BANGALORE INSTITUTE OF TECHNOLOGY

Principles of Communication
Systems
BEC 402

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

Module 2 (PCS)
Amplitude Modulation Fundamentals

Dr. Anupama H
Assistant Professor
Department of ECE, BIT
 BANGALORE INSTITUTE OF TECHNOLOGY

AM Concepts
• The information signal varies the amplitude of the carrier sine wave.
• The carrier frequency remains constant during the modulation process, but its
amplitude varies in accordance with the modulating signal.
• An increase in the amplitude of the modulating signal causes the amplitude of
the carrier to increase.
• Both the positive and the negative peaks of the carrier wave vary with the
modulating signal.
• An increase or a decrease in the amplitude of the modulating signal causes a
corresponding increase or decrease in both the positive and the negative peaks
of the carrier amplitude.
• An imaginary line connecting the positive peaks and negative peaks of the carrier
waveform gives the exact shape of the modulating information signal is known as
the envelope.
Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

AM Modulator
• A circuit that changes a lower-frequency baseband or intelligence signal to a higher-frequency signal is
usually called a modulator.
• A circuit used to recover the original intelligence signal from an AM wave is known as a detector or
demodulator

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

AM in Time Domain
• The sine wave carrier can be expressed as
vc =Vc sin2πfc t
where
vc - represents the instantaneous value of the carrier sine wave voltage at some specific time in the cycle
Vc- represents the peak value of the constant unmodulated carrier sine wave
fc- is the frequency of the carrier sine wave
t - a particular point in time during the carrier cycle.
• A sine wave modulating signal can be expressed as
vm = Vm sin2πfmt
where
vm = instantaneous value of information signal
Vm = peak amplitude of information signal
fm = frequency of modulating signal
the peak value of the carrier is the reference point for the modulating signal, the value of the modulating
signal is added to or subtracted from the peak value of the carrier. Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

• The instantaneous value of either the top or the bottom voltage envelope υ1 can be computed by
using the equation

• the instantaneous value of the complete modulated wave υ2 by substituting υ1 for the peak value
of carrier voltage Vc as follows:

• Substituting for v1 , we get

Using the trigonometric identity

v2 = vAM

where,
v2 is the instantaneous value of the AM wave (or υAM)
Vc sin 2πfc t is the carrier waveform, and
Dr. Anupama H
(Vm sin2πfmt) (sin2πfc t) is the carrier waveform multiplied by the modulating signal waveform. Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Sidebands and the Frequency Domain

• During modulation process, when a carrier is modulated by an information signal, new signals at
different frequencies are generated called side frequencies, or sidebands
• Occur in the frequency spectrum directly above and directly below the carrier frequency. i.e at
frequencies that are the sum and difference of the carrier and modulating frequencies.
• The upper sideband fUSB = fc + fm and lower sideband fLSB= fc - fm where fc is the carrier
frequency and fm is the modulating frequency.
υAM = Vc sin 2πfc t + (Vm sin 2πfmt) (sin 2πfc t)

• where the first term is the carrier; the second term, containing the difference fc - fm, is the lower
sideband; and the third term, containing the sum fc + fm, is the upper sideband.

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

AM - Frequency-Domain Representation
• plot of the carrier and sideband amplitudes with respect to frequency
• the horizontal axis represents frequency, and the vertical axis represents the amplitudes of the signals.
• The signals may be voltage, current, or power amplitudes
• A plot of signal amplitude versus frequency is referred to as a frequency-domain display.
• Spectrum analyzer instrument is used to display the frequency domain of a signal.
• Taking Fourier transform of the AM signal represents the frequency domain of a signal
• amplitudes in the frequency-domain display are the peak values of the carrier and sideband sine waves as
Vc/2 and mVc/4 respectively

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Modulation Index and Percentage

of Modulation
• For undistorted AM to occur, the modulating signal voltage Vm must be less than the carrier
voltage Vc
• modulation index m is the ratio of the amplitude of the modulating signal to the amplitude of the
carrier signal
m = Vm / Vc
• Multiplying the modulation index by 100 gives the percentage of modulation
• The modulation index value should be a number between 0 and 1
• The modulation index should be a number between 0 and 1.
• Based on value of m, three cases
• m<1 – under modulation
• m=1 – exact modulation
• M>1 – over modulation Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Modulation and Distortion

• Overmodulation - the amplitude of the modulating voltage is higher than the carrier voltage, m >1, causes
distortion of the modulated waveform. The waveform is flattened at the zero line, the received signal will
produce an output waveform whose negative peaks have been clipped off.
• Undermodulation -the amplitude of the modulating signal is less than the carrier amplitude, no distortion
will occur - m<1
• Exact modulation - The ideal condition for AM , Vm = Vc, or m = 1, gives 100 percent modulation. Results in
the greatest output power at the transmitter and the greatest output voltage at the receiver with no
distortion.

