[SI-LIST] building models from TDR measurements

Tom and others-
 

When looking at the rise time of a TDR to evaluate a discontinuity, you
should keep in mind the difference between resolving two discontinuities
that may be 50 mils apart (roughly 8 psec TD) compared with creating a model
for the two discontinuities.

 

In failure analysis, you may want to evaluate the two separate
discontinuities to note the structural features of the physical part you are
testing. This may tell you of manufacturing defects. However, if you are
trying to build a model of the interconnect, which might be used in a system
level simulation, you can easily build a model with a bandwidth comparable
to the bandwidth of the signal at the discontinuity, without resolving the
discontinuities. 

 

An Agilent or Tek TDR with a rise time of even 50 psec, easily has the
bandwidth to resolve a model of a discontinuity of 30 fF (the capacitance in
a 50 ohm line that is 10 mils long) or 90 pH (the loop inductance that is in
a 50 ohm line, 10 mils long). 

 

If you have a 35 psec rise time at the signal, you can build a ~ 10 GHz
bandwidth model. You may not resolve the model into two separate topological
structures- ie, transmission line elements or LC elements for each
discontinuity, but you could create a model for the composite structure that
would predict the performance of the interconnect, with its 50 mil
geometrical features, to 10 GHz. Is this what you are after?

 

With the output from a TDR measurement, tools such as Tek's new version of
Iconnect and Agilent's ADS are great tools to build topology based models,
optimized from the measurement, and then superimpose the measured result to
the simulation of the model to verify the accuracy of the model. If you
convert both the measurement and simulation into the frequency domain, you
can get a direct measure of the bandwidth of the model.

 

You might check my OLL-624 Characterizing discontinuities with a TDR and
OLL-691 High bandwidth models from measurements, both posted on my web site,
www.bethesignal.com <http://www.bethesignal.com/>  . The techniques for
building models from TDR and VNA measurements are outlined in these
lectures. My public class, Signal Integrity Characterization Techniques,
coming up in early June, also goes through these methods in great details,
with demos of the TDR and VNA measurements, how to build models, how to
optimize them and how to evaluate their bandwidth.

 

--eric

 

**************************************
Dr. Eric Bogatin, President
Bogatin Enterprises, LLC
Setting the Standard for Signal Integrity Training
26235 w 110th terr
Olathe, KS 66061
v: 913-393-1305
f: 913-393-0929
c:913-424-4333
e:eric@xxxxxxxxxxxxxxx
 <http://www.bethesignal.com/> www.BeTheSignal.com 
**************************************** 

tom_cip_11551 wrote:

> Hi to the Signal Integrity community.

> 

> I am looking for a set of equipment with wich to do TDR analysis on 

> high speed PCB structures. We are now building boards to run at speeds 

> of 3 Gb/s or faster and I would like to resolve discontinuites within, 

> say, 50 mils to 80 mils of each other.

> 

> I have gotten quotes from Tektronix and Agilent for TDR equipment.  

> 

> The Tektronix system consists of the DSA8200 digital sampling scope, 

> the 80E04 TDR sampling module and the 80SICON signal integrity and 

> failure analysis software (a $20K item by itself).

> 

> The Agilent system consists of the 86100C scope, the 54754A TDR module 

> and various other software programs for S parameters, Jitter analysis, 

> etc.

> 

> I am wondering what the Signal Integrity community at large is using. 

> Which system is more popular and, most important, cost effective?

> 

> What are some of the advantages and drawbacks of each?

> 

> Are there other solutions on the market that are more cost effective?

> 

> Thank You

> Tom

 



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