How to convert 50 Ohm S parameters to 75 Ohm?

[mhtplsh]

Designing a 75 ohm input/output amplifier will require S parameters with 75 ohm input output matching. Further the Smithchart will also changed to 75 ohm impedance.
All the S parameters r available in 50 ohms. How to convert them to 75 ohms.
Also input output impedance calculated will be of 50 ohms as all the data is available in 50 ohms, so this will also hv to be converted.
or
suggest anyother method for designing 75 ohm amplifier with s parameter & smith chart.

 

[biff44]

Re: 50 Ohm TO 75 Ohm

Assuming you have a modern RF analysis program to simulate the amplifier on, you do not have to worry, the program will do all that for you.

You transistor is modelled either:
1) as a lumped element model, which does not care what impedance your "system" is in

or
2) Uses an S parameter file (S2P file) where there is a line near the top that reads something like:

# GHZ S MA R 50

The 50 tells your simulation program that these are Sparameters measured in a 50 ohm system, and if you are doing a simulation with another system impedance other than 50 ohms that it will have to numberically convert the sparameters accordingly.

You DO have to change a global variable somewhere in the operating window of your analysis software to tell your software that they system impedance is now 75 ohms!

 

[mhtplsh]

Re: 50 Ohm TO 75 Ohm

Thanks biff44,
I hv carefully noted ur suggestions. I hv accordingly thought of following steps.
I saw a file in .xls from Maxim. Here they hv shown converted 50 ohm to 75 ohm S parameters. I can also fill in & 50 ohm s parameters & get 75 ohm S parameters.
I can get the 75 ohms input output impedance with the help of elekta programme.
Noise figure GO can be converted from 50 to 75 ohms.
In quicksmith ( smith chart software) Smith chart can be made 75 ohms.
In RFSIM99 simulator : We can define input output impedance as 75 ohms. Is it necessary to change the s parameters in .s2p file to 75 ohms or the same 50ohm .s2p file will be ok for simulation with 75 ohm input output impedance.
Will this be ok to go ahead?