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Phase Modulation Circuits

Published Date
Author Cadence PCB Solutions

Key Takeaways

  • The phase modulation circuit changes the phase of the carrier wave in accordance with the amplitude of the message signal.

  • In digital communication, phase modulation leads to efficient utilization of the available RF spectrum.  

  • Phase modulation is best for sound synthesizing applications due to its noise immunity characteristics. 

Amplitude modulation, frequency modulation, and phase modulation are some of the commonly used modulation techniques in wireless communication

Amplitude modulation, frequency modulation, and phase modulation are some of the commonly used modulation techniques in wireless communication

In wireless communication, signals are strengthened using modulation techniques before transmitting from the transmitter to the receiver. Amplitude modulation, frequency modulation, and phase modulation are some of the commonly used modulation techniques in wireless communication.

Similar to amplitude and frequency modulation circuits, a phase modulation circuit is used to change the phase of the carrier signal as required.

Comparing Phase Modulation With Amplitude and Frequency Modulation

Parameter

Amplitude Modulation (AM)

Frequency Modulation (FM)

Phase Modulation 

(PM)

Variable

Amplitude

Frequency

Phase

Constants

Frequency and phase

Amplitude and phase

Amplitude and frequency

Noise immunity

Poor

Good

Medium

Bandwidth

Narrow

Wide

Narrower than FM

Signal quality

Low SNR

High SNR

SNR less than FM

Application

Shortwave broadcasting, aviation, and marine communication

Commercial radio broadcasting

Mobile radio services

The Theory of Phase Modulation Circuits

Phase modulation is a modulation technique in which the phase of the carrier signal is changed with respect to the amplitude of the original message signal. The change in phase of the carrier wave is in a linear relationship with the instantaneous amplitude of the message signal. When the instantaneous value of the original signal is positive, the phase varies in a direction opposite to the direction of variation when the instantaneous amplitude is negative.

Mathematical Equation of the Phase-Modulated Signal

A is the amplitude of the carrier signal, ωc is the angular frequency of the carrier signal, and Φ(t) is the phase angle of the carrier signal, which is a function of the message signal.

Phase Modulation Circuit

Phase modulation circuit

Phase modulation circuit

As given by the aforementioned equation, the phase-modulated waveform changes the phase in accordance with the amplitude of the message signal. The phase-modulated signal can be generated using the phase-modulation circuit shown above.

The carrier signal sin ωct is converted to cos ωct by passing through the 90-degree phase shifter. The frequency-selective network superimposes the carrier signal and message signal and develops the output m(t)sin ωct, which is a function of sin ωct and sin ωmt, where ωm is the angular frequency of the message signal. The phase-shifted carrier signal and the output of the frequency-selective network are added together to form the phase-modulated signal.  

Phase Modulation in Digital RF Communication

In digital RF communication, phase modulation is more relevant than amplitude and frequency modulation. The table below gives some of the factors that influence the application of phase modulation in digital RF communication.

Factor

Explanation

Efficient use of available bandwidth

In digital communication, phase modulation leads to efficient utilization of the available RF spectrum. 

Noise immunity

In digital communication, signal integrity is compromised in noisy environments, as the amplitude varies with noise or interference. 

 

In phase modulation, the information is encoded as phase variations, and there is a resistance to vary the amplitude, which helps enhance the signal integrity compared to AM. 

Closely related to other digital modulation techniques

Phase modulation is the base of other digital modulation techniques such as quadrature phase shift keying, quadrature amplitude modulation, etc. It is easy to modify the phase modulation to other digital modulation techniques. 

Accurate estimation of phase at the receiving end

It is possible to accurately estimate the phase of the signal at the receiving end, and demodulation becomes easier. Signal recovery is reliable with phase modulation. 

Flexibility

Phase modulation offers the adjustment of the modulation index depending on digital RF communication scenarios such as noisy environments, channel impairments, etc. 

Phase Modulation Circuit Applications

  • Phase modulation circuits are the foundation of most digital modulation schemes. Phase modulation aids technologies used in satellite communication, GSM, WiFi, etc. 
  • Phase modulation is well-suited for sound synthesizing applications due to its noise immunity characteristics. 
  • Phase modulation helps generate waveforms and signals for various applications. 

Phase modulation circuits are widely used in digital synthesizers, signal generators, RF communications, etc. Cadence’s suite of design and analysis tools can be used to design mixed-signal environments that support analog and RF circuit development.

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