S-Parameters (Cascaded system) and a few questions on RF Design. (IIP3)

Thread Starter


Joined May 25, 2021
Please see attached figure for a few of the questions. I would love for the solutions to these questions.

Also, what are the effects when the width of the microstrip line is increased?



Joined Aug 25, 2019
Have you ever though in using the free and easy to use RF Sim99 simulator ?
Question 1: In a 50 Ohms system a S11 of -10dB independent of frequency is obtain by using a 96 or 26 Ohms load. Add a 5 dB attenuator (50 Ohms ref again) in front of the mismatched load and you'll get the answer.
Question 2: As IP3 is related to non linearities, it is no obvious to get an answer...
Question 3: the quarter wavelength line is valid only at a certain frequency (and some multiples). There is therefore only one answer: OPEN at the end of the line => SHORT at the input. Again, use RFSim99 if you want to get the input impedance when frequency is swept.


Joined May 2, 2019
Question 1: The S11 of the DUT(magnitude in dB) is -10dB, this means that any signal that is in input to the DUT, the reflected power (S11) will be 10dB lower than the input. You are inputting 0dBm into the attenuator, it gets reduced by 5dB to -5dBm. So the reflected power will be -10dB of -5dBm or -15dBm. The reflected power then flows back into the attenuator and is reduced by 5dB. therefore your reflected power is -20dBm, which is -20dB S11. You can do this without any actual power levels since all of this (-10dB S11 or 5dB of attenuator) are relative measurements. This is assuming magnitude only, actual impedances will be dependent on the phase of S11.

Question 2: You would need to know the assumptions of each of the pieces before you can determine the value.

Question 3: With a quarter wave transformer, the formula for the the characteristic impedance of the transformer is Z0 = (Zs * Zl)^0.5, to determine the Zs=Z0^2/Zl. Because Zl = infinite(open), you get zero(short) at Zs. Based on the formula, changing the characteristic impedance of the quarter wave has no effect(by changing the width of the line, you change its characteristic impedance. This only works for the case of Zl = 0 or infinite. If you have real impedances, then changing the width will change the Zs if Zl is constant.

I agree with Yffig, you should get RF Sim99. It is free and it works pretty good. You can do a bunch of what ifs to help you understand what is happening.


Joined Aug 25, 2019
For question 4, the width of the microstrip line affects mainly and mostly the characteristic impedance Zo of the line: the wider the track, the lower the Zo. RF Sim99 has a very simple tool to compute the Zo and the velocity on the stripline. Experiment with it (Menu = Tools/Transmission Line/ Microstrip) .