FREQUENCYMODULATION

INDATA TRANSMISSION

PRESENTED ON THE BASIS OF TECHNICAL REPORT BY :

RAJIB MODAKBISWAJIT MONDAL

&PRANAB KUMAR BANDYOPADHYAY

What is MODULATION ??

What is FREQUENCY ??

What is DATA ??

What is TRANSMISSION ??

Modulation

Continuous Wave Modulation

Pulse Wave Modulation

Linear Modulation

Non-linear Modulation

Amplitude Modulation

Frequency Modulation

Phase Modulation

What is Frequency Modulation ?? Carrie

r 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

GENARATION OF NARROW-BAND FM SIGNAL :

Integrator Product Modulator

Phase Shifter

_

+

NBFMem(t)

∑

Accos(2Πfct)

Narrow – Band Phase Modulator

Bandwidth Of FM: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)

Difference Between AM & FM :1>In case of AM instantaneous phase contains baseband

Signal. But in case of FM that contains baseband as wellas higher order odd harmonics.

2>AM signal is amplitude modulated , but FM is amplitudeAs well as frequency modulated.

3>

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

How we create FM Signal ?

i>Indirect Method

ii>Direct Method

Indirect Method of FM Generation :-----

IntegratorNarrowband

Phase Modulator

Crystal Oscillator

FrequencyMultiplier

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

Basebandsignal

Crystal Oscillator

f1 =0.1 MHz f2 =8.5 MHz

fc=100MHzΔf=75KHz

Direct Method of FM generation:----

VCOFrequency Multiplier

MixerFrequency 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 Discriminator

MixerCrystal

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 multiplier2. A loop filter3. 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 Modulator

C

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

The

STEREO FM

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, received and 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??Compatibility issue

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

WHAT IS A LIMITER DISCRIMINATOR ??

What is a preemphasizer

??

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.

A preemphasizer Network

What is a deemphasizer

??

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.

A Deemphasizer Network

QUESTIONS ARE WELCOME