# Measuring S parameter - RF Cafe Forums

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Amit
Post subject: Measuring S parameter Posted: Thu Jul 21, 2005 8:42 am
Hi everyone ,
I have one doubt , let me write that :-

Suppose that we design one circuit assuming that circuit is in stable condition ( K > 1 eg. LNA ) . For this we take its' S parameters and after fabriction of device / ckt. how we ensure that S parameters are same (as taken for calculation of K )? Second question is how to assume / take S parameters for initial calculations ?
Thankyou ,
Amit

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IR
Post subject: Posted: Thu Jul 21, 2005 1:49 pm

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello Amit,

S-parameters for device are taken from the data sheet of the device.
The S-parameters given for an amplifier (LNA in your example) are noted
per specific bias point (Vce, Ic in case of BJT device) and they are different at other bias level.

Any other element that you add to your system e.g. filter,pad etc, has its own S-parameter matrix. If you want to know the general S-parameter matrix you have to multiply the matrix of each element.

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Best regards,

- IR

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Guest
Post subject: Posted: Fri Jul 22, 2005 6:49 am
Actually you mind find it difficult to multiply S-parameter matrices. You need to convert them to ABCD or T-matrix to multiply , then convert them back to S. As far as the K-factor before and after. The S-parameters of the device are taken for a certain bias point and on a particular board material. Most manufactures of devices will de-embed the S-parameters to the package pins or die. However, unless the device is mounted onto a metal flange, there may be a slight differebce in the device S-parameters since you may be mounting it on a totally different board material and thickness which has a different parasitic inductance. I normally, take the extra step and measure and de-embedd the device myself insuring that I have the correct de-embedding parameters for the enviroment in which I will be using the device.

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IR
Post subject: Posted: Sat Jul 23, 2005 1:00 pm

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello,

Just another point to consider: There is also a lot-to-lot variation of S-parameters, that can be 5% or higher. You should consider that when you purchase a reel of a specific device. You can also ask the manufacturer to screen the units (In additional cost of course), it all depends on your application.

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Best regards,

- IR

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alvin_style
Post subject: s parameters calculationPosted: Sun Mar 05, 2006 10:38 am

Lieutenant

Joined: Sun Mar 05, 2006 10:32 am
Posts: 1
Hi to all,
I happen to come across this great website as I'm trying find an answer to my question on S-parameters. Hope you can be of a help. I'm designing a unequal power divider and i've managed to compute its impedances. I'm wondering with these impedances and its isolation resistance can we derive a 3x3 s-parameter matrix? thanks

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AlvinC

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jaya000
Post subject: Posted: Tue Feb 20, 2007 6:42 am

Captain

Joined: Tue Feb 20, 2007 5:43 am
Posts: 14
S parameter changes when the manufacturer change the mfg. process from time to time. Also they change in the condition where u use it.
So i think many times why not check S parameters under actual working condition with the pcb layout we r going to use. Then we must get real s parameters for our application under real conditions.
Many times nice circuit design with ideal s parameters actually oscillate in real world conditions.

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