Transcript ANALOG MODULATION AND DEMODULATION

MODULATION
AND
DEMODULATION
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• Modulation –
• It is the process of changing one or more
properties ( Amplitude, frequency or
phase) of the analog carrier in proportion
with the information signal.
• It is impractical to propagate information
as it is over standard transmission media.
• Reverse process of modulation and
converting the modulated carrier back to
the original information is known as
demodulation.
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• Why Modulation is necessary? –
• 1. It is difficult to radiate LF signal from
antenna in the form of EM energy.
• 2. Information signal often occupy the
same frequency band that would interfere
with each other.
• (Channel is a specific band of frequencies
allocated to a particular service.)
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• Types of Modulation –
• 1. Analog Modulation – (a) Amplitude
Modulation (AM) – If information signal is analog
and amplitude of the carrier is varied
proportional to amplitude information signal,
AM is produced.
(b) Frequency Modulation (FM) – If frequency of
carrier signal is varied proportional to
information signal, FM is produced.
(c) Phase Modulation (PM) –If phase of carrier is
varied proportional to amplitude of information
signal, PM is produced.
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• 2. Digital Modulation –(a) If the information signal
is digital and amplitude of carrier is varied
proportional to information signal, a digitally
modulated signal known as Amplitude Shift Keying
(ASK) is produced.
(b) If frequency of the carrier is varied, Frequency
shift Keying (FSK) is produced.
(c) If phase of the carrier is varied , Phase Shift
Keying (PSK) is produced.
• If both Amplitude and Phase are varied
proportional to the information signal, Quadrature
Amplitude Modulation QAM is produced.
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ANALOG
MODULATION
AND
DEMODULATION
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• Amplitude Modulation – It is a method of changing
the amplitude of relatively high frequency carrier
( normally sinusoidal wave) in proportion with
instantaneous value of modulating signal.
• It is to be noted that the carrier frequency remains
same and only amplitude varies.
• Let em represents the modulating signal.
em = Em sin ωm t and carrier signal is
ec = Ec sin ωc t
• Here Em & Ec – max. amplitude of modulating and
carrier signal and ωm & ωc – frequency of
modulating and carrier signal.
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Amplitude modulation
• A mathematical expression for modulated wave will
be Eam = Ec + em = Ec + Em sin ωm t.
• The instantaneous value of the amplitude
modulated wave can be e(am) = Eam sin ωc t
e(am) = (Ec + Em sin ωm t) sin ωc t . (##B4)
• Modulation Index and Percentage Modulation –
• Modulation Index m = Em/Ec. Term used to
describe the amount of amplitude change in AM.
• Should be always less than 1 to avoid any
distortion.(known as under modulation)
• m = 1 known as critical modulation.
• m > 1 known as over modulation. ###
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• AM power distribution• Total power of AM wave is
Ptotal = Pc +Pusb + Plsb
=E^2car/R +E^2usb/R +E^2lasb/R,
where R is characteristics impedance of
antenna and E is RMS voltages.
• Carrier power Pc = E^2carr/R = E^2c/2R
• Psb = E^2sb/R , Esb = (mEc/2) / √2.
= m^2 * E^2c/8R.
P total =E^2c/2R* {1+m^2/2) = Pc{1+m^2/2}
Ptotal/Pc = 1+m^2/2.
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• Frequency Spectrum and Bandwidth –
• The modulated carrier has new signals at
different frequencies called sidebands or side
frequencies, known as upper and lower
sidebands.
• f(lower sideband) = fc – fm
• F(upper sideband) = fc + fm.
• ###
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• Am Modulator Circuits –
• Two types• 1. Low level modulation – modulation takes
place at low carrier amplitude level. The
modulated carrier is then amplified and
transmitted.
• 2. High level modulation - The carrier is
amplified fully, then modulation takes place.
Modulator has to operate at high power level.
The modulated signal is then transmitted
directly.
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• Advantages and disadvantages of High and
low level modulation –
• High Level • Advantages – Power efficiency is practically
more than 80% and all the preceding linear
amplifiers operate at low power level.
• Disadvantages –
• Requires high amplitude of modulating signal.
Amplifier is non linear and hence generates
intermodulation frequency components
(Harmonics.)
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• Low level –
• Advantages – Less power consumption in the
modulator as it operates at low voltage.
• Being a class “A” amplifier, circuit is very
simple.
• Disadvantages – Modulator operates in class
A, hence power efficiency is very low.
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• AM High Level Transmitters Audio Input
Audio
Amplifier
Modulator
PA
Driver
Amp.
