Eastern Mediterranean University

Faculty of Engineering

Department of Electrical and Electronic Engineering

EENG 360 Communication System I Laboratory

LAB 9

FREQUENCY MODULATION & DEMODULATION

Objectives:

Understanding the principle of frequency modulation (FM) and

demodulation

Understanding the waveforms of modulated and demodulated signals

Designing frequency modulator and demodulator using Simulink

Theory

Phase modulation (PM) and frequency modulation (FM) are special cases of

angle-modulated signaling. FM is a process in which the carrier frequency is

varied by the amplitude of the modulating signal (i.e. intelligence signal).

Angle modulated signal can be expressed by the following equation:

( ) [ ( )] (1)

For FM, the phase is proportional to the integral of message signal ( ),

therefore

( ) [ ∫ ( )

] (2)

where is the frequency deviation constant and is the carrier frequency.

Fig.1: Frequency modulation waveforms

Frequency demodulator, also called frequency discriminator, is a circuit, which

converts instantaneous frequency variations to linear voltage changes. There are

many types of circuits used in communication systems such as FM to AM

conversion, phase-shift discriminators and phase-locked loop (PLL) frequency

demodulator. In this experiment we will introduce the operations of phase

discrimination method as demodulation. The illustration of FM demodulation is

shown in Fig.2.

Fig.2: Illustration of a frequency demodulation

Procedure

1. Arrange the functional blocks as shown in Simulink model.

2. Assign required parameters to each functional block.

3. Observe the outputs on scope.

Simulink Model

Parameters

Output

Assignment

1. Define FM and draw its spectrum.

2. Use freqspectrum function in MATLAB to verify your answer.

3. What are the advantages of angle modulation over amplitude

modulation?

4. What is the relationship between phase modulation (PM) and FM?