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Measuring S parameter - RF Cafe Forums
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Below are the old forum threads, including responses to the original posts.
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Test & Measurement
Post subject: Measuring S parameter Posted: Thu Jul 21, 2005
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 ?
Post subject: Posted: Thu Jul 21,
2005 1:49 pm
Joined: Mon Jun 27, 2005
for device are taken from the data sheet of the device.
given for an amplifier (LNA in your example) are noted
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.
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.
Post subject: Posted: Sat Jul 23, 2005 1:00 pm
Joined: Mon Jun 27, 2005 2:02 pm
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.
subject: s parameters calculationPosted: Sun Mar 05, 2006 10:38 am
Joined: Sun Mar 05, 2006 10:32 am
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
Post subject: Posted: Tue Feb 20, 2007 6:42 am
Joined: Tue Feb 20, 2007 5:43 am
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.