Author Topic: Some basic calculations from s parameters  (Read 142 times)

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Offline Cyberduke

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Some basic calculations from s parameters
« on: February 23, 2021, 03:00:46 pm »
Hi everyone, this is suppose to be simple, but I am somehow not getting the right answers.

Imagine you have the s Parameters, send power and Z0 of a 2 port network, and want to calculate the current flowing into the first port, and voltage over the second. Here is what I have:

  • Power going into the port is Pin = ptx(1-|s11|^2)
  • Z11 = Z0((1+s11)/(1-s11))
  • lastly the current I = sqrt(Pin/|Z11|)

and for the voltage

  • Prx = |s21|^2 Ptx
  • U = sqrt(Prx*Z0)

When I am doing the calculations, the answers that I am getting it most definitely very wrong, but I am not sure where. I am sure someone would point out something I did wrong quick! thanks
« Last Edit: February 23, 2021, 04:58:44 pm by Cyberduke »
 

Offline radiolistener

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Re: Some basic calculations from s parameters
« Reply #1 on: February 23, 2021, 05:07:01 pm »
  • Prx = |s21|^2 Ptx
  • U = sqrt(Prx*Z0)

When I am doing the calculations, the answers that I am getting it most definitely very wrong, but I am not sure where. I am sure someone would point out something I did wrong quick! thanks

Urx = sqrt(Prx * Zrx)

where:
Prx is a power on the load
Urx is RMS voltage on the load
Zrx is a load impedance (and it may not be equals to Z0 of port2 on your network analyzer)

Note, each port (network analyzer and network) has it's own Z and they can be different


Can you provide calculation example, because it's unclear what do you mean when you talking about wrong answer.

« Last Edit: February 23, 2021, 05:40:29 pm by radiolistener »
 

Offline radiolistener

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Re: Some basic calculations from s parameters
« Reply #2 on: February 23, 2021, 05:41:48 pm »
can you please provide calculation example? (with numbers)
Because it's unclear what you're talking about.

For example:
Zrx = 50 Ohm
Ptx = 10 W
|s21| = 0.31623

Prx = |s21|^2 * Ptx = 0.31623^2 * 10 = 0.1 * 10 = 1 W

Urx = sqrt(Prx * Zrx) = sqrt(1 * 50) = 7.071 Vrms

Note: |s21| depends on Zrx.
« Last Edit: February 23, 2021, 05:53:17 pm by radiolistener »
 

Offline Cyberduke

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Re: Some basic calculations from s parameters
« Reply #3 on: February 23, 2021, 05:58:47 pm »
Hi, thank you for the replies, I figured it out.

As embarrassing as it is, the Maths was fine, the issue was in the code that did the calculation.

For interest sake, the second example that you showed is exactly right and what I meant.
 


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