PLL

A PLL continuously compares the phase of a reference signal and a feedback signal, filters the error, and adjusts a VCO until the output frequency and phase are locked to the reference.

Input and output frequencies are locked in a relationship where output is some multiple/division of the input. PLL generates s stable frequency.

In most basic configuration, PLL compares the phase of a reference signal F_REF to the phase of an adjustable feedback signal RF_IN.

Reference signal -> phase detector -> low pass filter -> VCO -> output signal

output signal goes back into phase detector. this feedback input is where it does the multiplication by N

• Phase detector detects the difference in phase. It sees the phase error increasing, and if so, it generates error signal.
• VCO speeds up or slows down if it is told by the phase detector that there's error
• Low pass filter exists bc phase detector output is usually noisy or pulsed. It smooths the output into a smoother control voltage

§What does "lock" mean?

PLL forces reference frequency to be output_frequency / N Therefore: ouput_frequency = N * reference_frequency

N can be a decimal less than 1 and can be a very big whole number. But usually you can only set it to certain ratios. eg.

VCO = 1000 MHz

Divide by 2 -> 500 MHz
Divide by 4 -> 250 MHz
Divide by 8 -> 125 MHz

§Phase detector

This is needed because VCO might produce a different frequency despite receiving the same voltage as input due to several reasons:

• temperature changes
• supply voltage changes
• manufacturing variation
• aging
• noise It compares the VCO frequency to the reference signal. Reference signal comes from Quartz crystal which is likely gonna be reliable. Quartz has a very stable resonant frequency.

§VCO

VCO takes in a certain control voltage and outputs a frequency.

In control theory, it is modeled as

G(s)=k_v / s

k_v = VCO gain (hz/V or rad/s/V) s = Laplace variable

§Low pass filter

Needed for removing jitter or noise.

§Uses

Synchronization and demodulation circuits Clock recovery Noise and jitter reduction frequency synthesizers microprocessor