Thread: Ifft?


Permlink Replies: 6 - Pages: 1 - Last Post: Jul 4, 2012 1:41 PM Last Post By: mingal
biff44

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Registered: 08/29/07
Ifft?
Posted: Apr 22, 2012 3:02 PM
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What exactly does the ifft function display. For instance, if I measure an actual device and get its S2P file from a network analyzer, and them display the s2p file in genesys, what am I looking at when I ask for ifft(S1,1).

Is it the impulse response? It does not look like a step response.

I am trying to make S parameter measurements on a network analyzer, and do a Time Domain Reflectometer transformation with the measured data in Genesys. It is kind of working, but strangely.

Thanks all.

Edited by: biff44 on Apr 22, 2012 3:04 PM
wrivas

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Re: Ifft?
Posted: Apr 24, 2012 3:50 PM   in response to: biff44 in response to: biff44
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The ifft() is the inverse fast fourier transform. When you apply this to s-parameters you get the impulse response. However like all ifft() functions you will need to fix the time scale since it will most likely think it is in seconds and not the proper units based on the delta frequency, if I recall correctly.
mingal

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Re: Ifft?
Posted: May 1, 2012 9:44 AM   in response to: biff44 in response to: biff44
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Attachment FFT_IFFT.wsx (129.8 KB)
Dear Rich-

See if this helps.

Respectfully submitted,
Mark W. Ingalls
biff44

Posts: 34
Registered: 08/29/07
Re: Ifft?
Posted: Jun 27, 2012 11:36 AM   in response to: mingal in response to: mingal
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OK, more questions!

I am getting S2P parameters from a network analyzer using a program called "painless extraction".
I save the S2P file to a computer. I then use genesys to show the frequency response of this file by loading it into a 2 port data file circuit element. I then use some negative line lengths to correct for some reference plane extensions so that a short circuit right at the end of the two network analyzer cables looks like a short circuit on Genesys smith chart of S11 and S22.

Now I use variable in the linear graph pop up box: ifft(S1,1) to compute and display the impulse response on a linear graph.

I am fairly happy. I can see the magnitude of discontinuities in the processed data of a microstrip unit I am testing. I can hold tuning sticks along the microstrip lines, and see their discontinuity in the genesys processed data.

But one thing that is confusing me is...if I see a discontinuity some time from zero, how do I tell if it is inductive or capacitive? I can plot in genesys ang(ifft(S1,1)) and the impulse response of an open circuit at the end of a line goes from positive phase before it hits the discontinuity to a negative phase after the discontinuity. Similarly, a short at the end of a line has an impulse response that goes from negative phase before the discontinuity to a positive phase after the discontinuity. So there is some phase information in the ifft function.

But I would understand this a little better if I could post process a STEP function. A step function is simply the integral of an IMPULSE function. But I am a little unclear on how I can get genesys to add an integration, or a multiplication by S, after the ifft is computed. Any ideas?

Edited by: biff44 on Jun 27, 2012 11:36 AM
mingal

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Re: Ifft?
Posted: Jul 1, 2012 7:27 PM   in response to: biff44 in response to: biff44
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Have you tried using "Math Language"? Pretty much anything you can do with Matlab you can do with Math Language. It's a little gewfy getting stuff from Eng. Lan. into math lang., but once you do, woo! hoo!

Edited by: mingal on Jul 1, 2012 7:27 PM
biff44

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Registered: 08/29/07
Re: Ifft?
Posted: Jul 3, 2012 1:03 PM   in response to: mingal in response to: mingal
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Well, you gave me a good example to follow. I will give it a try. Is this "math language" covered in one of the user manuals?
mingal

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Re: Ifft?
Posted: Jul 4, 2012 1:40 PM   in response to: biff44 in response to: biff44
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Very sparse documentation. Try here instead: http://www.mathworks.com/help/techdoc/. My hunch is that Genesys is using a subset of Matlab under license,,,

Edited by: mingal on Jul 4, 2012 1:40 PM

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