Mod class
C AMP
Antenna
Antenna
matching
network.
Buffer
AMP
Crystal
Osc.
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• Carrier generation is by crystal oscillator.
• Buffer and driver amplifiers brings power level
of carrier to desired level and given to C class
modulator amplifier.
• Modulating signal is amplified by audio and
audio power amplifier at a level suitable for
modulation.
• Modulating amplifier modulates carrier
according to the modulating signal.
• Output of modulator is fed to antenna via
matching network, consisting of LC circuit in
collector circuit of modulating amplifier.
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• AM low Level Transmitters Audio Input
Audio
Amplifier
Driver
Amp.
Antenna
Modulator
PA
Mod class
C AMP
Linear
Class B
Power
Amp.
Antenna
matching
network.
Buffer
AMP
Crystal
Osc.
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• A linear class B PA is used, for power
amplification and amplified signal is fed to
antenna.
• Modulator amplifier performs modulation at
relatively low power level and modulated AM
signal is amplified by class B power amplifier
to avoid distortion in the out put.
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• AM Broadcast Transmitter –
• Crystal osc generates carrier frequency signal.
• Power level of carrier is raised by buffer
amplifier.
• Driver stage C class modulator is for partial
modulation and further modulated by power
amplifier.
• Advantages- 1. If one or more components in
main amplifier fails, driver stage still provides
modulated output for transmission. Thus total
shut down is avoided.
•
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• 2. Distribution of total power in driver and
main amplifier.
• The modulating signal is amplified and fed to
modulator driver amplifier.
• The modulating signal is coupled to class C
driver and modulator through transformer
coupling.
• Part of transmitted signal is demodulated by
the linear detector and given as negative
feedback to audio amplifier to linearize the
modulation chracteristics.
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• DSB-SC and SSB-SC Amplitude Modulation –
• In AM, information is contained in two side
bands and not in carrier. Hence most of the
energy is wasted.
• In suppressed carrier system, carrier is not
transmitted but only sidebands are
transmitted, thus saving lot of transmitter
power.
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• DSB-SC Modulator – Product Modulator –
• Modulating
Signal f(t)
DSB-SC Signal
Product
Modulator
F(t )cosωc.t
Carrier
cos ωc.t
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• By modulation theorem, the frequency
spectrum of f(t) cos ωct will be
• f(t) cos ωct FT ½{F(ω + ωc) +F (ω – ωc)}
• Above equation shows two components –
1/2F(ω+ ωc) and 1/2F(ω- ωc) which are
termed as sidebands and are located at + ωc
and – ωc.
• BW of each sideband is 2 ωm.
• ###
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•
•
•
•
Balanced Modulator using Diodes (Ring)Refer circuit diagramLet modulating input is zero initially.
In the positive half cycle of carrier signal, D1 &
D2 are forward biased and D3&D4 are
reversed biased.
• Due to center tapping of T2, current is equally
divided in upper and lower portion of primary
winding.
• Hence the magnetic field produced by upper
and lower winding are equal but opposite and
hence output is zero.
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• When a sinusoidal modulating signal is
applied to the primary of T1, which appears
at secondary of T1.
• In the positive half cycle, D1 and D2 are
forward biased connecting secondary of T1 to
primary of T2.
• In the negative half cycle, D3 and D4 are
forward biased connecting secondary of T1 to
primary of T2. When D3 & D4 conduct, the
polarity of the signal is opposite to that of
modulating signal.
• Thus carrier is totally supressed.
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• Balanced Modulator using FETs – (##)
• The carrier signal is applied to the center tap
of secondary of T1 from primary of T2 and
output is from T3.
• The signal is applied to two gates of FETs in
phase through the secondary of T2.
• Modulating signal appears out of phase (180
degrees) at the gates .
• If there is no modulating signal, FET current
due to carrier is equal and opposite in
direction and no out put is produced at the
secondary of T3, thus carrier is suppressed.
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• When modulating signal is applied, currents
id1 and id2 flows in the primary of T3.
• Since modulating signal is applied 180 degrees
out of phase at the gates of FETs, the FET
currents due to modulating signal are equal
but not opposite and hence do not cancel
each other.
• Hence output produced at the secondary of
the transformerT3 is DSB out put produced by
FET balanced modulator.
• To prove that BM produces DSB Out put?(##)
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• Switching Modulator –(##)
• In this type of modulator, multiplication is
replaced by simple switching operation.
