Total Harmonic Distortion calculation problems in matlab.

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recently i was working on matlab simulink basically on converter application i came to the conclusion after running the fft analysis (i know how to save signal and run it in fft gui analysis) that the thd of the signal is not correct what is outputted by matlab. i tested single pwm inverter which produces square wave output it gave THD of 48.3% ... which is the actual thd of the single pwm inverter but when i use sinusoidal pulse width modulation having carrier frequecny of 1080 and operative frequency of 60 hz and modulation index 0.8. keeping the aspect that the primary frequency is 60hz same as with simple pwm inverter i get higher thd of 76% thd which is totally wrong. thd specifies the closeness of the control wavefrom with the sine waveform, applying spwm increases the switching frequency and hence increases the closeness to the sine waveform. but this results are not helpful the sampling time of the powergui block and the modulation scheme was equal. also the bars in the fft analysis were not showing just the word file which gives thd and harmonics 1 to 1600.
so i wanted to ask if i am doing something wrong .. but eventough except me other colleagues of mine are having the same problem of incorrect thd.
wanted help on this topic as soon as possible !

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