ISLAMIC UNIVERSITY OF TECHNOLOGY
ORGANISATION OF ISLAMIC COOPERATION (OIC)
GAZIPUR, BANGLADESH
EEE 4541
Wireless Communication
Superheterodyne Receiver (Superhet)
Dr. Mohammad Tawhid Kawser
Professor, EEE Dept.
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)
Audio Bandwidth
• Different audio sources have different
bandwidth “W”
– Speech- 4 kHz
– High quality music- 15 kHz
– AM radio limits “baseband” bandwidth W to about
5 kHz (capturing some music components)
– FM radio uses “baseband” bandwidth W to 15 kHz
Bandwidth
– Bandwidth approximately same as transmission
bandwidth, BT
– For AM: BT = 2W
– For FM: BT = 2( D + 1)W
ITU Regions (Primarily)
• ITU Region 1: Europe, Africa, the former Soviet
Union, Middle East
• ITU Region 3: The rest of Asia, Australia
• ITU Region 2: North/South America, Greenland
ITU Regions
AM Radio
Double Sideband (with carrier) Amplitude
Modulation is used allowing low receiver cost.
• Channel Bandwidth in ITU Regions 1 and 3: 9
kHz
• Channel Bandwidth in ITU Region 2: 10 kHz
AM Radio Frequencies
Low frequencies are used allowing wide coverage
• ITU Regions 1 and 3
– Carrier frequency: from 531 to 1602 kHz, with 9
kHz spacing
– Frequency range: 526.5–1606.5 kHz
• In ITU Region 2
– Carrier frequency: 530 to 1700 kHz, with 10 kHz
spacing
– Frequency range: 525–1705 kHz
FM Radio
• Channel Bandwidth: 200 kHz, and therefore the
carrier spacing is 200 kHz
• Frequency range: 88 MHz – 108 MHz
Superheterodyne Receiver
Superheterodyne Receiver
• The RF amplifier can amplify any channel
captured by the antenna and thus, its bandwidth
must be very wide. Consequently, it can offer very
poor Gain.
• For the amplifier/demodulator to work with any
radio channel, the carrier frequency of any radio
channel is converted to Intermediate Frequency
(IF)
• This allows the IF amplifier to have operating
bandwidth very small and thus, its Gain can be
very high.
Superheterodyne Receiver
• A radio receiver consists of the following:
– A Radio Frequency (RF) section
– An RF-to-IF converter (mixer)
– An Amplifier for IF frequency
– Demodulator for IF frequency
– Filter for IF frequency
– There can be further amplification for IF
frequency (e.g. power amplifier)
Superheterodyne Receiver
• Intermediate Frequency (IF) frequencies
– AM: 455 kHz
– FM: 10.7 MHz
Superheterodyne 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
Superheterodyne Receiver
• The minimum bandwidth of RF filter:
BRF > BT
• Passes the desired radio channel, and
adjacent channels
Converts carrier frequency IF
frequency
• Local oscillator with a center frequency f LO
• f is a function of RF carrier frequency
LO
fLO = fc + fIF
Superheterodyne 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.
Superheterodyne Receiver
• Two frequencies are generated at the output
of mixer:
fLO + fc = 2 fc + fIF
fLO − fc = fIF
• The higher frequency component is
eliminated through filtering
• We are left with IF frequency
Image Signal: A Problem
• Image signal has a center frequency:
fi = fc + 2 fIF
Superheterodyne Receiver
• Example: Incoming carrier frequency 1000 kHz
• Local oscillator = 1000 + 455=1455 kHz
• Consider another carrier at 1910 kHz. If it uses
the same oscillator, it will also create a 1910-
1455=455 kHz component
• Therefore, both carriers will be passed through
RF-to-IF converter
Superheterodyne Receiver
• Therefore, RF filter should be designed to
eliminate image signals
• The frequency difference between a carrier
and its image signal is: 2 fIF
• Therefore,
BT < BRF < 2 fIF
