How to draw the power spectral of the PWM and calculate THD in Matlab?

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clarken

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i am doing the final year project about class D amplfier , use some algorithm to produce PWM , then pass to the power amplifier and low pass filter to generate sound.

Since i finish to build the class D amplifier in fypga and also able to see the Power spectral of the generated PWM to measure the THD of the board. The only thing i need to do now is use the matlab to see the Power spectral of the PWM inside the matlab.

I already can produce the PWM in matlab.But i did not know how to draw the power spectral of the PWM and calculate THD. Anyone know, please help me .

My system start with the signa sine wave 5khz, carrier frequency is 200khz , so the PWM had the frequency is 200khz, and will contain the information of 5khz.
Thanks
 

Re: matlab simulation

Hi you First try this with simulink
 

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matlab simulation

i knew something about the fft function , but i do not know how to choose the N for my application. Each period (1/200khz) will have 4096 point ( some will be 1 and some will be zero--> PWM) .... The duty cycle is not fixed but changed.

So what i should choose N value for my system and how to draw the power density spectral ?

All i read is a example for the sine or cosine wave, i do not know how to apply it
 

matlab simulation

Normally by default 512 point fft is seen at the ouput. You can see psd at the output using following command
where x is filtered output


y=fft(x,512);
df=fs/512;
m=fftshift(abs);
m=m.^2;
m1=20*log10(m);
p=unwrap(angle);
f = -(fs/2)+df:dffs/2);
plot(f,m1);title('magnitude')
 

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