Pulse Position Modulator Using 555 IC

Pulse position modulation is similar to pulse width modulation, but the frequency is not constant.  Like pulse width modulator circuit, pulse position modulator can be easily constructed using 555 IC.  Here is the schematic diagram of the circuit:

If you look at the output signal plot in the above timing chart, and compare it with our previous pulse width modulation, you can find the key difference: the pulse width modulator modulate the active factor (the duty cycle), t(on)/(t(on)+t(off)), without changing the total period of  t(on)+t(off), while pulse position modulator modulate the t(on) and keep t(off) fixed.  You can see that in the pulse-position-modulation, the frequency of the pulse is modulated as well as the duty cycle. [Schematic diagram source: National Semiconductor Application Notes]