Communication Systems/Phase Modulation

What is Phase Modulation?

Similar to FM (frequency modulation), is Phase modulation. (We will show how they are the same in the next chapter.) If we alter the value of the phase according to a particular function, we will get the following generalized PM function:

$s_{PM}=A\cos (2\pi f_{c}t+\alpha s(t))$

It is important to note that the fact that $-\pi <\alpha s(t)\le \pi$ for all values of t. If this relationship is not satisfied, then the phase angle is said to be wrapped.

Wrapped/Unwrapped Phase

The phase angle is a circular quantity, with the restriction $0=2\pi$. Therefore, if we wrap the phase a complete 360 degrees around, the receiver will not know the difference, and the transmission will fail. When the phase exceeds 360 degrees, the phase value is said to be wrapped. It is highly difficult to construct a communication system that can detect and decode a wrapped phase value.

PM Transmitter

PM signals can be transmitted using a technique very similar to FM transmitters. The only difference is that we need to add a differentiator to it:

Signal ---->|Differentiator|---->|VCO|----> PM Signal

PM Receiver

PM receivers have all the same parts as an FM receiver, except for the 3rd step:

1. A limiter, to remove abnormal amplitude values
2. bandpass filter, to separate the out-of-band noise.
3. A Phase detector, to convert a phase back into a voltage
4. A lowpass filter, to remove noise added by the discriminator.

Phase detectors can be created using a Phase-Locked-Loop (again, see why we discussed them first?).