Question

6.2 Name the three key components of a phase locked loop and write a brief explanation of how each component works.

Answer 1

The Three elements of a Phae Locked Loop are:

1. The Phase Detector

2. The Loop Filter

3. Voltage Controlled Oscillator (VCO)

The Phase Detector

The Phase Detector takes two input signals and generates a voltage as output that represents the phase difference between the two input signals. In case of PLL, the two inputs are The reference input and Feedback from VCO. The output voltage of the Phase Detector is used to control VCO such that the phase difference between the two inputs is maintained constant., making it a negative feedback system.

The Loop Filter

The primary function of Loop Filter is to determine loop dynamics, also called stability. This is how the loop responds to disturbances, such as changes in the reference frequency, changes of the feedback divider, or at startup. Depending on the application, this may require one or more of the following: a simple proportion (gain or attenuation), an integral (low pass filter) and/or derivative (high pass filter). Loop parameters commonly examined for this are the loop's gain margin and phase margin. Common concepts in control theory including the PID controller are used to design this function.

The second common consideration is limiting the amount of reference frequency energy (ripple) appearing at the phase detector output that is then applied to the VCO control input. This frequency modulates the VCO and produces FM sidebands commonly called "reference spurs".

Voltage Controlled Oscillator

All phase-locked loops employ an oscillator element with variable frequency capability. This can be an analog VCO either driven by analog circuitry in the case of an APLL or driven digitally through the use of a digital-to-analog converter as is the case for some DPLL designs. Pure digital oscillators such as a numerically controlled oscillator are used in ADPLLs.