[SI-LIST] Re: TDR Resolution
- From: "Moeller, Merrick" <mmoeller@xxxxxxxxxxxxxxx>
- To: "Scott McMorrow" <scott@xxxxxxxxxxxxx>
- Date: Tue, 18 Jan 2005 13:39:37 -0600
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Scott,
I see what you are saying... My purpose is just to create a well
represented
high speed pulse that will depict effects of features that would not
normally be seen at
much lower operating frequencies. It is interesting to see how a slower
signal will=20
average out discontinuities that are much shorter than min. resolved
feature.=20
CST will use the bandwidth you enter to determine the "Real" square wave
excitation
used to create a TDR profile. Helping to limit over shoot and better
matching the=20
max dv/dt of the signal.=20
Any frequency domain information from the "Real" pulse that is above
the usable=20
frequency range of the structure would seem to not be considered.
However, data created by=20
running that 150GHz pulse does a fairly good job of picking out
features. I may doubt the
accuracy, but it makes a nice picture. :)
=20
Regards,
Merrick
=20
=20
________________________________
From: Scott McMorrow [mailto:scott@xxxxxxxxxxxxx]=20
Sent: Tuesday, January 18, 2005 12:36 PM
To: Moeller, Merrick
Cc: si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Re: TDR Resolution
Merrick
Well, you are going to need to be very careful with your simulation
setup. Assuming that you have a constant coaxial section leading into
the retention structure, then you will be able to perform a good
waveport match to the coaxial mode. You'll also need to perform port
deembedding with quite a few samples across the entire broadband
spectrum to be sure that in simulation of the waveport you don't
inadvertently launch additional modes. I would highly suggest that you
model all materials as either PEC metals or lossless dielectrics,
otherwise there will be slight errors in the mode match that will cause
mode converstion at the launch. If you do all of these things, then you
may be able to reasonably resolve the retention ring.
However, this seems to be a purely academic exercise. If it takes 150
GHz bandwidth in order to resolve the structure, and the operational
bandwidth is sigificantly lower than this, for all purposes your
retention structure is invisibile. However, if you are trying to do
this in order to optimize the mechanical structure in some way, you are
probably using the wrong method. There are better ways to do this in
CST.
best regards,
scott
Moeller, Merrick wrote:
Scott,
In this case lets assume that the retention feature is
surrounded on either side
by uniform impedance section. Although these sections have more
length than the
retention feature itself they are less than 1mm, and the
retention feature itself=20
is less than .3mm. Would it be feasible to run the simulation to
a bandwidth that
would allow for that kind of resolution? 150GHz? More? Also
assume meshing
is sufficient. Would Max frequency of the structure permit such
a high frequency limit?
Would frequency cutoff limit simulation?=20
=20
Merrick
________________________________
From: Scott McMorrow [mailto:scott@xxxxxxxxxxxxx]=20
Sent: Monday, January 17, 2005 4:50 PM
To: Moeller, Merrick
Cc: si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Re: TDR Resolution
=09
=09
Merrick
=09
That's what I suspected. In that case, you need to reduce your
problem size to that of retention structure surrounded by uniform
impedance crosssections. Otherwise, you will obscure resolution of the
retention structure with other reflections. If you want to TDR the
assembly without any spurious high frequencies then you will need to
create a custom TDR waveform that is created from a low pass filtered
trapizoidal edge. You will also need to be sure that there sufficient
mesh cells to correctly discretize the structure you are simulating.
Depending upon the geometry, this might be as few as two cells in each
direction, but quite posibly it could be more. You can increase the
mesh density across the retention structure locally by selecting the
component, selecting its mesh properties and changing them. Finally,
you will need to adjust your simulation frequency for meshing and your
minimum mesh step size to guarantee that the simulation will be
performed at a high enough bandwidth to resolve your TDR rising edge.
=09
If you don't want to reduce your problem size by slicing the
model, then you can create the necessary waveforms in CST and then pass
them back through software the performs the reverse scattering algorihm
and provides a true impedance profile. With a bit of data manipulation
you should be able to pass your data back through TDA iConnect.
=09
In either case, your key to good results is to create a well
filtered TDR waveform at the appropriate edge rates, to control the mesh
density to gain the resolution you desire across your structure, and to
set the frequency and smallest mesh step size so that you have enought
simulation bandwidth.
=09
scott
=09
=09
Moeller, Merrick wrote:
=09
Scott,
3D solver. CST Microwave Studio...=20
TDR impedance profile at various rise times.
=20
M
________________________________
From: Scott McMorrow [mailto:scott@xxxxxxxxxxxxx]=20
Sent: Monday, January 17, 2005 2:02 PM
To: Moeller, Merrick
Cc: si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Re: TDR Resolution
=09
=09
Merrick,
=09
It depends on the type of simulation you are performing.
Is this a spice simulation based on extracted cascaded model sections,
or is it a 3D time domain field solver/simulation environment?
=09
Also, it depends on what you are trying to resolve:
final impedance of the retention feature, electrical length of the
retention feature, signal distortion due to retention feature at various
risetimes?
=09
scott
=09
=09
Moeller, Merrick wrote:
=09
Thanks for all the help everyone. I found
Kipp's solution suitable for my needs..
=09
To continue the discussion:
This scenario is taking place in a simulation.
It is
Possible to create increased frequency bandwidth
simulations
That may handle faster rise times... What are
some=3D20
Of the limiting factors when considering a
MAXIMUM FREQUENCY
BANDWIDTH for a simulation? For example, I
wanted to=3D20
Resolve some retention feature in a coaxial type
structure.
Is there a Max frequency.... Strictly speaking
of TDR and
Square wave reproduction. I realize that the
structure will
Have cutoff frequencies for particular modes.
=09
M
=09
-----Original Message-----
From: Ed Sayre III [mailto:esayre3@xxxxxxxx]=3D20
Sent: Monday, January 17, 2005 12:17 PM
To: Moeller, Merrick
Cc: si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] TDR Resolution
=09
Merrick,
As you can see from the responses the
"Rule-of-Thumb depends on your
signaling needs. The risetime of the TDR really
depends on the the data
rate of the signals. R. K Poon's book has an
excellent section on
sampling and risetime effects. I believe it is
in an appendix in
chapter 5.
Also a word to the warning, don't use a ramp
function, it tends to
give you unrealistic results and lots of ringing
and convergence
problems.
=09
Regards
-Ed
=09
=09
At 10:21 AM 1/17/2005 -0600, Moeller, Merrick
wrote:
=20
Experts,
I am in the process of running a
simulation In order to=3D20
resolve signal path discontinuities. Is
there a Rule of thumb that=3D20
would give the minimum rise time signal
To resolve a min length=3D20
discontinuity?
Regards,
Merrick
=09
=09
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Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax
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Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax
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Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax
http://www.teraspeed.com
Teraspeed is the registered service mark of=20
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