# Help to plot a frequency response of a function

7 views (last 30 days)
Jimi Cunningham on 5 Dec 2022
Commented: Jon on 8 Dec 2022
I need to plot the following S21 function:
S11= 1/(2RojωC -1)
S21 = Ro/((-1/jωC)+Ro) * (1+S11)
I then need to plot the following S21 function:
S11= -RojωC/(RojωC-2)
S21= 1+ S11
The above euqations need to have:
C = 1.5pF
Ro = 50Ω
ω = 2π*(0GHz to 20GHz)
Intervals: 0.1 GHZ
x axis = Freqency
y axis = dB
Jimi Cunningham on 5 Dec 2022
It's doesn't have to be 0.1 steps, it can be continuous if that's an option

Jon on 5 Dec 2022
Edited: Jon on 5 Dec 2022
Here is a basic approach (not using any toolbox functions for frequency response), that might give you some ideas of how this can be done in MATLAB
% parameters
Ro = 50 % [Ohms]
fRange = [0 20e9] % [frequency range Hz]
interval = 0.1e9 % [plotting interval]
% create vector of frequency values to be evaluated
f = fRange(1):interval:fRange(2)
omega = 2*pi*f;
j = 1i; % define j (just in case j is used somewhere else as an index)
% evaluate functions at each frequency (vectorized)
S11 = 1.0 ./(2*Ro*j*omega*C -1);
S21 = Ro ./((-1.0 ./(j*omega*C))+Ro) .* (1+S11);
% find the magnitude of S21 and convert to dB
dB = 20*log10(abs(S21));
% plot results
plot(f,dB)
xlabel('frequency [Hz]')
ylabel('S21 [dB]')
Here is what running this code will give you
% parameters
C = 1.5000e-09
Ro = 50 % [Ohms]
Ro = 50
fRange = [0 20e9] % [frequency range Hz]
fRange = 1×2
1.0e+10 * 0 2.0000
interval = 0.1e9 % [plotting interval]
interval = 100000000
% create vector of frequency values to be evaluated
f = fRange(1):interval:fRange(2)
f = 1×201
1.0e+10 * 0 0.0100 0.0200 0.0300 0.0400 0.0500 0.0600 0.0700 0.0800 0.0900 0.1000 0.1100 0.1200 0.1300 0.1400 0.1500 0.1600 0.1700 0.1800 0.1900 0.2000 0.2100 0.2200 0.2300 0.2400 0.2500 0.2600 0.2700 0.2800 0.2900
omega = 2*pi*f;
j = 1i; % define j (just in case j is used somewhere else as an index)
% evaluate functions at each frequency (vectorized) note ./, .* for element
% wise operations
S11 = 1.0 ./(2*Ro*j*omega*C -1);
S21 = Ro ./((-1.0 ./(j*omega*C))+Ro) .* (1+S11);
% find the magnitude of S21 and convert to dB
dB = 20*log10(abs(S21));
% plot results
plot(f,dB)
xlabel('frequency [Hz]')
ylabel('S21 [dB]')
Jon on 8 Dec 2022
You can use semilogx for this. A quick way to see the documentation for functions is to type on the command line doc followed by the name of the command. So in this case type on the command line doc semilogx, and you will get all of the details on how to use the function.

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