Communication System

Super Heterodyne Receive of AM and FM

VCO
PLL
Emphasis (Pre Emphasis And De Emphasis)
 Problem Statement
At transmission side: In Transmitter circuit the
signal is process and transmit at higher
frequency which save cost
Example. The low frequency required high size
antenna thus high frequency reduce antenna
size which reduce cost
At receiver side: The High frequency signal
processing required higher circuit cost so it is
preferred to processing it in low frequency.
 Problem Statement
• The Super Heterodyne Receiver convert high
Radio Frequency into low Intermediate
frequency and then demodulate the signal
• The Heterodyne mean mixing of two
frequencies. In this high Modulated Radio
frequency is mixed with Local oscillator
frequency to form an Intermediate frequency.
Super Heterodyne Receive Demodulate AM
Signal
 RF Amplifier
• This block catches the signal through antenna.
• The incoming signal is very weak and noisy so
its filter and amplify it for further processes
 Mixer
The mixer multiply the incoming RF Frequency
with local oscillator frequency so that it create
two different frequency signals
cos(α) * cos(β) = (cos(α + β) + cos(α - β) )/2
One is addition term
cos(α + β) /2
One is Difference term
cos(α - β) / 2
 Mixer
Example: If the local oscillator frequency is FOSC =
1455 kHz, and incoming radio signal frequency is FRF =
1000 kHz, then the mixer block will produce two IF
frequencies,
FOSC + FRF = 1455 + 1000 = 2455
FOSC – FRF = 1455 – 1000 = 455
The frequencies for IF is 455 kHz is pass through IF Filter
and other frequency 2455 kHz will be rejected by filters.
FIF = FOSC – FRF
 IF Amplifier and Filter
• It is Band Pass Filter with pass band frequency
of 455 KHZ.
• This block Filter and Amplify the incoming
frequency

455 KHz IF Filter Crystal

(Ceramic resonator)
 Detector
In Detector Block the AM Demodulation can be
from anyone of the following method
• Diode Detector (Non Coherent demodulation)

• Product Detector (Coherent demodulation)

 Audio Frequency Amplifier
The Audio Frequency Amplifier amplify the weak
signal and give of speaker
Super Heterodyne Receive of AM Signal
Views at different stages
 Super Heterodyne Receive Demodulate FM
Signal

The FM receiver process is almost similar as Super

Heterodyne Receive of AM except few block
 RF Amplifier and Mixer
The first two block is same as discuss in AM. The
only difference is the IF frequency is 10.7MHz
 IF Amplifier
The working of IF amplifier is also same except
the IF frequency is 10.7 MHz

multipole ceramic resonator

 Automatic Gain Control
• AGC (Automatic Gain Control) maintain the
amplitude of signal. It generate DC Voltage
according to the signal voltages variation and
give this DC voltage to its previous block which
maintain the incoming signal
 Limiter
• This part is used to remove noise
• Also it remove any amplitude variation in the
receiver signal.
 Discriminator
The FM Demodulation is done here. There are
several type of circuit used for FM Demodulation
• Phase Locked Loop
•Radio Detector
•Foster Seeley FM
•Quadature Detector
 De Emphasis and Audio Amplifier
• After Demodulation of FM the De emphasis is
done to reduce the effect of Pre Emphasis
which was done at transmitter side
• At last the Audio signal is amplify and sent to
Speaker
 VCO
The VCO is Voltage Control Oscillator circuit
produce oscillation depend upon following
consideration:
a. The VCO Circuit produce output oscillation
proportional to the input voltages that’s why
called Voltage Control
b. Another way to control oscillation of VCO is
by RC (Resistor and Capacitor) circuits.
VCO
 FM Modulation using VCO
VCO can be used to generate FM signal. First the
carrier frequency in absent of Input voltages (by
setting voltage to 0V) and set the value of
Resistor and Capacitor (This condition is called
Free Running Mode). Now by applying message
signal the output frequency is varying with
respect to its voltages level.
 Phase Locked Loop (PLL)
• A phase-locked loop or phase lock loop (PLL) is
a control system that generates an output
signal whose phase is related to the phase of
an input signal.
Phase Locked Loop
 PLL (Phase Locked Loop)
PLL Free Running Frequency: At starting the when no
input is applied at reference signal the Phase detector
and LPF produce none. The VCO frequency is running
on R and C values.
Lock Range: The range of frequencies over which PLL
track the input frequency and remain lock is known as
Lock Range.
Capture Range: When PLL is in lock range and any
variation in input signal will detect by Phase detector
and it force VCO set its frequency equal to incoming
signal. This is known as Capture range
Phase Locked Loop
 PLL Application
FM Demodulation: The FM demodulation is done through PLL. First
the carrier frequency of FM is set on RC circuit of VCO. When FM
signal is applied the Phase detector compares it constant carrier
frequency and generate voltages according to the difference
between them.
 PLL Application
Frequency Multiplier: The PLL can be used to
generate frequency multiples of 2N of the Input
frequency. To generate 2N frequency the
feedback divided by N counter.
 Noise in AM and FM
• The Noise distribution in AM is constant thus known
as Rectangular AM Distribution of Noise
• In FM the effect of noise is increase with increase of
frequency. Mean higher frequency has more noise
effect the lower frequency.
 Noise in AM and FM
• As Noise in AM is uniform so it can filter out
easily.
• The noise in FM has non uniform response its
SNR (Signal to Noise Ration) is not constant
At Lower Frequency SNR > 1
At Higher Frequency SNR < 1
To over this problem the Emphasis is used in FM
 Emphasis in FM Signal
Pre Emphasis is done on transition side before FM

At receiver side after demodulation of FM the De-

Emphasis is used to cancel out the effect of pre-
Emphasis
 Pre-Emphasis
Pre-Emphasis:
• Applies a high-pass filter to the signal before
transmission
• Boosts or amplifies the high-frequency components
• Commonly boosts frequencies above 2-3 kHz
• Provides up to 10 dB of gain to high frequencies
• Done prior to transmission or recording
• Takes advantage of high-frequency noise immunity
Pre-Emphasis
 De-Emphasis
• De-Emphasis:
• Applies a low-pass filter to the received signal
• Attenuates or reduces the boosted high frequencies
• Rolls-off highs above 2-3 kHz
• The reverse process of pre-emphasis
• Provides gain reduction equal to the pre-emphasis
• Restores original frequency spectrum
• Reduces noise and distortion picked up during
transmission
De-Emphasis