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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