MODULATION AND

DEMODULATION

BDG(xx) 1

• 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.

BDG(xx) 2

• 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.)

BDG(xx) 3

• 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.

BDG(xx) 4

• 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.

BDG(xx) 5

ANALOG MODULATION

AND DEMODULATION

BDG(xx) 6

• 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.

BDG(xx) 7

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. ###

BDG(xx) 9

• 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.

BDG(xx) 10

• 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.• ###

BDG(xx) 11

• 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.

BDG(xx) 12

• 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.)

BDG(xx) 13

• 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.

BDG(xx) 14

• AM High Level Transmitters -

BDG(xx) 15

Audio Amplifier

Modulator PA

Mod class C AMP

Driver Amp.

Audio Input

Antenna matching network.

Crystal Osc.

Buffer AMP

Antenna

• 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.BDG(xx) 16

• AM low Level Transmitters -

BDG(xx) 17

Audio Amplifier

Modulator PA

Mod class C AMP

Driver Amp.

Audio Input

Antenna matching network.

Crystal Osc.

Buffer AMP

Antenna

Linear Class B Power Amp.

• 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.

BDG(xx) 18

• 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.

• BDG(xx) 20

• 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.

BDG(xx) 21

• 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.

BDG(xx) 22

• DSB-SC Modulator – Product Modulator –

• Modulating DSB-SC Signal Signal f(t)

Carrier cos ωc.t

BDG(xx) 23

Product Modulator

F(t )cosωc.t

• 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.• ###

BDG(xx) 24

• Balanced Modulator using Diodes (Ring)-• Refer circuit diagram-• Let 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.

BDG(xx) 25

• 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.BDG(xx) 26

• 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.BDG(xx) 27

• 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?(##) BDG(xx) 28

• 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. BDG(xx) 29

• 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,

BDG(xx) 30

• Filter method to generate SSB –

BDG(xx) 31

Balanced Modulator

SB suppression

filter

Balanced Mixer

Crystal Osc.

Carrier Signal

Modulating Signal

Linear Amplifier

• 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. BDG(xx) 32

• 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.

BDG(xx) 33

• Phase Shift Method to generate SSB -

BDG(xx) 34

90 degrees Phase Shifter

Balanced Modulator

M1

Summing Amplifier

Carrier Generator

Balanced Modulator

M2

90 degrees Phase Shifter Linear

Power Amplifier

Modulating Signal

• 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.BDG(xx) 35

• 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.• ##

BDG(xx) 36

• Synchronous Detection of DSB SC Signal –• DSB- SC signal can be detected by

synchronous detector.

BDG(xx) 37

Multiplier LPFf(t) cosωc t

Carrier cosωc t

1/2f(t)

• 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).

BDG(xx) 38

• Synchronous Detection of SSB SC Signal –• Synchronous detector –

BDG(xx) 39

MultiplierSSB

Signal essb(t)

Carrier cosωct

LPF Output 1/2f(t)

• 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. BDG(xx) 40

• 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. ##

BDG(xx) 41

• 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. .BDG(xx) 42

• Double conversion AM Receiver -

BDG(xx) 43

1st IF Conv.

Antenna

RF Amp.

2nd IF Conv.

2nd IF Amp.

AM Detector

1st LO 2nd LO

• 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.

BDG(xx) 44

• Advantages – 1. Easy amplification • 2. Better image frequency rejection.

• Disadvantages – 1. Critical design of first conversion stage. 2. Additional stage of IF conversion.

BDG(xx) 45

• 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.

BDG(xx) 46

• 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”

BDG(xx) 47

• 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.

BDG(xx) 48

• Generation of VSB using filter method-

BDG(xx) 49

Product Modulator

Message signal m(t)

Carrier Ac cos(2*pi*f*t)

BP filter H(f)

VSB signal s(t)

• 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.

BDG(xx) 50

• 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). ###

BDG(xx) 51

Product modulator

A’c cos 2*pi*fc*t

Input LPF Output

• 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.

BDG(xx) 52

Simple Diode / Envelope Detector / Demodulator

Diagonal Clipping

54

Negative Peak Clipping

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

65

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.

66

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 noise-

plus-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.

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 Too Low

Poor Selectivity 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

Coupling

Unit

First Mixer

2-16 MHz

2-stage

IF amplifier

1.7MHz

Second Mixer

1.7MHz

2-stage

IF amplifier

200kHz

Local

Oscillator

3.7-17.7 MHz

Crystal

Oscillator

1.5 MHz

Beat

frequency

Oscillator

AGC detectorAGC

amplifier

DetectorSquelch

Circuit

Audio

Amplifer

Power

Amplifer

RF

amplifier

2-16 MHz

Antenna

OFF ON

Delay AGC

Loudspeaker

Important features

• Fine tunning.

• Variable Sensitivity.

• Variable Selectivity.

• Noise limiting.

• Better image frequency rejection.

• Better adjacent channel rejection.

• Automatic frequency control.

74

DSB-SC Receivers

Requirements : High ReliabiltyExcellent suppression of adjacent signals. High signal to noise ratio. Ability to demodulate SSB

75

Types of DSB-SC Receiver

Product demodulator – used for fixed position application.

Diode balanced modulator – used as portable SSB transceiver.

76