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

• Computation of the modulation index from measurements taken on the composite modulated wave
• The peak value of the modulating signal Vm is one-half the difference of the peak and trough values
• Vmax is the peak value of the signal during modulation, and Vmin is the lowest value, or trough, of the
modulated wave. The Vmax is one-half the peak-to-peak value of the AM signal, or Vmax (p 2p)/2.
• Subtracting Vmin from Vmax produces the peak to-peak value of the modulating signal.

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

AM Power
• In radio transmission, the AM signal is amplified by a power amplifier and fed to the antenna with
a characteristic impedance that is ideally, pure resistance.
• The total transmitted power PT is sum of the carrier power Pc and the power in the two sidebands
PUSB and PLSB

• original AM equation

• Vc and Vm are peak values of the carrier and modulating sine waves, respectively. For power
calculations, rms values must be used for the voltages.

• Using coefficients on the sine and cosine terms in the power formula
Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

• Expressing the modulating signal Vm in terms of the carrier Vc

Vm = mVc
• Expressing the sideband powers in terms of the carrier power, the total power becomes

• Since the term Vc2/2R is equal to the rms carrier power Pc, giving

• In terms of current

where

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Advantages of AM
• Simple and effective.
• Used in AM radio broadcasting, CB radio, TV broadcasting, and aircraft tower communication.
• Simple control radios use ASK because of its simplicity. Ex: garage door openers and remote
keyless entry devices on cars.
• AM is also widely used in combination with phase modulation to produce quadrature amplitude
modulation (QAM) which facilitates high-speed data transmissions in modems, cable TV, and
some wireless applications.

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Single-Sideband Modulation
• two-thirds of the transmitted power is in the carrier, which itself conveys no information.
• The real information is contained within the sidebands.
• One way to improve the efficiency of amplitude modulation is to suppress the carrier and
eliminate one sideband, which results in single-sideband (SSB) signal.
• SSB is a form of AM that offers unique benefits
• To generate an SSB signal, suppress the carrier, leaving the upper and lower sidebands. This type
of signal is referred to as a double-sideband suppressed carrier (DSSC or DSB) signal.
• DSB is the algebraic sum of the two sinusoidal sidebands, is the signal produced when a carrier is
modulated by a single-tone sine wave information signal. The carrier is suppressed, and the time-
domain DSB signal is a sine wave at the carrier frequency, varying in amplitude
• A unique characteristic - phase transitions occur at the lower-amplitude portions of the wave
• spectrum space occupied by a DSB signal is the same as that for a conventional AM signal
• DSB AM saves considerable power, not widely used because the signal is difficult to demodulate
at the receiver
Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

DSB

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

SSB

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

SSB Signals-four major benefits

1. The spectrum space it occupies is only one-half that of AM and DSB signals- greatly conserves spectrum
space and allows more signals to be transmitted in the same frequency range.
2. All the power previously devoted to the carrier and the other sideband can be channelled into the single
sideband, producing a stronger signal that should carry farther and be more reliably received at greater
distances.
Thus, SSB transmitters can be made smaller and lighter than an equivalent AM or DSB transmitter because less
circuitry and power are used.
3. Because SSB signals occupy a narrower bandwidth, the amount of noise in the signal is reduced.
4. There is less selective fading of an SSB signal over long distances. AM signals are affected in slightly different
ways by the ionosphere and upper atmosphere. The carrier and sidebands may arrive at the receiver at slightly
different times, causing a phase shift that can, in turn, cause them to add in such a way as to cancel one
another rather than add up to the original AM signal. Such cancellation, or selective fading, is not a problem
with SSB since only one sideband is being transmitted
• In a standard AM transmitter, the carrier is still transmitted even though it may not be modulated, but in
SSB, no carrier is transmitted if the information signal is zero, sidebands are generated only during the
modulation process. SSB is so much more efficient than AM

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Disadvantages of DSB and SSB

• They are harder to recover, or demodulate, at the receiver.
• The carrier must be regenerated at the receiver and reinserted into the signal to recover the
message signal, the reinserted carrier must have the same phase and frequency as those of the
original carrier. This is a difficult requirement.
• When SSB is used for voice transmission, the reinserted carrier can be made variable in frequency
so that it can be adjusted manually while listening to recover an intelligible signal.

SSB Power
• The peak envelope power =

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Amplitude Modulators

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Low-Level AM
• Signals are generated at very low voltage and power amplitudes.
• The voltage is typically less than 1 V, and the power is in milliwatts
• class A, class AB, or class B—raise the level of the signal to the desired power level before the AM
signal is fed to the antenna.

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Low-Level AM - Diode Modulator

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

• consists of a resistive mixing network, a diode rectifier, and an LC tuned circuit.

