TABLE OF CONTENTS Phase Locked Loops (PLL) are very important blocks in a linear system. They are used for synchronization in radar applications, satellite communication, airborne navigational systems, frequency modulation communication systems, and more.
An input signal is given as one input of the phase detector. The other input to the phase detector is the output of a voltage- controlled oscillator. The output of the phase detector V e becomes the input of a low-pass filter. The output of that filter V e ? becomes the input of an amplifier. The output of the amplifier V c becomes the control voltage for the voltage-controlled oscillator.
The voltage controlled oscillator (VCO) mentioned here is nothing but a free-running multivibrator, i.e., astable multivibrator. Its normal frequency of oscillation is f 0 and its normal voltage is V 0. By applying a DC voltage as control voltage for the VCO, the frequency of its oscillation can be shifted in both directions. The frequency deviation of this device is proportional to the DC control voltage, hence its name.
Operation. Let the assumption be made that the input signal has an amplitude V s and a frequency f s.
A phase detector compares two input signal frequencies and the phase of them. If they differ, it will produce an output error voltage, otherwise not. If f s ? f 0, then V e is produced. In general,...
