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Thread: Using a single port S1P file to simulate a load impedance in R+JX format ?


Permlink Replies: 4 - Pages: 1 - Last Post: Aug 31, 2012 8:00 PM Last Post By: paulh
paulh

Posts: 22
Registered: 08/02/07
Using a single port S1P file to simulate a load impedance in R+JX format ?
Posted: Aug 26, 2012 4:05 PM
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Hello I am trying to use a single port s1p file to generate a matching network but am stuck on how to format the s1p file correctly so perhaps someone can help.

I measured the input impedance of a FET using a test fixture and a VNA at various points across the band of
interest. The data I have is in R+JX as read from the VNA display. What I would like to do is use this measured impedance data as the basis of a Load so I can create a broadband matching network.

I tried to format the S1P file like this but it does not appear to work. I believe the syntax is not correct.

#MHZ Z RI R 50
175Mhz 2.6-J23.5
190Mhz 2.9-J22
210Mhz 3.1-J20

The basic idea I guess is to have a dynamic Term that is not a fixed impedance. I believe the S1P symbol is the correct one to use in ADS but I am not 100% sure. I could take the average of the measured data and put that into the term but was thinking there might be a better way to do it.

Thanks Paul
eyeung

Posts: 50
Registered: 01/22/09
Re: Using a single port S1P file to simulate a load impedance in R+JX format ?
Posted: Aug 27, 2012 6:53 PM   in response to: paulh in response to: paulh
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Hello,

The format of the 1 port touchstone file of impedance requires three columns of data: frequency, real part of Z, imag part of Z.

Please refer to the ADS documentation for further details

http://edocs.soco.agilent.com/display/ads2011/Working+with+Data+Files#WorkingwithDataFiles-TouchstoneSnPFormat
paulh

Posts: 22
Registered: 08/02/07
Re: Using a single port S1P file to simulate a load impedance in R+JX format ?
Posted: Aug 29, 2012 12:52 PM   in response to: eyeung in response to: eyeung
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eyeung thanks for the information. I took a look at that appnote and was able to format the S1P file correctly
but the problem is I would have to convert the R+jx impedance to S-Parameters first in order for the data to be of any use.

This can be done but it is kind of a pain to do. I am used to working with large signal parameters which usually
come in the form of R + JX or R - JX as read from a VNA. Typically anyone working with a agilent VNA will be able to read the Real and imaginary parts of the measured impedance right off the display. It is interesting that
ADS does not support a term symbol that would allow direct entry of R+JX impedance data at specific frequency points for use in circuit simulation. In the real world a term or load rarely is just purely resistive but more a combination of both real and or inductive or capacitive parts.

There is proabably a way to do what I want to do using a parameter sweep of the load or an equation but I was unable to find any information on how to do this.

Perhaps in the next revision of ADS they might include a Term Port to handle real world impedance data at specific frequency's and may be do a interplot of the data similar to the options in the S1P symbol. That would be a handy thing to have for us designers that just work with R+JX impedance. : )

Thanks Paul
sdgrant


Posts: 110
Registered: 06/08/09
Re: Using a single port S1P file to simulate a load impedance in R+JX format ?
Posted: Aug 31, 2012 5:30 PM   in response to: paulh in response to: paulh
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The method you are using to define an S1P file to model the frequency dependant load data is the most direct method available. The only problem is finding the best way to enter the data. It sounds like the conversion of the measured complex impedance data to the required format is the obstacle.

The data you have is complex impedance data already. The conversion you are doing is to normalize the values to the defined 50 Ohm reference impedance. If you change the normalizing impedance to 1 Ohm then the impedance values should be able to be entered directly. Simply change the header line from #MHZ Z RI R 50 to #MHZ Z RI R 1.
paulh

Posts: 22
Registered: 08/02/07
Re: Using a single port S1P file to simulate a load impedance in R+JX format ?
Posted: Aug 31, 2012 8:00 PM   in response to: sdgrant in response to: sdgrant
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That worked and it is a perfect solution for me. Thank you very much for the insight. Normalizing the S1P file to 1 ohm as opposed to 50 ohm as is shown in most application notes was the fix.

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