AM/FM Receiver
Communication Systems
We have studied the basic blocks of any
communication system
Modulator
Demodulator
Modulation Schemes:
Linear Modulation (DSB, AM, SSB, VSB)
Angle Modulation (FM, PM)
AM/FM Radio System
Principles:
Frequency Spectrum Sharing (many
transmitters using one medium)
Demodulating desired signal and rejecting other
signals transmitted at the same time
AM/FM Radio System
The source signal is audio
Different sources have different spectrum
Voice (speech)
Music
Hybrid signals (music, voice, singing)
AM/FM Radio System
Different audio sources have different
bandwidth W
Speech- 4kHz
High quality music- 15kHz
AM radio limits baseband bandwidth W to
5kHz
FM radio uses baseband bandwidth W to
15kHz
AM/FM Radio System
Radio system should be able to receive any
type of audio source simultaneously.
Different stations with different sources
transmit signals simultaneously.
Different listeners tune to different stations
simultaneously.
AM/FM Radio System
The different radio stations share the frequency
spectrum over the air through AM and FM
modulation.
Each radio station, within a certain geographical
region, is designated a carrier frequency around
which it has to transmit
Sharing the AM/FM radio spectrum is achieved
through Frequency Division Multiplexing (FDM)
Example of AM Radio Spectrum
Different radio stations, different source
signals
Fc0 Fc1 Fc2 F
Carrier spacing- 10kHz (AM)
Bandwidth (3-5kHz)
AM/FM Radio System
For AM radio, each station occupies a
maximum bandwidth of 10 kHz
Carrier spacing is 10 kHz
For FM radio, each station occupies a
bandwidth of 200 kHz, and therefore the
carrier spacing is 200 kHz
AM/FM Radio System
Transmission Bandwidth: B T
BT is the bandwidth occupied by a
message signal in the radio frequency
spectrum
B T is also the carrier spacing
AM: BT = 2W
FM: BT = 2( D + 1)W (Carsons Rule)
AM/FM Radio Receiver
Design of AM/FM radio receiver
The radio receiver has to be cost effective
Requirements:
Has to work with both AM and FM signals
Tune to and amplify desired radio station
Filter out all other stations
Demodulator has to work with all radio stations
regardless of carrier frequency
AM/FM Radio Receiver
For the demodulator to work with any radio
signal, we convert the carrier frequency
of any radio signal to
Intermediate Frequency (IF)
Radio receiver design can be optimized for
that frequency
IF filter and a demodulator for IF frequency
AM/FM Radio Spectrum
Recall that AM and FM have different radio
frequency (RF) spectrum ranges:
AM: 540 kHz 1600 kHz
FM: 88 MHz 108 MHz
Therefore, two IF frequencies
AM: 455 kHz
FM: 10.7 MHz
AM/FM Radio Receiver
A radio receiver consists of the following:
A Radio Frequency (RF) section
An RF-to-IF converter (mixer)
An Intermediate Frequency (IF) section
Demodulator
Audio amplifier
R F Tuner IF Filter Audio
Demodulator
Amplifier
AM/FM Radio Receiver
This is known as the Superheterodyne
receiver
Two stages: RF and IF
(filtering and amplification)
The receiver was designed by Armstrong
AM/FM Radio Receiver
RF Section
Tunes to the desired RF frequency, fc
Includes RF bandpass filter centered around fc
The bandwidth BRF
Usually not narrowband, passes the desired
radio station and adjacent stations
AM/FM Radio Receiver
The minimum bandwidth of RF filter:
BRF > BT
Passes the desired radio channel, and
adjacent channels
AM/FM Radio Receiver
RF-IF converter:
Converts carrier frequencyIF frequency
How can we convert signals with different
RF frequencies to the same IF frequency?
AM/FM Radio Receiver
Local oscillator with a center frequency f LO
f is a function of RF carrier frequency
LO
fLO = fc + fIF
R F Tuner IF Filter Audio
Demodulator
Amplifier
AM/FM Radio Receiver
RF-to-IF receiver includes:
An oscillator with a variable frequency f LO
(varies with RF carrier frequency)
By tuning to the channel, you are tuning the
local oscillator and RF tunable filter at the same
time.
AM/FM Radio Receiver
All stations are translated to a fixed carrier
frequency for adequate selectivity.
Fc
X FIF
FLO
AM/FM Radio Receiver
Two frequencies are generated at the output
of product modulator:
fLO+ fc = 2 fc + fIF
f LO f c = f IF
The higher frequency component is
eliminated through filtering
We are left with IF frequency
AM/FM Radio Receiver
One problem with this receiver:
Image Signal
Image signal has a center frequency:
f i = f c + 2 f IF
AM/FM Radio Receiver
If an image signal exists at the input of
the RF-to-IF converter, then the output of
the converter will include the desired signal
+ image signal
Fc
X FIF
FLO
fLO + fi = ( fc + fIF) + ( fc + 2 fIF) = 2 fc + 3 fIF
fLO fi = ( fc + fIF) ( fc + 2 fIF) = fIF
AM/FM Radio Receiver
Example: Incoming carrier frequency
1000 kHz,
Local oscillator = 1000+455=1455 kHz
Consider another carrier at 1910 kHz
If this is passed through the same oscillator,
will have a 1910-1455=455 kHz component
Therefore, both carriers will be passed
through RF-to-IF converter
AM/FM Radio Receiver
Therefore, RF filter should be designed to
eliminate image signals
The frequency difference between a carrier
and its image signal is: 2 f IF
RF filter doesnt have to be selective for
adjacent stations, have to be selective for
image signals
Therefore,
B T < B RF < 2 fIF
AM/FM Radio Receiver
IF filter:
Center frequency fIF
Bandwidth approximately same as transmission
bandwidth, B T
For AM: B T = 2W
For FM: BT = 2(D+1)W
AM/FM Radio Receiver
Depending on the type of the received
signal, the output of IF filter is
demodulated using AM or FM
demodulators.
For AM: envelope detector
For FM: frequency discriminator
