Without Test equipment one can also cut the board, measure the cross section,
organize the materials Datasheet and put it into a free of charge impedance
calculator.
That’s also the next step in troubleshooting a trace that was found to have the
wrong impedance.
BR
Gert
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Von: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] Im
Auftrag von Chris.Scholz@xxxxxxxxxxxxxxxxxxxxx
Gesendet: Donnerstag, 13. August 2015 06:03
An: tom@xxxxxxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Betreff: [SI-LIST] Re: Impedance measurement
Hi Tom,
you can do a lot of good measurements with TDR. What I like about TDR is that
this are very intuitive measurements.
What I still have not seen yet is any information on the measurement accuracy
of a TDR. The figures I remember are in the order of +/- 6 dB uncertainty for
frequencies above 30 GHz. Not to mention that when you add anything that messes
with the directivity of your measurement setup, like cables, probes or God
forbid a switch matrix, things can get a lot worse.
We (or at least I) don't know anything about the OP's frequency range,
requirements for accuracy, reproducibility, etc.
Maybe a step generator and an cheap oscilloscope is all he needs. For
reproducible measurements with defined measurement uncertainty at high
frequencies, a VNA will fit the bill.
Chris
From: "Tom Dagostino" <tom@xxxxxxxxxxxxxxxxx>
To: <rdawson16@xxxxxxxxxxx>, <jeff.loyer.si@xxxxxxxxxxx>,
<dharanidhar.signalintegrity@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>,
Date: 08/12/2015 07:21 PM
Subject: [SI-LIST] Re: Impedance measurement
Sent by: si-list-bounce@xxxxxxxxxxxxx
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
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