[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: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: heidi_barnes@xxxxxxxxxxx
- Date: Fri, 11 Jul 2014 13:01:53 -0400
Please stop feeding the engineering troll.
Steve Ham needs to do his own research, rather that continue to ask very
basic questions about multiple topics. In my opinion, this is not an
engineering classroom
kindest regards,
Scott
Scott McMorrow
Teraspeed® Consulting - A wild and wacky company
16 Stormy Brook Rd
Falmouth, ME 04105
(401) 284-1827 Business
http://www.teraspeed.com
We're growing.
On Fri, Jul 11, 2014 at 12:40 PM, <heidi_barnes@xxxxxxxxxxx> wrote:
> 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#61581&cname=AGILENT_EDITORIAL
>
> http://www.home.agilent.com/agilent/redirector.jspx?action=ref&lc=eng&cc=US&nfr=-34346.1078495.02&ckey#60915&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
>
>
> ----------------------------------------
> Absender ist HARTING Electronics GmbH, Marienwerderstraße 3, D-32339
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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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