Phase Locked Loop System Working

To understand the working of the phase locked loop system, let us consider the FM transmitter, which can be considered as one of the most frequently used PLL applications.

PLL circuit in FM transmitter is a closed loop feedback control system. This phase locked loop keeps the generated signal and reference signal in a fixed relationship. The main impracticality of using an FM transmitter can be counteracted by using this phase locked loop subsystem. If we use a simple transistor based oscillator circuit, then we can observe the drift in frequency. This drift in frequency is caused due to different instabilities such as load-induced in power rails, inherent oscillator, thermally induced, inertial forces, and so on. This drift in frequency causes problem at the receiving end and thus, the receiving person has to retune the radio or FM transmitter radio like car-radio or handheld radio.

The figure shows the block diagram of the phase locked loop system in FM transmitter that consists of different blocks such as a crystal oscillator, phase detector, loop filter, voltage controlled oscillator (VCO), and frequency divider.

Crystal Oscillator

It is the most important part of the phase locked loop system. The shortcomings of a common FM transmitter can be overwhelmed with stability. This stability can be provided by using a crystal oscillator.

The crystal oscillator circuit diagram is shown in the figure that is used in the FM transmitter system.

Voltage-Controlled Oscillator

Due to the recurring nature of the phase locked loop system, the performance of the system is analyzed starting from output to input. Thus, the most obvious thing to consider in PLL system is output which would be the VCO.

The voltage controlled oscillator circuit/FM modulator circuit is shown in the figure which is used to adjust the resonant frequency based on the error voltage received from preceding components of the PLL circuit. The varactor D1 (MV2101) acts as a voltage controlled capacitor, error voltage is incident upon this varactor, hence, changing the resonance. The general purpose NPN bipolar transistor PN2222A is configured as a Colpitts oscillator based on which voltage controlled oscillator works. If we remove this varactor and its associated inputs, then the oscillator operates as a general FM transmitter.

Oscillation frequency with varactor is given by equation

From the above equation, we can observe that, if the capacitance of the varactor D1 changes, then the resonant frequency also changes. And, by varying the variable capacitor C3 we can adjust to tune the voltage controlled oscillator circuit as the system that ranging from 88 MHz to 108 MHz (which is usually commercial radio’s bandwidth) based on the input to the varactor. In general, 2 turns of 22 gauge wire inductor is used.

Frequency Divider

A high speed 12-stage binary counter 74HC4040 is used as frequency divider in the phase locked loop system. The output of voltage controlled oscillator of PLL circuit is fed to this frequency divider as shown in the schematic diagram. To divide the FM input signal by 512 effectively, output Q8 of the binary counter is selected. Thus, the order of the original signal is 100 MHz, order of the quotient is 200 KHz, and crystal oscillator circuitry implements the proximity of the reference signal.

Phase Detector

If we consider two input signals, then to quantify the frequency difference and/or phase difference of these input signals a phase detector is used. In phase locked loop system, a simple EXOR gate is used to generate the error signal.

The error signal generated is a variable duty square wave that depends on the diversity of applied two input signals.

Loop Filter

The loop filter used in the phase locked loop system is equivalent to first order low pass filter. The error signal generated by phase detector is fed to loop filter for averaging or smoothing. This smoothing causes the charging and discharging of capacitor, as essential, in contradiction of varactor for adjusting the voltage controlled oscillator of PLL circuit. This process gets aborted, if and only if the error signals duty cycle becomes zero and voltage controlled oscillator & crystal based oscillator frequency are efficiently locked.

Thus, by using a phase locked loop in FM transmitter circuit and PLL in FM demodulator facilitates to carry highly stable and clean audio. Furthermore, by using the phase locked loop system in FM transmitter we can tune common FM transmitter even keeping crystal stability.

Hope this article given brief information about phase locked loop system used in FM transmitter. For more detailed information regarding the PLL IC, PLL circuits, and PLL applications you can approach us by posting your queries in the comments section below.