Load from a VNA Cal Kit.
This sets the reference impedance (voltage) that is the basis of a TDR
measurement. This will also calibrate out any aberrations in the step
output of the TDR pulse or sampling head. And with the reference step
waveform recorded also removes most of the measurement system's step height
and sampling head gain errors.
Remember the equation p = Vreflected/Vincident = (Z - Zo)/(Z + Zo). The
reflected voltage is a fraction of the incident voltage so the amplitude of
the incident voltage does not matter. It is easy to see the difference in
50 Ohm terminations and you can cross check these measurements with an Ohm
meter. It is easy to see 0.1 Ohm differences accurately.
So as long as the difference in impedance of the unknown is large enough to
cause a measureable voltage you can make very accurate measurements. And
for the best measurements it is best to use a calibration standard close to
the unknown impedance. I would not use a 50 Ohm standard if I'm trying to
measure a 28 Ohm trace for example. I'd likely use a 25 Ohm standard.
Tools like IConnect also employ a peeling algorithm that compensates for the
reduction in step amplitude after a reflection. Part of the incident
waveform is reflected back to the source at a discontinuity thus the
amplitude of the step after the discontinuity is lower.
If you don't have a lot of losses (poor launches, discontinuities, lossy
material) in your DUT you can make good measurements with just the TDR scope
in an Ohm display mode.
A TDR measurement can be made with a matched load, an arbitrary load or an
open at the end of the trace. The reflected signal prior to the
"termination" looks the same prior to seeing the effects of the termination.
Monitoring the voltage at the end of the trace will give you the output
voltage of the DUT, TDT signal, and this can be used to calculate the
insertion loss of the DUT. The TDT signal is not needed for a calculation
of impedance.
You are correct, the measurement you make on one trace is not necessarily
the measurement you will have on another trace, board material properties,
manufacturing tolerances, etc. will give different answers on different
boards. And within a single trace you will see variation in impedance.
These are caused by etching, height and fiber weave effects among others.
If we are trying to measure Zo of a trace it cannot be coupled to any other
trace or we will be seeing the effect of additional metal causing a change
of impedance.
Discontinuities in the launch, the transition from coax to board, can cause
issues. It is best to minimize these. Well-designed launches should be
used in all high quality measurements. Discontinuities in launches act like
low pass filters and limit the bandwidth of the signal propagating down the
trace. They destroy the return loss of a system and reduce the bandwidth of
the output of the DUT, the TDT measurement. The higher the bandwidth of the
signal getting into the DUT the better time resolution of the TDR
measurement.
Speaking of time resolution there is a trade-off between a TDR measurement
and a VNA in TDR mode. With a TDR measurement the location of a fault or
discontinuity is limited by the sampling spacing of the scope's timebase.
If you are looking to isolate a fault in a BGA package you can easily see
the difference in time between an open before a via or one after a via.
With a VNA your time resolution is based on the maximum frequency of the
measurement. With my 20 GHz VNA I see measurement samples every 50 psec if
I measure out to 20 GHz.
So I think the original question was, "I need to measure a trace's impedance
and I don't have the standard equipment (TDR or VNA) to make a measurement.
How can I do this with alternative measurement equipment?" I think Jeff's
answer and the one I posted this morning (which I'm not sure made it to the
list) say that if you have a scope and a pulse source you have a poor man's
TDR system.
Regards,
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 Randy Dawson
Sent: Wednesday, August 12, 2015 3:54 PM
To: jeff.loyer.si@xxxxxxxxxxx; dharanidhar.signalintegrity@xxxxxxxxx;
si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Impedance measurement
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@xxxxxxxxxxxit's better than nothing.
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
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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