My aim to is compute the modulation index(u) of a amplitude modulated waveform but I only have access to its power spectrum from the spectrum analyser.
I attached an image from the spectrum analyser.
(Note: The spectrum analyser has an input impedance of 50 ohms).
The carrier has a power of -26.6 dBm and the two sidebands have power of -38.5 dBm each.
Quite obviously we know the modulation index must be less than 1 because the carrier has more power - but how do I find the exact modulation index?
My thought process:
- First I converted the dBm to watts.
- Then I found the voltage using P=(V^2)/R, for carrier and sidebands and we know R would be like 50.
- Then I just did modulation index= mp/Ac where mp is the voltage in the message signal and Ac is the voltage in the carrier signal which is computed in step 2.
After doing all this I get a u of 0.25. This sounds kind of correct but what do you guys think?
***clear;clearvars;clc;clearAllMemoizedCaches m_db= -38.5 ; % message power dBm mpow= (10^(m_db/10))/1000 % message power in watts c_db= -26.6; % carrier power dBm cpow=(10^(c_db/10))/1000 % carrier power in watts mv= sqrt(mpow*50) % message voltage cv= sqrt(cpow*50) % carrier voltage mu= mv/cv % modulation index= message/carrier