• Diode is assumed to act as a ideal switch i.e. it
presents zero impedance in forward biased
condition and infinite impedance in reverse
biased condition.
• The diode switch is controlled by carrier c(t).
• When c(t) is more than zero, diode is forward
biased and reverse biased when c(t) is less
than zero.
• Ec>>Em.
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• Single Side Band – Suppressed Carrier (SSB-SC)
• We have studied techniques of suppressing
carrier.
• Now let us study techniques of suppressing one
sideband.
• These techniques are
(i) Filter Method
(ii) Phase shift method,
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• Filter method to generate SSB –
Linear
Amplifier
Carrier
Signal
Balanced
Modulator
SB
suppression
filter
Balanced
Mixer
Crystal
Osc.
Modulating
Signal
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• Balanced modulator produces DSB output
containing both the sidebands.
• It is given to sideband suppressing filter to
remove unwanted sideband.
• Filter must have a flat passband and extremely
high attenuation outside the passband.
• To get this type of response the Q of the tuned
circuits must be very high.
• Required value of Q factor increases as the
difference between modulating frequency and
carrier frequency increases.
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• SSB suppression filter suppresses one of the
sidebands
• Frequency of the SSB signal at the output of
the filter is very low as compared to the
transmitter frequency.
• Hence the frequency is boosted up to the
transmitter frequency by the balanced
modulator and crystal oscillator. ( up
conversion).
• SSB signal is then amplified by the linear
amplifiers.
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• Phase Shift Method to generate SSB Modulating
Signal
Balanced
Modulator
M1
90 degrees
Phase Shifter
90 degrees
Phase Shifter
Summing
Amplifier
Linear
Power
Amplifier
Carrier
Generator
Balanced
Modulator
M2
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• The carrier signal is shifted by 90 degrees and
applied to M1.
• Modulating signal is directly applied to M1.
• Carrier signal is directly applied to M2.
• Modulating signal is phase shifted & applied to
M2.
• Both modulators produce an output consisting
of only sidebands.
• M1 generates USB and LSB but each one is
shifted by +90 degrees.
• M2 generates USB shifted by +90 and LSB
shifted by - 90 degrees.
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• The outputs of the BM are added in summing
amplifier.
• Since USBs of both BMs are phase shifted by
+90 they are in phase and add to produce
double amplitude signal.
• But LSBs of BM are (+90 and -90) 180 degrees
out of phase and hence cancels each other,
giving only USB SSB signal.
• Carrier is suppressed by BM.
• Let us prove this mathematically.
• ##
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• Synchronous Detection of DSB SC Signal –
• DSB- SC signal can be detected by
synchronous detector.
f(t)
cosωc t
Multiplier
LPF
1/2f(t)
Carrier
cosωc t
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• The modulated signal f(t) cosωct is given to
the multiplier.
• The modulated signal is multiplied with locally
generated carrier cosωc t.
• Multiplier output signal is f(t) cos^2. ωct.
• ###
• LPF has cutoff frequency just more than that
ωm. Hence passes the signal 1/2f(t).
• Since 2 ωc >> ωm, the signal 1/2f(t) cos2ωct is
not passed by LPF.
• So output is fo(t) = 1/2f(t).
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• Synchronous Detection of SSB SC Signal –
• Synchronous detector –
SSB
Signal
essb(t)
Multiplier
LPF
Output
1/2f(t)
Carrier
cosωct
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• Advantages of DSB-SC and SSB- SC –
• 1.BW is reduced hence more number of
channels can be transmitted in same available
BW.
• 2. As the carrier is suppressed the power
requirement is also reduced.
• 3. Noise is reduced as BW is reduced.
• Disadvantages of DSB-SC and SSB- SC –
• 1. Complicated process.
• 2. High quality filters with good stability and
selectivity are required for narrow BW
operation.
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• Amplitude Modulation by multiple sine waves • In reality, modulation takes place due to
multiple modulating waves.
• Hence consider what will happen if two or
more sine waves modulate carrier
simultaneously. ##
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• AM Receiver with double conversion –
• In superheterodyne receivers, Intermediate
Frequency (IF) is used for amplification and
uniform gain.
• For stability of IF amplifier, the IF shall be as
low as possible.
• For good image frequency rejection IF shall be
as high as possible.
• To achieve these two extreme requirements,
two IFs can be used, first high IF for image
frequency rejection and second low IF for
stable amplification. .
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• Double conversion AM Receiver Antenna
RF
Amp.
1st IF
Conv.
2nd IF
Conv.
