In pulse position modulation, the amplitude and width of the pulses are kept constant, while the position of each pulse with reference to position of reference pulse, is changed according to the instantaneous sampled value of the modulating signal. Below shown circuit diagram is the simplest pulse position modulation circuit using IC-555.
With this type of circuit, the position of each pulse changes. Both width and period of the pulses vary with the modulating signal. Due to modulating signal at pin 5 ( Control voltage pin ), the UTP level changes to (2Vcc/3)+Vmod.
When Vmod increases, the UTP level increases and hence pulse width also increases. If Vmod decreases, UTP level decreases and pulse width also decreases. Thus the pulse width varies.
The pulse width is given by,
W = – (R1+R2) Cin [ (Vcc-UTP)/(Vcc-0.5UTP) ].
The period is given by,
T = W+0.693R2C
The space between the pulses which is constant is given by 0.693 R2C.
This circuit has many application a quick example is, it is used in communication application for transferring voice or data.