An Introduction to Phase-Locked-Loops and RF Oscillators

Dec. 28, 2018

In most RF systems, the production, transmission, and reception of a radio frequency signal is accomplished by way of a transmitter which amplifies and modulates the signal and a receiver system which feeds the signal into the mixer circuits to enable frequency conversion and demodulation. An oscillator is a circuit that generates a repetitive waveform, often an amplifier, with positive feedback. To control the rate of waveform repetition, frequency selection is accomplished by using capacitors and inductors or resistor and capacitor combinations. Phase-Locked-Loops (PLL) are an important class of oscillator which compares the oscillator output with a reference signal and generates a control voltage based upon the phase difference. The control voltage is then used to “lock” in the phase of the output signal.

Basic PLL Components

In other words, a PLL, is an electronic circuit that locks the phase of the output to the input by way of a negative feedback system that synchronizes the voltage controlled oscillator (VCO) output signal to the phase of a reference input signal. A PLL is made of a variable frequency oscillator which generates a periodic signal and a phase detector which compares the phase of that signal with the phase of the input periodic signal, adjusting the oscillator to keep the phases matched by way of a feedback loop. PLLs can be either analog or digital circuits and have a similar structure made up of four basic elements:

• Phase comparator/detector which is a circuit block component that detects and compares  a phase of two signals and generates a voltage based on the phase difference between the two signals; it generates the phase difference between input signals,

• Loop filter that filters phase comparator output and filters out signals from the voltage controlled oscillator (VCO) line and moderates the stability of the PLL by canceling high frequencies and keeping DC components away from the error signal,

• Voltage controlled oscillator (VCO) is the circuit block that generates the output radio frequency signal and is controlled by the error signal from the phase comparator/detector, and

• Feedback path which may include a frequency divider that directs the operational frequency band for the loop from the output signal to the phase detector.


PLL Applications

PLLs are used in many radio frequency applications and are necessary components in radio frequency equipment including radio receivers, test equipment, demodulators, and frequency synthesizers. Applications include:

• FM demodulation/AM demodulation

• Indirect frequency synthesizers.

• Signal recovery

• Timing distribution

Types of PLLs

• Analog Phase-Locked Loops (PLL/APLL)

First-Order PLL where the loop Filter frequency response is equal to single pole LP filter

Second-Order PLL has greater flexibility and complexity

Used in communication applications

• Digital Phase-Locked Loops (DPLL/ADPLL)

A DPLL has a digital phase detector and an analog VCO and loop filter

An All-Digital PLL (ADPLL) has all digital components

Used in non-communication applications

• Software Phase-Locked Loops (SPLL/ASPLL)

Mostly used for clock/data recovery

High flexibility such that many PLLs can be software implemented

• Neuronal Phase-Locked Loops (NPLL)

Artificial Neural Network (ANN) is a component in the feedback loop

Used for automatic speech recognition and temporal coding in biomedical simulations of the brain

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