Duty cycle and spectra components

[Emil09]

I'm observing the signal spectra of a square wave signal by using the FFT function of the oscilloscope. I'm trying to understand what's going on when I change the duty cycle. Does anyone understand the relationship between duty cycle and the spectra components? The amplitude doesn't change but the peaks in-between the components change a little....

 

[MikeMl]

Duty Cycle effects the amount of energy in each of the harmonics of the fundamental frequency. For example, a 50/50 duty cycle has almost no energy at the EVEN harmonics; all of the energy is only at the ODD harmonics. Zoom the spectrum so you are only looking up to the tenth harmonic, and then play with the duty cycle...

 

[Emil09]

Duty Cycle effects the amount of energy in each of the harmonics of the fundamental frequency. For example, a 50/50 duty cycle has almost no energy at the EVEN harmonics; all of the energy is only at the ODD harmonics. Zoom the spectrum so you are only looking up to the tenth harmonic, and then play with the duty cycle...

Thanks for the reply. It looks like the fundamental component and the harmonics become larger as the duty cycle is increased. Sound reasonable?

 

[MikeMl]

There should be some duty cycles which enhance the even harmonics, particularly second, fourth, etc. Other duty cycles should enhance the odds, like third, fifth, etc.