[SI-LIST] Re: AW: I am using HFSS TDR and compare with Agilent TDR in measurement of a sliced via, I found usually HFSS has 2 or 3 ohm higher than Agilent, why?
- From: <heidi_barnes@xxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>, <issue.lu@xxxxxxxxxxx>
- Date: Fri, 11 Jul 2014 16:40:42 +0000
Hi Steve,
As Gert mentions it is very instructional to play with the wide variety of
windowing functions to deal with the iFFT on bandlimited frequency domain data
and see how it affects the rising and falling edges which in turn has a direct
impact on the peaks and valleys of impedance discontinuities that are shorter
than the rise time length. Starting with simple test structures like a series
resonant Beatty standard (ie 50ohms to 25ohms to 50ohms) and varying the length
of the discontinuity can be very instructional when looking at time and
frequency domain and matching measurement with simulation.
Some interesting observations to explore:
1) HFSS is an FEM simulator which is a frequency domain solution, so it is
similar to a network analyzer measurement with extrapolation to DC and
bandlimited with a max frequency. Ideally this max frequency should be higher
than the bandwidth of the DUT, but for simple passive structures this is not
always easy to do. In simulation increasing the max frequency increases the
mesh and the simulation time, in measurement not only does the instrument have
a max frequency, but the ability to design a wide bandwidth fixture and
calibrate it out also has its bandlimiting max frequency. As the max frequency
gets lower and closer to the DUT frequencies of interest, then one also becomes
more sensitive to the "windowing" that is being used for the conversion to the
time domain.
2) On the measurement side with a TDR/TDT instrument one now has a broad band
step edge which can be calibrated to correct for fixture losses, but
identifying an industry accepted standard for the shape of that rising and
falling edge is not a simple thing. So in short if you send an "unknown" step
edge shape into a black box then it will be difficult to match with simulation.
Also, the TDR/TDT measurement does not require an FFT transformation which is
typically non-causal due to the required windowing.
3) Matching the TDR from an iFFT of a frequency domain simulation with the TDR
from an iFFT of a frequency domain measurement is much easier to do since one
can insure that the same windowing is being done and identical reference planes
are being used.
4) To get simulation to match time domain TDR/TDT measurements can be done with
a few different approaches. One is to use a time domain EM simulator (Finite
Difference Time Domain FDTD) and stimulate with a stored TDR step edge waveform
that was obtained from measurement of the output from the TDR box at the
reference plane of interest. Second is to use this stored waveform and run it
through the frequency domain simulation data using a causal Hilbert transform
instead of the iFFT that requires non-causal windowing techniques.
As with anything it always helps to start with something simple that is easy to
understand. Al Neves of Wild River Technolgy has some great connectorized test
structure boards to 50GHz, and I am working with him to do an ADS simulation
starter kit to demonstrate these issues and how to get simulations and
measurements to match. One of my favorite structures is the simple series
resonant Beatty structure (ie 50 ohms to 25 ohms to 50 ohms) that works great
for validating both simulations and measurements in the time and frequency
domains.
If you can't measure and simulate a simple series resonant test structure
correctly, then how do you expect to do a complicated SERDES channel with
packages, PCBs, connectors, and cables?
... and in response to the differences you are seeing #1 is that the fixture is
slowing down the risetime so that the Z peak is lower for a via discontinuity
that is shorter than the rising edge and you need to do fixture removal
calibration techniques or, #2 the HFSS max frequency is allowing higher
amplitudes at the higher frequencies then the high frequency roll-off that is
typical in a TDR step edge, or #3 the as-fabricated material properties are not
the same as used in simulation (ie drill size vs finished hole size, etching,
lamination thicknesses, etc).
Best Regards,
Heidi Barnes
Agilent EESof High Speed Digital SI/PI
PS- Published References:
http://www.home.agilent.com/agilent/redirector.jspx?action=ref&lc=eng&cc=US&nfr=-34346.1056823.02&ckey=2361581&cname=AGILENT_EDITORIAL
http://www.home.agilent.com/agilent/redirector.jspx?action=ref&lc=eng&cc=US&nfr=-34346.1078495.02&ckey=2360915&cname=AGILENT_EDITORIAL
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Havermann, Gert
Sent: Thursday, July 10, 2014 11:52 PM
To: si-list
Subject: [SI-LIST] AW: I am using HFSS TDR and compare with Agilent TDR in
measurement of a sliced via, I found usually HFSS has 2 or 3 ohm higher than
Agilent, why?
Steve,
change the window function in ADS and look how the results are changing. Then
think about how TDR is calculated from Frequency data. And finally use your own
brain to figure out if your 3 Ohm difference make sense.
BR
Gert
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-----Ursprüngliche Nachricht-----
Von: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] Im
Auftrag von steve ham
Gesendet: Freitag, 11. Juli 2014 05:02
An: si-list
Betreff: [SI-LIST] I am using HFSS TDR and compare with Agilent TDR in
measurement of a sliced via, I found usually HFSS has 2 or 3 ohm higher than
Agilent, why?
I am using HFSS TDR and compare with Agilent TDR in measurement o= f a sliced
via, I found usually HFSS has 2 or 3 ohm higher than A= gilent, why?
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