Input-Output characteristics of phase detectors (PD) circuits.

i) A phase detector(PD) is a circuit whose average output, $V_{out}$, is linearly proportional to the phase difference, $\Delta \phi$, between its 2 inputs.

ii) In the ideal case, the relationship between $\bar{V_{out}}$ and $\Delta \phi$ is linear, crossing the origin for $\Delta \phi=0$, called the 'gain' of the PD, the slope of the line, $K_{PD}$ is expressed in V/rad.

iii) A familiar example of phase detector is the exclusive OR(XOR) gate. As shown in fig, as the phase difference between the input varies, so does the width of the output pulses, thereby providing a dc level proportional to $\Delta \phi$. While the XOR circuit produces error on both rising and falling edge.

iv) The operation of phase detector is similar to that of differential amplifier in that both sense the difference between 2 inputs, generating a proportional output.