[SI-LIST] Re: tips of using TDR probe
- From: "Chen, Sherman" <sherman.chen@xxxxxxx>
- To: "tom@xxxxxxxxxxxxxxxxx" <tom@xxxxxxxxxxxxxxxxx>, "Charles.Grasso@xxxxxxxxxxxx" <Charles.Grasso@xxxxxxxxxxxx>, "heidi_barnes@xxxxxxxxxxxx" <heidi_barnes@xxxxxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Thu, 22 Jan 2015 15:45:03 +0000
Tom,
Agree with you on all you said.
My point is:
a. launch the step at the closest location to the UUT thus to reduce the
peeling effect.
b. reduce the preceding peak or dips as much as could. Better to be even with
Z0.
Then you will obtain quite accurate impedance profile of UUT.
We taught our JDM this method and they applied it in measuring vias. The
results are pretty good.
Best Regards,
Sherman Chen
Signal Integrity
EMC Global Hardware Engineering
Tel: +86 21 60951100-3329
-----Original Message-----
From: Tom Dagostino [mailto:tom@xxxxxxxxxxxxxxxxx]
Sent: Wednesday, January 21, 2015 3:20 AM
To: Chen, Sherman; Charles.Grasso@xxxxxxxxxxxx; heidi_barnes@xxxxxxxxxxxx;
si-list@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] Re: tips of using TDR probe
Sherman
The resolution of any measurement system is dependent on the measurement
bandwidth at the DUT. Just about anything between the instrument and the DUT
will cut the available bandwidth - and discontinuities can pack a big punch.
And in many cases you will have two, one at the input and one at the output of
the DUT for insertion loss type measurements and once going in and once going
out for return loss measurements.
It is well known that TDR's ability to resolve features is related to the
risetime of the TDR edge at the DUT, and that the TDR edge has to go through
the system twice, once getting to the DUT and once getting back to the scope.
The other thing you will run into the reflections in the system will make the
downstream measurements inaccurate. This can be compensated for with IConnect
from Tek. At every discontinuity some of the energy is reflected back to the
scope. This implies there is less energy propagating past the discontinuity.
If you look at the equations for calculating the impedance based on the amount
of reflected voltage you will see that both the system Zo and the incident
voltage are used.
Vreflected = Vincident(Zx-Zo)/(Zx+Zo)
Thus for accurate measurements the incident voltage must be known at the input
to every discontinuity. The peeling algorithm in IConnect does this
computation to accurately calculate the impedance.
Tom Dagostino
Teraspeed Labs
9999 SW Wilshire Street
Suite 102
Portland, OR 97225
tom@xxxxxxxxxxxxxxxxx
www.teraspeedlabs.com
971-279-5325 office
503-430-1065 cell
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Chen, Sherman
Sent: Tuesday, January 20, 2015 8:25 AM
To: tom@xxxxxxxxxxxxxxxxx; Charles.Grasso@xxxxxxxxxxxx;
heidi_barnes@xxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: tips of using TDR probe
Hi Tom,
Thanks for sharing the valuable experiences. To verify my measurement results,
I did numerous simulation in ADS. Here is what I got:
1. an inductive peak or a capacitive dip at the launch will smear the peaks and
dips at the following discontinuities. But looks they don't distort the average
impedance much.
2. a lossy tline also has the similar impact.
So the point here is: when there exist peaks or dips, or a segment of lossy
tline in front of the UUT, the peaks and dips displayed on the TDR profile are
very likely distorted. Meanwhile the average impedance of the UUT mostly is
still trustable.
As I said in my last mail, to accurately evaluate the impedance of a UUT, I
will first try to set the launch point closest to the UUT. If it's not possible
to do so, above conclusion can be used to estimate the average impedance of the
far off UUT.
