[SI-LIST] Re: tips of using TDR probe

  • From: "Tom Dagostino" <tom@xxxxxxxxxxxxxxxxx>
  • To: <sherman.chen@xxxxxxx>, <Charles.Grasso@xxxxxxxxxxxx>, <heidi_barnes@xxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
  • Date: Tue, 20 Jan 2015 11:20:20 -0800

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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