How to transfer RF signal to Frequency Domain?

Hi Experts,

I tried to create a similar signal below. Now the Time Domain is worked only, it's strange to me.

I don't understand why that Frequency is incorrect when I plot on Frequency Domain.

Would you please share your comments?

Fin = 200e3; % Frequency Signal

DutFs = 200e6; % Sampling frequency

Impedance = 50;

t = 0:(1/DutFs):(1/DutFs)*3000;

OriSignal = (0.49505106597641246)*sin(2*pi*Fin*t); % Signal, 3.893 dBm.

HarmonicFin2 = (0.0012331048332289098)*sin(2*pi*Fin*2*t); % Harmonic2, -48.19 dBm.

HarmonicFin3 = (0.0015631476426409539)*sin(2*pi*Fin*3*t); % Harmonic3, -46.13 dBm.

HarmonicFin4 = (0.00011953635256737178)*sin(2*pi*Fin*4*t); % Harmonic4, -68.46 dBm.

MixedSignal = OriSignal + ... % Amplitude is from 1.15V to 0.15V.

+ HarmonicFin2...

+ HarmonicFin3...

+ HarmonicFin4 + 0.65;

subplot(2,1,1);

plot (t,MixedSignal), grid on;

title("TimeDomain");

ylabel("amplitude(V)");

xlabel("time(s)");

Power_dBm = 10*log10((((MixedSignal*0.707).^2)/Impedance)*1000); % Calculate dBm from Voltage.

NumberofSample = length(Power_dBm);

FreqRange = linspace(100e3,1000e3,3001); % Define X-Axis range.

power = fft(Power_dBm);

subplot(2,1,2);

plot(FreqRange,Power_dBm);

Answer 1

[Nithin]

Hi Simo,

I understand that you were able to plot the RF signal in the time domain and want to know how to plot the same in the frequency domain.

Kindly modify your code with the following steps to implement the same:

1. To plot the signal in the frequency domain, you should plot “abs(power)” against the corresponding frequency axis.
2. Modify the frequency axis based on the sampling frequency and the number of samples.

To plot the required signal in the frequency domain, kindly refer to the following code snippet:

Fin = 200e3;  % Frequency Signal
DutFs = 200e6;  % Sampling frequency 
Impedance = 50;
t = 0:(1/DutFs):(1/DutFs)*3000;                                                                                   
OriSignal = (0.49505106597641246)*sin(2*pi*Fin*t);  % Signal, 3.893 dBm.
HarmonicFin2 = (0.0012331048332289098)*sin(2*pi*Fin*2*t);  % Harmonic2, -48.19 dBm.
HarmonicFin3 = (0.0015631476426409539)*sin(2*pi*Fin*3*t);  % Harmonic3, -46.13 dBm.
HarmonicFin4 = (0.00011953635256737178)*sin(2*pi*Fin*4*t);  % Harmonic4, -68.46 dBm. 
MixedSignal = OriSignal + HarmonicFin2 + HarmonicFin3 + HarmonicFin4 + 0.65;

subplot(2,1,1);
plot(t, MixedSignal), grid on;                                  
title("Time Domain");
ylabel("Amplitude (V)");
xlabel("Time (s)");

Power_dBm = 10*log10((((MixedSignal*0.707).^2)/Impedance)*1000);  % Calculate dBm from Voltage.
NumberofSample = length(Power_dBm);    

% Calculate the frequency axis
deltaF = DutFs / NumberofSample;  % Frequency resolution
f = (-DutFs/2 : deltaF : DutFs/2 - deltaF);  % Frequency axis

power = fftshift(fft(Power_dBm));

subplot(2,1,2);
plot(f, abs(power));
title("Frequency Domain");
ylabel("Magnitude");

xlabel("Frequency (Hz)");
xlim([-1e5, 1e5]);

For more information regarding “abs”, “fft” and “fftshift”, kindly refer to the following documentation:

https://www.mathworks.com/help/matlab/ref/abs.html

https://www.mathworks.com/help/matlab/ref/fft.html

https://www.mathworks.com/help/matlab/ref/fftshift.html

I hope this answer provides you with the required information regarding your query.

Regards,

Nithin Kumar.