The common approach is to include a test coupon in the board design and let
the PWB manufacturer test it with proper equipment. Traces of other widths
can be scaled from that measurement. For traces only 1/2" long, unless your
risetimes are really that fast (<100 ps), worrying about the precise
impedance is not very meaningful. If they are that fast, you can't get a
useful measurement using other than proper equipment. Sure, a TDR is "less
accurate" but if you need better than a few percent, that's no longer an SI
problem and you are up against manufacturing tolerances anyway. The bottom
line is that if you can't get the proper equipment you don't need our advice
-- you need a new job with better support.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of dharanidhar chatrathy
Sent: Saturday, August 15, 2015 12:49 PM
To: tom@xxxxxxxxxxxxxxxxx
Cc: Chris.Scholz@xxxxxxxxxxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Impedance measurement
I want to measure the impedance of PCB traces (Differential and single
ended). The trace lengths are around 0.5". I don't have any components
mounted on the board. Don't have the TDR and Network analyzer on my bench.
So, i thought if i could measure the impedance in any other way.
Looking at one of the technique, using a high speed source and oscilloscope,
this seems doable but as pointed by few this seems to be the technique with
least accuracy. If i want to measure the impedance of a newly built board,
this method doesn't work as i will not be sure if the technique caused the
issue or the board really has a impedance issue.
On Fri, Aug 14, 2015 at 3:35 AM, Tom Dagostino <tom@xxxxxxxxxxxxxxxxx>
wrote:
Chrisinstruments.
I've done some comparisons of a 40 GHz VNA to the Tek 80E10 out to 40
GHz about 7 years ago. The DUT measured was designed for 20 GHz
bandwidth, it used SMA connectors. The results overlaid to about
34GHz. Above that there was about a dB of noise difference out to 40
GHz.
The difference between the VNA and the TDR system was dynamic range.
The TDR gets about 60-70dB dynamic range out to 5 GHz and 30 or so dB
out to 40GHz for the setup I was using. More dynamic range can be had
by using more sampling points and averaging than I used for my
measurements that day.
The VNA was getting about 50dB at 25GHz. Again, those were likely
limited by my setup that day. I was not going for best performance
but looking at throughput times for a typical application with both
setup.
Using any TDR system should get Dharanidhar Chatrathy's measurement
made to much better than the 5-10% variability between circuit boards
that will be seen in production lots. Even with the poor man's TDR I
outlined. A well calibrated TDR should get 1% or better. You will
see more change in impedance of the trace than 1%.
I have the TDR option on my VNA and prefer to use a TDR system for
making impedance measurements. If I have a very wide dynamic range
measurement or a tuned circuit the VNA will do better. For the
run-of-the-mill measurements I like a scope with TDR. A well-equipped
lab should have both.
The user needs to understand each instrument's limitations and how to
make the trade-offs among accuracy, measurement time, dynamic range and
Kit.
Regards,
Tom Dagostino
Teraspeed Labs
9999 SW Wilshire Street
Suite 102
Portland, OR 97225
971-279-5325
tom@xxxxxxxxxxxxxxxxx
From: Chris.Scholz@xxxxxxxxxxxxxxxxxxxxx
[mailto:Chris.Scholz@xxxxxxxxxxxxxxxxxxxxx]
Sent: Wednesday, August 12, 2015 9:03 PM
To: tom@xxxxxxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: Re: [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
This sets the reference impedance (voltage) that is the basis of a TDRI'd likely use a 25 Ohm standard.
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.
see variation in impedance.
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
These are caused by etching, height and fiber weave effects among others.the scope's timebase.
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
If you are looking to isolate a fault in a BGA package you can easilyvia.
see the difference in time between an open before a via or one after a
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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