1st LO
2nd LO
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2nd IF
Amp.
AM
Detector
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• Signal received from antenna is passed
through RF amplifier to increase amplitude of
received signal and to improve signal to noise
ratio.
• Then signal is passed through 1st IF convertor,
to generate IF of 10.625Mhz. This enable us to
push image frequency (IF x 2) away from RF
range.
• First IF is then converted to 2nd IF of 455khz,
passed through IF amplifier and then to
detector.
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• Advantages – 1. Easy amplification
• 2. Better image frequency rejection.
• Disadvantages – 1. Critical design of first
conversion stage.
2. Additional stage of IF conversion.
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• Vestigial Sideband Transmission –
• SSB works satisfactorily for an information bearing
signal(speech signal) with energy gap around zero
frequency.
• But for transmission of wideband signals, we have to
look to a new method of modulation for two
reasons.
• 1. The spectra of wideband signal contain significant
low frequencies, which makes it impractical to use
SSB modulation.
• 2. The spectral characteristic of wideband data befit
the use of DSB-SC. But DSB-SC requires a
transmission BW equal to twice the message BW,
which violates the BW conservation
requirements. 46
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• To overcome these limitations, we need a
compromise method of modulation that lies
between SSB and DSB-SC in its spectral
characteristics.
• Vestigial sideband (VSB) is that compromise
scheme.
• VSB modulation distinguishes itself from SSB
modulation in two practical aspects –
• 1. Instead of completely removing a sideband,
a trace or vestige of that sideband is
transmitted, hence the name “vestigial
sideband”
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• Instead of transmitting the other sideband in
full, almost the whole of this second band is
also transmitted.
• Transmitting BW of a VSB modulated signal is
defined as BT =fv + W, where fv is the vestige
BW and W is the message BW.
• Typically fv is 25% of W which means that the
VSB BW BT lies between the SSB BW ‘W’ and
DSB – SC BW 2W.
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• Generation of VSB using filter methodMessage
signal m(t)
Product
Modulator
BP filter
H(f)
VSB signal
s(t)
Carrier Ac
cos(2*pi*f*t)
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• The BP filter is designed in such a way that it is
suppresses one side band partially and passes
a portion (vestige) of other sideband.
• Output of BP filter is VSB signal.
• Magnitude Response of VSB Filter - ##
• Advantages of VSB -1.equencies near fc are
transmitted without any attenuation. 2. BW is
reduced as compared to DSB.
• VSB is mainly used for TV transmission.
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• Generation of VSB using Phase discrimination
Method –
• Principle of this method will become easy to
understand once we determine the time
domain description of the VSB wave. ###
• Demodulation of VSB – Input VSB signal s(t),
output signal v0(t), Product modulator output
is v(t). ###
Input
Product
modulator
LPF
Output
A’c cos
2*pi*fc*t
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• AM Receivers –
• What is Demodulation?
 Exactly opposite procedure of
Modulation.
 Process of retaining the Original Signal
from a modulated signal is called
“DEMODULATION”
 Method to detect the AM (DSB-FC)
signal  Diode Detector / Envelope
Detector.
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Simple Diode / Envelope Detector /
Demodulator
Negative Peak
Diagonal Clipping
Clipping
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Radio Receivers
Definition:
Radio receiver is an electronic equipment
which pick ups the desired signal, reject the
unwanted signal and demodulate the carrier
signal to get back the original modulating
signal.
Functions of Radio Receivers
• Select desired signal and reject unwanted
signal
• Amplify the desired R.F. signal
• Demodulate the selected amplified signal
• After demodulation,the original modulating
signal is obtained which must be amplified.
• Apply the amplified demodulated signal to
the loudspeaker.
Classification of Radio Receivers
Radio receivers are classified according to
the type of traffic they are designed to
handle.
• A.M. broadcast receivers
• F.M. broadcast receivers
• T.V. receivers
• Radar receivers
TRF (Tuned Radio frequency)
Receiver
Speaker
Advantages and Drawbacks of TRF Receiver
 ADVANTAGES:
• TRF receivers are simple to design as it does not involve
mixing and IF operation.
• Good sensitivity and very much suitable for single frequency.
• Allow the broadcast frequency 535 KHz to 1640 KHz.

•
•
•
•
•
DRAWBACKS:
Instability
Variation in the bandwidth over the tuning range
Insufficient Selectivity at higher frequencies
At the higher frequency, it produces difficulty in design.
It has poor audio quality.
