Frequency Modulation In Data Transmission

Modulation

Continuous Wave

Modulation

Pulse Wave

Modulation

Linear Modulation

Non-linear

Modulation

Amplitude Modulation

Frequency Modulation

Phase Modulation

Carrier Signal

Baseband Signal

Modulated Signal

There is two types of Frequency Modulation

with respect to bandwidth:---

i> Wide Band Frequency Modulation

ii> Narrow Band Frequency Modulation

IntegratorProduct

Modulator

Phase Shifter

_

+

NBFMem(t)

∑

Accos(2Πfct)

Narrow – Band Phase Modulator

Theoretically the Bandwidth Of FM is infinite (for

Sinusoidal Modulation). But practically FM wave is

effectively limited to a number of significant side

frequencies (almost 200 kHz).

From Carson’s rule approximation Bandwidth of FM - -

BT = 2 f + 2fm

BT = 2 f (1 + 1/β)

In case of AM instantaneous phase contains baseband

Signal. But in case of FM that contains baseband as well

as higher order odd harmonics.

AM signal is amplitude modulated , but FM is amplitude

As well as frequency modulated.

The bandwidth of FM wave is much larger than AM wave.

Indirect Method of FM Generation :-----

Integrator

Narrowband

Phase

Modulator

Crystal

Oscillator

Frequency

Multiplier

Base Band

Signal

FM

Signal

Indirect Method to Generate WBFM for Practical

use:-

IntegratorNarrow

Band Phase

Modulator

Frequency

Multiplier

(*100)

Frequency

Multiplier

(*75)

Crystal

Oscillator

Mixer FM

signal

Baseband

signal

Crystal

Oscillator

f1 =0.1 MHz f2 =8.5 MHz

fc=100MHz

Δf=75KHz

Direct Method of FM generation:----

VCOFrequency

MultiplierMixer

Frequency

Multiplier

Fixed

Oscillator

Bandpass

Filter

WBFMBase band

Signal

DISADVANTAGE: very poor frequency stability due to unstable basic oscillator unlike

crystal oscillator.

Feedback Scheme For Frequency Stabilization

of a Direct Frequency Modulator:-

VCO

LPF &

Amplifier

Frequency

DiscriminatorMixer

Crystal

Oscillator

Base Band

Signal

Frequency stabilized

FM wave

Demodulation of Frequency Modulated Wave:-

Frequency Demodulation is the process that enables

us to extract the original modulating signal.

There are two basic methods for demodulating the FM

waves:

1. Based on Frequency Discrimination

2. Based on Phase Locked Loop

Frequency discriminator:-

A frequency discriminator is an FM demodulator which

must produce an output voltage linearly dependent on

input frequency.

Frequency Discrimination(Slope Detection)

technique:

Envelope

Detector

H(f)

s(t)sc(t) sd(t)

Sc(t)S(t)

Sd(t)

t

t

t

|H(f)|

f0 fcf

Problems:-

1>The detector also responds to spurious

amplitude variation of the input FM.

2> The range of linear slope is quite small.

Frequency Demodulation using Phase Locked Loop:-

PLL is useful in FM demodulation in presence of large noise and

low signal power. Thus it finds application in space vehicle to

earth data links.

Recently it is being used in Commercial FM receiver.

Phase Locked Loop:-A PLL is basically a negative feedback system. It consists of three major

components:

1. A multiplier

2. A loop filter

3. A voltage controlled oscillator(VCO)

These are connected together in the form of a feedback loop.

X Loop Filter

VCO

FM wave

s(t)

e(t)v(t)

r(t)

Circuit for Direct FM generation:-

OSCILLATORSound Wave

Condenser

Microphone

CL

FM

Varactor Diode Modulator:-

FM

-Vcc

Cc

C LVaractor

Diode

em(t)

Practical Reactance Modulator:-

+Vcc

RFCR1RFCR1

OscilatorReactance

ModulatorC

R2

R

Modulating

signal

CB

CE

CC

R2

CE

RE RE

C1

C2 C

FM

output

FM Receiving System:-

Frequency

changer

circuit

FM receiver circuit:-

The application circuit to generate FM(88-108 MHz) with stereo headphone amplifier

What Stereo Means :-

How then comes the virtualization ??

What do we need for the transmission?• We need two different signals L(left) &R(right) to be modulated, transmitted, receivedand demodulated duly without getting distorted

We need :----

a.An input signalb.Two preemphasizersc.A DSB-SC modulatord.A frequency doublere.A pilot signalf. A summerg.An FM modulator

Why do we need L + R & L – R??

A mono phonic receiver will work with only the L+R component and there will no loss of AUDIO(so the signal will have the backward compatibility)

We need :----

a. An input signal (through a LIMITER DISCRIMINATOR)

b. A Low Pass Filter(0-15 KHz)c. A Bandpass Filter(19 KHz)d. A Bandpass Filter( 23 – 53 KHz)e. A pilot signalf. A Frequency Doublerg. A Synchronous Detectorh. Two Deephasizers

Preemphasizer refers to a system

process designed to increase, within a band of frequencies, the magnitude of some (usually higher) frequencies with respect to the magnitude of other (usually lower) frequencies in order to improve the overall signal-to-noise ratio by minimizing the adverse effects of such phenomena as attenuation distortion or saturation of recording media in subsequent parts of the system.

Deemphasizer refers to a system process designed to decrease, within a band of frequencies, the magnitude of some (usually higher) frequencies with respect to the magnitude of other (usually lower) frequencies in order to improve the overall signal-to-noise ratio by minimizing the adverse effects of such phenomena as attenuation distortion or saturation of recording media in subsequent parts of the system.

QUESTIONS ARE WELCOME