• The carrier is applied to one input resistor and the modulating signal, mixed signals appear across R3, two
signals are linearly mixed i.e added.
• The composite waveform is applied to a diode rectifier. The diode is connected so that it is forward-biased
by the positive-going half-cycles of the input wave.
• During the negative portions of the wave, the diode is cut off and no signal passes.
• The current through the diode is a series of positive-going pulses whose amplitude varies in proportion to
the amplitude of the modulating signal
• These positive-going pulses are applied to the parallel-tuned circuit made up of L and C, which are resonant
at the carrier frequency.
• Each time the diode conducts, a pulse of current flows through the tuned circuit. The coil and capacitor
repeatedly exchange energy, causing an oscillation, or “ringing,” at the resonant frequency.
• The oscillation of the tuned circuit creates one negative half-cycle for every positive input pulse.
• The resulting waveform across the tuned circuit is an AM signal
• The Q of the tuned circuit should be high enough to eliminate the harmonics and produce a clean sine wave
and to filter out the modulating signal, and low enough that its bandwidth accommodates the sidebands
generated.
Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Transistor Modulator
• uses a transistor instead of the diode
• The circuit has gain. The emitter-base junction
is a diode and a nonlinear device.
• Modulation occurs as described in Diode
Modulator
• Except that the base current controls a larger
• collector current, the circuit amplifies.
• Rectification occurs because of the emitter-base junction.
• Causing larger half-sine pulses of current in the tuned circuit.
• The tuned circuit oscillates (rings) to generate the missing half-cycle.
• The output is a classic AM wave.

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Differential Amplifier Modulator

• excellent amplitude modulator.
• Transistors Q1 and Q2 form the differential pair, and Q3 is a constant-current source. Transistor Q3 supplies a
fixed emitter current IE to Q1 and Q2, one-half of which flows in each transistor.
• The output is developed across the collector resistors R1 and R2
• The output is a function of the difference between inputs V1 and V2; that is,
• Vout = A(V2 - V1), where A is the circuit gain.
• if V1 is zero, the output is Vout = A(V2). If V2 is zero, the output is Vout= A(-V1) = -AV1, the circuit inverts V1.
• The two outputs are 180° out of phase with each other. If the balanced output is used, the output voltage
across the load is twice the single-ended output voltage.

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

Differential amplifier modulator
 BANGALORE INSTITUTE OF TECHNOLOGY

High-Level AM
• Modulator varies the voltage and power in the final RF amplifier stage of the
transmitter.
• Results is high efficiency in the RF amplifier
• High-quality performance.

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Collector Modulator
• The modulator is a linear power amplifier that takes the low-level modulating signal and amplifies it to a high-power level.
• The modulating output signal is coupled through modulation transformer T1 to the class C amplifier.
• The secondary winding of the modulation transformer is connected in series with the collector supply voltage VCC of the class C amplifi er.
• With a zero-modulation input signal, there is zero-modulation voltage across the secondary of T1, the collector supply voltage is applied
directly to the class C amplifi er, and the output carrier is a steady sine wave.
• When the modulating signal occurs, the ac voltage of the modulating signal across the secondary of the modulation transformer is added to
and subtracted from the dc collector supply voltage.
• Then applied to the class C amplifi er, causing the amplitude of the current pulses through transistor Q1 to vary.
• Hence the amplitude of the carrier sine wave varies in accordance with the modulated signal.
• When the modulation signal goes positive, it adds to the collector supply voltage, increasing its value and causing higher current pulses and
a higher-amplitude carrier.
• When the modulating signal goes negative, it subtracts from the collector supply voltage, decreasing it
• class C amplifi er current pulses are smaller, resulting in a lower-amplitude carrier output.
• For 100 percent modulation, the peak of the modulating signal across the secondary of T1 must be equal to the supply voltage. When the
positive peak occurs, the voltage applied to the collector is twice the collector supply voltage
• When the modulating signal goes negative, it subtracts from the collector supply voltage. When the negative peak is equal to the supply
voltage, the effective voltage applied to the collector of Q1 is zero, producing zero carrier output
• The collector current pulses cause the tuned circuit to oscillate (ring) at the desired output frequency.
Dr. Anupama H
• The tuned circuit reproduces the negative portion of the carrier signal Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Series Modulator
• major disadvantage of collector modulators is the need for a modulation transformer
that connects the audio amplifi er to the class C amplifi er in the transmitter. The higher
the power, the larger and more expensive the transformer.
• Transistorized version of a collector modulator in which a transistor is used to replace the
transformer approach is series modulator
• Replaces the transformer with an emitter follower.
• The modulating signal is applied to the emitter follower Q2, which is an audio power
amplifier.
• Q2 simply varies the supply voltage to Q1.
• If the modulating signal goes positive, the supply voltage to Q1 increases; thus, the
carrier amplitude increases in proportion to the modulating signal.
• If the modulating signal goes negative, the supply voltage to Q1 decreases, thereby
decreasing the carrier amplitude in proportion to the modulating signal.
• For 100 percent modulation, the emitter follower can reduce the supply voltage to zero
on maximum negative peaks.
Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Amplitude Demodulators- Diode Detectors