SuperHeterodyne Receiver
AGC
Characteristics of Radio Receivers
•
•
•
•
Sensitivity
Selectivity
Fidelity
SINAD
Sensitivity:• Ability to amplify weak signals.
• Broadcast receivers/ radio receivers should have
reasonably high sensitivity so that it may have good
response to the desired signal.
• But should not have excessively high sensitivity
otherwise it will pick up all undesired noise signals.
• It is function of receiver gain ( decided by RF & IF
amplifier stage) and measures in decibels.
 Selectivity:• Selectivity of radio receiver is its ability to reject unwanted
signals and select the desired signal.
• It decides the adjacent channel rejection capability of a
receiver
• Radio receiver should have good selectivity
 Fidelity:• This represents variation ofthe
response (output) of the
receiver (in dB) with the
modulating frequency, when
output loadis resistive.
• Fidelity means the ability of the
receiver to reproduce correctly
the various modulating
frequncies components
present in the input signal
• Radio receiver should have high fidelity or accuracy.
• It is determined by the high frequency response. Therefore
it should have high AF amplifier frequency response over
entire audio frequency range.
• In an A.M. broadcast the maximum audio frequency is 5
KHz hence receiver with good fidelity must produce entire
frequency up to 5KHz.
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 SINAD:• It stands for Signal-to-noise and distortion ratio.
• It is a measure of the Quality of a signal from a
communication device,defined as;
1. The ratio of total received power i.e., the signal to the noiseplus-distortion power.
2. The ratio of the power of original modulating audio
signal, i.e., from a modulated radio frequecy carrier to the
residual audio power, i.e., noise-plus-distortion powers
remaining after the original modulating audio signal is
removed.
• SINAD level less can be than one.
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Image Frequency and its Rejection
 Definition: In radio reception using heterodyning in the
tuning process, an undesired input frequency that is
capable of producing the same intermediate frequency
(IF) that the desired input frequency produces is called
“IMAGE FREQUENCY” as that frequency is the IMAGE
of the signal frequency.
• It is given by signal frequency plus twice the intermediate
frequency
fsi = fs + IF
 Remedy: Its rejection before IF stage High Selectivity
• The Image frequency rejection ratio can be defined as a
ratio of the gain at the signal frequency to the gain at the
image frequency.
• This gives the degree of image frequency rejection.
Image Frequency Rejection Ratio
Double Spotting
• In this, receiver picks up same station at
two nearby frequencies or points on the
receiver dial.
• This phenomenon of Double spotting
occurs at higher frequencies due to poor
front end selectivity of receivers.
• Double spotting is harmful because a
weak station may be masked by the
reception of a strong station at the same
point,on the dial.
Tracking
• The process of tuning circuit to get the
desired output is called tracking.
• Any error that exists in the frequency
difference will result in an incorrect
frequency being fed to the IF amplifier.
Such errors are known as tracking errors.
• To avoid tracking errors ganged capacitors
are used.
Choice of IF
Too High
Poor Selectivity
Too Low
Image Frequency Rejection is
poorer
Tracking Problem increase
Selectivity too sharp,cuts the
sidebands
Need Frequency stability of
Local oscillator too high
Automatic Gain Control (AGC)
in Superheterodyne Receiver
• AGC or AVC (Automatic Volume Control)
is a system by means of which the overall
gain of radio receiver is varied
automatically with the variations in the
strength of received signals, to maintain
the output constant.
Double Conversion Receiver
Antenna
Antenna
Coupling
Unit
RF
amplifier
2-16 MHz
First Mixer
2-16 MHz
2-stage
IF
amplifier
1.7MHz
Second Mixer
1.7MHz
2-stage
IF amplifier
200kHz
OFF
Local
Oscillator
3.7-17.7 MHz
Delay AGC
Power
Amplifer
Loudspeaker
Audio
Amplifer
ON
Crystal
Oscillator
1.5 MHz
Beat
frequency
AGC
detector
AGC
amplifier
Squelch
Circuit
Oscillator
Detector
Important features
•
•
•
•
•
•
•
Fine tunning.
Variable Sensitivity.
Variable Selectivity.
Noise limiting.
Better image frequency rejection.
Better adjacent channel rejection.
Automatic frequency control.
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DSB-SC Receivers
 Requirements :
 High Reliabilty
Excellent suppression of adjacent signals.
 High signal to noise ratio.
 Ability to demodulate SSB
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Types of DSB-SC Receiver
 Product demodulator – used for fixed
position application.
Diode balanced modulator – used as
portable SSB transceiver.
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