FM Transmitter

• FM Broadcast Range 88 to 108 MHz.

• Channel Separation 200 KHz

• Frequency Deviation ∆ = 75 KHz • Frequency Deviation ∆ = 75 KHz

• FM Broadcast

� Monophonic

� Stereophonic ( Left and Right Channel)

FM Stereo Transmitter

• Signals L + R and L – R are Pre-Emphasized.

• Input Audio Signal (L-R)’ is Converted to NBFM by DSBSC

Modulator. Uses Armstrong Method of FM Generation

• Carrier Used for this Modulation is 38 KHz.

• This carrier is Obtained from Pilot Source of 19 KHz via

Frequency Doubler.

• Signal (L+ R)’ is used Directly

FM Transmitter

Composite Baseband Signal for Final Wideband FM

Modulation is

Pilot is Kept at 19 KHz as on its both side there is no signals up to 4 KHz.

FM receiver• FM receiver is similar to the

superheterodyne layout

• Only Difference is Envelope Detector is replaced by Limiter – Discriminator and De-Emphasis Circuits

• RF is 88 to 108 MHz, IF is 10 MHz.

RF

mixer

LO

limiterDiscrimi-

natordeemphasis

AF power

amp

IF

Receiver components:

RF amplifier

• AM may skip RF amp but FM requires it

• FM receivers are called upon to work with

weak signals (~1µV or less as compared to 30

µ

weak signals (~1 V or less as compared to 30

µV for AM)

• An RF section is needed to bring up the signal

to at least 10 to 20 µV before mixing

Limiter

• A limiter is a circuit whose output is constant

for all input amplitudes above a threshold

• Limiter’s function in an FM receiver is to

remove unwanted amplitude variations of the remove unwanted amplitude variations of the

FM signal

Limiter

Limiting and sensitivity

• A limiter needs about 1V of signal, called

quieting or threshold voltage, to begin

limiting

• When enough signal arrives at the receiver to • When enough signal arrives at the receiver to

start limiting action, the set quiets, i.e.

background noise disappears

• Sensitivity is the min. RF signal to produce a

specified level of quieting.

Sensitivity example

• An FM receiver provides a voltage gain of

200,000(106dB) prior to its limiter. The limiter’s

quieting voltage is 200 mV. What is the

receiver’s sensitivity?receiver’s sensitivity?

200 mV/200,000= 1µV->sensitivity of receiver

Discriminator

• The heart of FM is this relationship

• What we need is a device that linearly follows

inst. frequency

fi(t)=fc+kfm(t)

inst. frequency

Disc.output

f

Deviation limits

+75 KHz-75 KHz

fcarrier

fcarrier is at the IF frequency

Of 10.7 MHz (Fixed for all

channels)

Examples of discriminators

• Slope detector - simple LC tank circuit

operated at its most linear response curve

output

fc fo

output

f

Phase-Locked Loop

• PLL’s are increasingly used as FM

demodulators and appear at IF output

Phase Lowpassfin Error signal

Output proportional toDifference between fin and fvcoPhase

comparatorLowpass

filter

VCO

fin Error signal

fvcoVCO input

Control signal:constantWhen fin=fvco

Zero crossing detector

Hard

limiter

Zero

Crossing

detector

Multi-

vibrator

Averaging

circuit

FM Output

FM input

Hard limiter

ZC detector

multiV

more frequent

ZC’s means

higher inst freq

in turn means

Larger message

amplitudes

Averaging circuit

FM Receiver• After Limiter and Discriminator, Signal is Passed through various Filter

Circuits.• Output of Filters is De-Emphasized and Audio Signals obtained as L and

R separate channels of Stereophonic Receiver.• Pilot Freq. Signal after extracted from Filter Used as Synchronous

Detector Carrier for DSBSC Modulated Waves.

Noise In Communication System

A. External Noise:

(1)Atmospheric Noise

(2)Extraterrestrial Noise

B. Internal Noise:

(1) Thermal Agitation

Noise(2)Extraterrestrial Noise

• Solar Noise

• Cosmic Noise

(3) Industrial Noise

(2) Shot Noise

(3) Transit Time Noise

Noise Calculations

(1) Addition of Noise Due to Several Sources

(2) Addition of Noise Due to Several Amplifier

NOISE IN ANALOG MODULATION

AMPLITUDE MODULATION

Channel model

•Distortionless

•Additive White Gaussian Noise (AWGN)

No/2Flat noise spectrum:white noise

W-W

No/2Flat noise spectrum:white noise

Noise power=hatched area

Receiver Model

• The objective here is to establish a relationship

between input and output SNR of an AM

receiver

BPF detector

Noise n(t)

Modulated signal s(t)

output

filter

fc-fc

BT=2W

Establishing a reference SNR

• Define “channel” SNR measured at receiver

input.

=

The output of the product modulator isThe output of the product modulator is

Therefore, any reduction in input SNR is

linearly reflected in the output.

• Following a similar approach,

( )( )

index modulation AM:

11

2

2

k

Pk

Pk

SNR

SNR

a

a

c

o <+

=

• Best case is achieved for 100% modulation index which, for tone modulation ( Ka

2 = P/2), is only 1/3

power message avg.:P

DSB-AM and DSB-SC noise

performance

• An AM system using envelope detection needs

3 times as much power to achieve the same

output SNR as a suppressed carrier AM with

coherent detectioncoherent detection

• This is a result similar to power efficiency of

the two schemes

Threshold effect-AM

• In DSB-AM (not DSB-SC) there is a

phenomenon called threshold effect

• This means that there is a massive drop in

output SNR if input SNR drops below a output SNR if input SNR drops below a

threshold

• For DSB-AM with envelope detection, this

threshold is about 6.6 dB