Bandpower function question/problem

I am trying to use the Matlab bandpower function to measure power from noise sources. I originally did my own function but would prefer to use a function from Matlab. I have a problem with the results when I pass any additional inputs into the function to use the frequency space option.

I have included my code below, and hopefully as you look at the code you will understand what I am doing and my question.

_ %%CODE Starts below _

%Clear the workspace

close all

clear all

%Set the clock at 100MHz

fs=100E6;

period = 1/fs;

%Create 1ms time axis based on 100MHz clk

t=period:period:1E-3;

%Create random frequency for noise

f = rand(1,length(t))-.5;

sig1 = cos(2*pi*f.*t);

sig2 = sin(2*pi*f.*t);

%Subtract the median value to remove the DC bias

sig1 = sig1 - median(sig1);

sig2 = sig2 - median(sig2);

%Make it complex

sig1 = complex(sig1,sig2);

N = length(sig1);

fullPower = bandpower(sig1);

%Sig1 is even length and complex 

fullPower2 = bandpower(sig1,fs,[-(N-2)*fs/(2*N), fs/2] );

%%End of code

I think the variables fullPower and fullPower2 should have the same results because I feel I am including the entire frequency spectrum in both functions. Unfortunately they are not the same. What am I missing?

Answer 1

[Honglei Chen]

The calculation of the fullPower is simply the square of the signal's rms value. As to the fullPower2, the command line help indicates that it is computed using periodogram with a Hamming window. That's why they are slightly different.

HTH

[Paramonte]

and why is fullPower2 higher than fullPower.Should be the other way round since bandpower(sig1,fs,[-(N-2)*fs/(2*N), fs/2] ) is suposed to use a window over the signal which effectively decreases its power