Best Regards,
Sherman Chen
Signal Integrity
EMC Global Hardware Engineering
Tel: +86 21 60951100-3329
-----Original Message-----
From: Tom Dagostino [mailto:tom@xxxxxxxxxxxxxxxxx]
Sent: Tuesday, January 20, 2015 9:42 AM
To: Charles.Grasso@xxxxxxxxxxxx; Chen, Sherman; heidi_barnes@xxxxxxxxxxxx;
si-list@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] Re: tips of using TDR probe
Hi Sherman and others
I've worked with TDR and all kinds of interfacing to the DUT. Coaxial and
microprobes tend to be best IF the launches into the test board are well
designed. If not you have created a low pass filter ahead of your DUT.
This low pass filter will either be an RC like filter if your launch if
capacitive or a series inductance if your launch is inductive. This filter
does two things, first it limits the insertion loss measurement. Secondly is
can severely limit the return loss performance. At higher frequencies you are
seeing the launch not the DUT. With a coaxial interface you can calibrate out
the effects of the launch if the launches have consistent
performance, i.e., they have identical TDR responses. If you apply the
rule of thumb of insertion loss minus return loss should be greater than 15 dB
for a quality measure at a given frequency you can see how important the return
loss effect of a poor launch can impact your measurements.
I ran some experiments with some launches we have to illustrate this effect.
There were three cases, a very good launch designed by Teraspeed, a published
launch by the connector vendor and a "hand soldered" launch typical of someone
trying to use a piece of coax soldered to the board. The discontinuity cause
by the launches were +2 Ohms, +15 Ohms and +40 Ohms inductive respectfully if
my memory serves me. The corresponding effective
bandwidths were about 20 GHz, 6 GHz and 1 GHz. I defined effective
bandwidth as a 15 dB spread between the insertion and return loss.
With hand held probes you will run into a consistency issue and unless you
design a consistent and effective return path you will have an inductive launch
into your DUT. And if you have a differential probe the bandwidth of that
probe is determined by the spacing of the probe tips. The wider the spacing
the larger the inductive loop is at the probe/DUT interface. For single ended
probes the launch will also have an inductive characteristic caused by the loop
area of the probe/ground return path. And who knows what the characteristics
of the DUT's interface looks like. Unlike a designed launch you have with the
coax interface you may be probing a couple of test points 0.100" apart. They
will in all likelihood look inductive or if the pads are large and use large
drill sizes - capacitive.
If you used something to keep the impedance constant between the probe and the
DUT then you will have a better shot at a meaningful measurement. But this has
to be consistent from test to test and during any de-embedding calibration you
do. If they do not then the difference between the two measurements
(calibration and measurement) will be placed between your measurement system
and the DUT.
Many of the TDR probes are measure for differential measurements and don't
really use a ground return. These can do an excellent job of making
differential insertion or return loss measurements if the interface to the DUT
is electrically clean. And they are do a very respectful job of differential
Zo measurements.
I've made test jigs to hold the hand held probes so that I got consistent
placement of the probe onto the circuit board. This helps, the geometry is the
same from measurement to measurement and it frees up the hands to push buttons
and capture the waveforms. And trying to hold a probe on a test point
consistently while waiting for a VNA sweep or multiple average TDR measurement
is a pain.
As Heidi points out for quality measurements you need quality cables. The same
things applies to the interface between the probe and the DUT, it has to be
consistent.
Hope this helps.
Tom Dagostino
Teraspeed Labs
9999 SW Wilshire Street
Suite 102
Portland, OR 97225
tom@xxxxxxxxxxxxxxxxx
www.teraspeedlabs.com
971-279-5325 office
503-430-1065 cell
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Grasso, Charles
Sent: Monday, January 19, 2015 1:09 PM
To: sherman.chen@xxxxxxx; heidi_barnes@xxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: tips of using TDR probe
Hello Chris - I am intrigued. It sounds like you solved a tricky problem.
Are you permitted to expand on your " enhancing the shielding" solution? I am
wondering how you avoided changing the impedance of the probe.
Best Regards
Charles Grasso
Compliance Engineer
Echostar Communications
(w) 303-706-5467
(c) 303-204-2974
(t) 3032042974@xxxxxxxxx
(e) charles.grasso@xxxxxxxxxxxx
(e2) chasgrasso@xxxxxxxxx
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Chen, Sherman
Sent: Monday, January 19, 2015 1:06 PM
To: heidi_barnes@xxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: tips of using TDR probe
Hi Heidi,
Thanks for sharing the tips. I was able to figure out the way how to accurately
do TDR measurement by enhancing the shielding around the very ends of the probe
tips. Now we are using this method for correlating simulation to measurement
even on very short structures such as vias. And it works fine.