• Demodulators, or detectors, are circuits that accept modulated signals and recover the original modulating information
• AM signal is transformer-coupled and applied to a basic half wave rectifier circuit consisting of D1 and R1.
• The diode conducts when the positive half-cycles of the AM signals occur. During the negative half-cycles, the diode is
reverse-biased and no current flows through it.
• As a result, the voltage across R1 is a series of positive pulses whose amplitude varies with the modulating signal.
• A capacitor C1 is connected across resistor R1, effectively filtering out the carrier and thus recovering the original
modulating signal.
• On each positive alternation of the AM signal, the capacitor charges quickly to the peak value of the pulses passed by the
diode.
• When the pulse voltage drops to zero, the capacitor discharges into resistor R1. The time constant of C1 and R1 is chosen
to be long compared to the period of the carrier.
• As a result, the capacitor discharges only slightly during the time that the diode is not conducting.
• The resulting waveform across the capacitor is a close approximation to the original modulating signal.
• The circuit is called an envelope detector as the diode detector recovers the envelope of the AM signal, which is the
original modulating signal.
• The dc component in the output is removed with a series coupling or blocking capacitor C2 Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Balanced Modulators - Diode Ring or Lattice

Modulators
• Circuit that generates a DSB signal- suppressing the carrier and leaving only the sum and difference
frequencies at the output.
• consist of an input transformer T1, an output transformer T2, and four diodes connected in a bridge circuit.
• The carrier signal is applied to the center taps of the input and output transformers
• modulating signal is applied to the input transformer T1. The output appears across the secondary of the
output transformer T2.
• The carrier sine wave, higher in frequency and amplitude than the modulating signal, is used as a source of
forward and reverse bias for the diodes.
• The carrier turns the diodes off and on at a high rate of speed, and the diodes act as switches that connect
the modulating signal at the secondary of T1 to the primary of T2.
• Assume that the modulating input is zero. When the polarity of the carrier is positive, diodes D1 and D2 are
forward-biased. At this time, D3 and D4 are reverse-biased and act as open circuits.
• current divides equally in the upper and lower portions of the primary winding of T2.
• The current in the upper part of the winding produces a magnetic field that is equal and opposite to the
Dr. Anupama H
magnetic field produced by the current in the lower half of the secondary. Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

• The magnetic fields thus cancel each other out. No output is induced in the secondary, and the carrier is
effectively suppressed.
• When the polarity of the carrier reverses, diodes D1 and D2 are reverse-biased and diodes D3 and D4
conduct the current flows in the secondary winding of T1 and the primary winding of T2.
• The equal and opposite magnetic fields produced in T2 cancel each other out. The carrier is effectively
balanced out, and its output is zero.
• phase reversal of the signal in the very center of the waveform

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Frequency-Division Multiplexing
• Multiplexing is the process of simultaneously transmitting two or more individual signals over a single
communication channel, cable or wireless, it increases the number of communication channels so that more
information can be transmitted.
• The two most common types of multiplexing are frequency-division multiplexing (FDM) and time-division
multiplexing (TDM).
• FDM systems are used for analog information and TDM systems are used for digital information

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

FDM Transmitter-Multiplexers
• In (FDM), multiple signals share the bandwidth of a common communication channel
• Each signal to be transmitted feeds a modulator circuit. The carrier for each modulator (fc) is on a different frequency.
• The carrier frequencies are usually equally spaced from one another over a specific frequency range. These carriers are
referred to as subcarriers.
• Each input signal is given a portion of the bandwidth.
• Any of the standard kinds of modulation can be used, including AM, SSB, FM, PM, or any of the various digital modulation
methods.
• The FDM process divides up the bandwidth of the single channel into smaller, equally spaced channels, each capable of
carrying information in sidebands.
• The modulator outputs containing the sideband information are added algebraically in a linear mixer; no modulation or
generation of sidebands takes place.
• The resulting output signal is a composite of all the modulated subcarriers.

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Receiver-Demultiplexers
• A receiver picks up the signal and demodulates it, recovering the composite signal.
• This is sent to a group of bandpass filters, each centered on one of the carrier frequencies.
• Each filter passes only its channel and rejects all others. A channel demodulator then recovers
each original input signal.

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

 BANGALORE INSTITUTE OF TECHNOLOGY

Dr. Anupama H
Assistant Professor
Dept. of ECE, BIT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING