ANGLE MODULATION Angle Modulation Introduction Types of Angle

ANGLE MODULATION

Angle Modulation • Introduction • Types of Angle Modulation – FM & PM • Definition – FM & PM • Signal Representation of FM & PM • Generation of PM using FM • Generation of FM using PM

Angle Modulation Consider again the general carrier represents the angle of the carrier. There are two ways of varying the angle of the carrier. a) By varying the frequency, c – Frequency Modulation. b) By varying the phase, c – Phase Modulation

Frequency Modulation

Phase Modulation • One of the properties of a sinusoidal wave is its phase, the offset from a reference time at which the sine wave begins. • We use the term phase shift to characterize such changes. • If phase changes after cycle k, the next sinusoidal wave will start slightly later than the time at which cycle k completes.

Introduction to Angle Modulation • High degree of noise immunity by bandwidth expansion. • They are widely used in high-fidelity music broadcasting. • They are of constant envelope, which is beneficial when amplified by nonlinear amplifiers.

Introduction to Angle Modulation

FM and PM

FREQUENCY MODULATION

Frequency Modulation (FM) • Types of FM • Spectrum of FM • Transmission Bandwidth of FM • Deviation Ratio • Generation of FM • Detection of FM • Signal to Noise Ratio of FM • Pre-Emphasis & De-Emphasis in FM

Types of FM • Basically 2 types of FM: – NBFM (Narrow Band Frequency Modulation) – WBFM (Wide Band Frequency Modulation)

NBFM • Derivation • Difference between NBFM & AM DSB FC • Generation of NBFM

WBFM • Generating WBFM from NBFM • Derivation – Bessel Function

Spectrum of FM

Transmission Bandwidth of FM

Carson’s Rule

Deviation Ratio

Power in FM

Generation of FM • Mainly there are 2 methods to generate FM Signal. They are: 1. Direct Method 1. Hartley Oscillator 2. Basic Reactance Modulator 2. Indirect Method 1. Amstrong Modulator (Using NB Phase Modulator) 2. Frequency Multiplier

Generation of FM • Basically two methods: 1. Direct method • Build a voltage controlled oscillator (VCO) where the frequency is varied in response to an applied modulating voltage by using a voltage-variable capacitor • The main difficulty is that it is very difficult to maintain the stability of the carrier frequency of the VCO when used to generate wide-band FM. 2. Indirect method • Use a narrow-band FM modulator followed by frequency multiplier and mixer for up conversion. • Allows to decouple the problem of carrier frequency stability from the FM modulation.

Edwin Howard Armstrong ( 1890 - † 1954) Edwin Howard Armstrong received his engineering degree in 1913 at the Columbia University. He was the inventor of the following basic electronic circuits underlying all modern radio, radar, and television: Ø Regenerative Circuit (1912) Ø Superheterodyne Circuit (1918) Ø Superregenerative Circuit (1922) Ø FM System (1933).

Indirect Method – Amstrong Modulator

Indirect Method

Narrow Band Phase Modulator (NBPM)

Detection of FM • Types of FM Detectors: 1. 2. 3. 4. RL Discriminator Tuned FM Discriminator Balanced Slope Detector Centre Tuned Discriminator / Phase Discriminator / Foster – Seeley Discriminator 5. Phase Locked Loop (PLL) Demodulator 6. Ratio Detector

RL Discriminator

Signal to Noise Ratio of FM • Introduction to Signal to Noise Ratio (SNR) • Noise in FM – Capture Effect – FM Threshold Reduction – Pre-Emphasis & De-Emphasis in FM