Cascade S parameters - RF Cafe Forums
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Post subject: Cascade S parameters Posted: Thu Jul 17, 2008 8:53 am
Joined: Tue Sep 04, 2007 7:54 am
I am trying to analyze the casecade S parameters of a passive filter cascaded with a LNA. The passive filter is followed by the LNA. S11 of the passive filter is 22dB and S22 is around 21dB. LNA S11 is around 18dB. Wht will be the S11 of filter LNA cascade?. Cascade S11 will vary depending on the phase of filter S22 and LNA S11. If the phase varies from 0 to 360 what will be the maximum and minimum casecade S11?
Post subject: Posted: Fri Jul 18, 2008 10:46 am
Joined: Tue Feb 27, 2007 6:16 am
To determine the cascaded S parameters of the resultant two port network, it is easiest to first convert from S parameters to the appropriate Z, Y or T parameters, work out the resulting matrix and then convert back to S parameters.
For parallel connection of two port networks, convert to Y parameters and sum the resultant. For series connection of two port networks, convert to Z parameters and then sum the resultant. For a cascaded connection, convert to T parameters then multiply the last network by the one previous and so forth (order is important). Refer to pg 169 Intro to Radio Frequency Design by Wes Hayward (beware typo pg 172, this one held me up for many months)
Most books on RF design will include some section on how to convert between parameters. If you use Matlab I can post some code I was using to convert between parameters.
Regards, Darcy Randall, Perth, Western Australia
Post subject: Posted: Fri Jul 18, 2008 5:09 pm
Joined: Tue Jul 08, 2008 7:15 pm
Location: Boulder Colorado
If you put a passive filter in front of an LNA, the S11 of combined network will be dependent on the match between filter output and the LNA input. I don't think it is possible to predict composite S11 without full set of S parameters for each network.
I am not sure what objective is, but if filter were perfect (S11=S22=0, S21=S12=1) in passband, then S11 of network would be S11 of LNA, in other words, the S11 of the amp can't be improved by the filter unless an attempt is made to match the filter to the amp for optimum return loss in passband.
If you are inclined to do some math, HP app note #95-1 gives a very good tutorial on S-Parameter design.
If you are like most of us, and would prefer for computer to do math, there are a number of free s-parameter design tools available. I have not used it, but I understand that Ansoft has a "student" version available. There used to be a program called ARRL designer that was quite capable of doing simple s-parameter calculations. I would bet that if you check this (RF Cafe) site you will probably find a program that is capable of doing this analysis.
- Mark -