For the method of using VNA to measure the sparam of the UUT, I think although
theoretically any types of probe can be used with ISS (impedance substrate
standard) to calibrate themselves out, despite they are highly reflective or
not, due to the calibration error, a highly reflective probe may not deliver a
satisfying calibration result. I will do some experiments to verify this when I
get a chance to play with ISS.
Best Regards,
Sherman Chen
Signal Integrity
EMC Global Hardware Engineering
Tel: +86 21 60951100-3329
-----Original Message-----
From: heidi_barnes@xxxxxxxxxxxx [mailto:heidi_barnes@xxxxxxxxxxxx]
Sent: Saturday, January 17, 2015 5:45 AM
To: Chen, Sherman; si-list@xxxxxxxxxxxxx
Subject: RE: tips of using TDR probe
Hi Sherman,
Investing time in set-up and training to use probes can pay off when it comes
to making measurements, but it does require a methodical process for
repeatability.
1) Connecting to probes with an instrument tends to leave the connecting cables
unsupported with multiple bends. This means that high performance phase stable
cables (and new ones that have not yet been damaged) are needed to avoid errors
in the calibration due to phase changes from cable movement.
Investing in the setup to minimize cable movement and provide support for the
cables can reduce the phase errors which should help with a cleaner
de-embedding.
2) Effervescent waves love to travel on the outside of a coax cable ( or probe
tip) so getting the fields to go into the PCB and not back up the cable does
require as continuous a ground as possible. GSG probes are better than GS,
and if you look at some of the vertical launch SMA's that are compression
mount, they could be considered the ideal probe with a continuous 360 ground
and no possibility for fields to travel on the outside jacket. Some
manufacturers add polyiron at the probe tip to reduce this problem, others try
to improve the coax to planar transition of the probe.
4) Clean surfaces are also important to achieve repeatable contacts with the
same force and flexing of the probe tip. Ideally, one should verify each probe
landing by looking at the TDR to confirm the impedance profile is the same as
that used for calibration.
5)Using a 2-Tier calibration with NIST traceable coaxial calibrations to the
end of the cables and then de-embedding of the probes is my preferred method.
This way I always have the NIST traceable calibrated data set, and I can always
come back later and improve the probe model for de-embedding if needed.
De-Embedding probes is not easy since the probe is low loss and measured
S-Parameters can suffer from passivity and causality problems (usually cable
movement). However, one can also look at creating deconstructed measurement
based models in simulation to provide adjustable probe S-parameters to match
with a given measurement.
Goodluck with the probing,
Heidi Barnes
EDA Software for SI/PI Applications
Keysight Technology
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Chen, Sherman
Sent: Thursday, January 15, 2015 8:42 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] tips of using TDR probe
Hi,
We're evaluating the performance of our TDR probe to determine if it can be
used in VNA measurement. Here is some observations:
1. Although in most people's opinion the contact quality of probe is
far less reliable & trustable compared to SMA, based on my experience, it's
feasible to achieve a stable contact with probe, at least for the bandwidth up
to 20GHz.
2. Besides measuring the sparam of the tline or other components on
PCB without the trouble of fabricating the test fixture, another benefit of
directly probing is the masking effect due to the fanout traces on the fixture
is avoided-more details will be shown on the TDR profile.
3. Experiments showed that the contact of ground collar on the TDR probe can
reduce the inductive peak at the very beginning of the TDR curve. It seems the
ground conductor must be surrounding the signal - a single ground wire won't
have much effect.
4. The deembeding of the probe is challenging - lots of glitches appeared on
the deembedded sparam. We're still working on that trying to find out the best
method.
I would appreciate if you can share your experience with using TDR probe either
for TDR measurement, or for sparam measurement.
Best Regards,
Sherman Chen
Signal Integrity
EMC Global Hardware Engineering
Tel: +86 21 60951100-3329
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