Why does increasing duty cycle decrease the separation between spectral components? I know that the duty cycle measures the width of the signal in percentages. But why does it decrease the signal width with decreasing duty cycle?
At 50 % duty cycle a pulse wave is a square wave. A square wave has only odd harmonics (and a possible DC bias). At other values of the duty cycle there will also be even harmonics. The emergence of even harmonics can be seen as halving of the separation between spectral components.