Can anyone help me to explain the following

1. What is phase comparator
2. What is the purpose of phase comparator
3. What is the component involve in phase comparator as in the symbol shown below

 

Hello,

Have a look at the attached PDF.

The shown schematic consists of two parts.
The gate in the first part creates a delay, this will result in small pulses on the output of the XOR gate.
These small pulses are filtered in the second part to create the multiplied frequency.

Bertus

 

Thanks this gives me an idea i will search further