MODULE 2- PULSE MODULATION TECHNIQUES & ANTENNAS

SAMPLING
 The process of conversion of analog signals in to
discrete form is called sampling.
 The discrete form of analog signal is called as
samples.

SAMPLING RATE
 To discretize the signals, the gap between the
samples should be fixed. The gap between each samples in discrete form is called sampling
period Ts.
Where,
 Ts is the sampling time
 fs is the sampling frequency or the sampling

 Sampling frequency is the reciprocal of the sampling period. It is also called sampling rate.
 The sampling rate denotes the number of samples taken per second.
NYQUIST RATE
 The effective reproduction of the original signal is possible , if the sampling frequency should be
twice the highest frequency. This rate of sampling is called as Nyquist rate.

Where,
𝒇𝒔 = 𝟐𝒇𝒎
 fs is the sampling rate
 fm is the highest frequency
SAMPLING THEOREM

 Sampling theorem states that ,a Continuous time signal which is band limited to fm hertz, can be
represented in its samples and it can be recovered back if the sampling frequency is greater than
or equal to twice the highest frequency component fm of the message signal.
𝒇𝒔 ≥ 𝟐𝒇𝒎

SIGNIFICANCE OF SAMPLING
• Generally signals are analog in nature.
Eg: Speech, weather signals
• To process the analog signal by digital means, it is essential to convert them into discrete-time signal.
• Then from discrete-time to digital signals.
• The process of converting analogue signal to digital signal is Analogue to Digital Conversion(ADC)
• There are three steps involved in ADC:
1. Sampling
2. Quantization
3. Encoding

 To reconstruct the original analog signal we are considering fs ≥ 2fm

 If fs < 2fm; it is called as under sampling.
 Under sampling results in aliasing.
 Aliasing is the effect in which overlapping of a frequency components takes place at the frequency higher
than Nyquist rate.
 The phenomena in which a high frequency component in the frequency spectrum of a signal takes identity
 of a lower frequency component in the same spectrum of the sampled signal is called as aliasing
 Distortion of signal happens due to aliasing effect.
 The data will be lost and cannot be recovered.
When fs < 2fm
Messsage signal sampled at a rate lower
than 2fm
Overlapping of information contents.
Loss of information or aliasing ( overlapping of
frequency spectrum) occurs.

When fs = 2fm
The information is reproduced without any
loss.
Perfect sampling

No overlapping of message components

When fs > 2fm

The information is reproduced without
any loss.
Over sampling
No overlapping of message
components

ANALOG PULSE MODULATION TECHNIQUES

 In Pulse modulation methods, the carrier is pulse train.

 Type of modulation in which the signal is transmitted in the form of pulses is called as Pulse
modulation.
Pulse Analog modulation
1. Pulse amplitude modulation (PAM)
2. Pulse width modulation (PWM)
3. Pulse position modulation (PPM)

1) PULSE AMPLITUDE MODULATION (PAM)

It is the process of changing the amplitude of carrier pulse in accordance with the instantaneous
amplitude of the modulating signal.
 Generation of PAM
The circuit shows the generation of PAM.
Here modulating signal is applied to the source of
the FET, and the carrier pulses are applied to the
gate of the FET.
When a pulse is applied to the gate the FET
becomes ON and then the input is passed passed to
the output.
So we will get the output whenever the pulse
applied to the gate terminal.
A low pass filter is used to get the demodulated
output.

Advantages of PAM
 Both Modulation and demodulation are
simple.
 Easy construction of transmitter and
receiver circuits.
Disadvantages of PAM
 Large bandwidth is required for
transmission.
 More noise.
 Here the amplitude is varying. Therefore,
the power required will be more.
Applications of PAM
 Mainly used in Ethernet communication.
 Many microcontrollers use this technique
in order to generate control signals.
 It is used in Photo-biology.
 It acts as an electronic driver for LED circuits.


PAM Generation Circuit PAM Demodulation Circuit

 2) PULSE WIDTH MODULATION
 Pulse Width Modulation (PWM) is also called as Pulse Duration Modulation (PDM) or Pulse Time
Modulation (PTM) technique.
 It is the process of changing the width of carrier pulse in accordance with the instantaneous amplitude
of the modulating signal.
 The width of the pulse varies in this method, but the amplitude of the signal remains constant.
 Amplitude limiters are used to make the amplitude of the signal constant.
 These circuits clip off the amplitude to a desired level, and hence the noise is limited.
Advantages of PWM
 Low power consumption.
 It has an efficiency of about 90 per
cent.
 Noise interference is less.
 High power handling capacity.
Disadvantages of PWM
 The circuit is more complex.
 Voltage spikes can be seen.
 The system is expensive as it uses
semiconductor devices.
 Switching losses will be more due to
high PWM frequency.
Applications of PWM
 Used in encoding purposes in the
telecommunication system.
 Used to control brightness in a
smart lighting system.
 Helps to prevent overheating in
LED’s while maintaining it’s
brightness.
 Used in audio and video amplifiers.

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PULSE POSITIONMODULATION

 It is the process of changing the position of carrier pulse in accordance with the instantaneous amplitude
of the modulating signal.
 In PPM synchronization pulses are needed to synchronize transmitter and receiver , it helps to
maintain position of pulses.
 It is done in accordance with PWM.
 Trailing edge of the PWM pulse is the starting edge of PPM signal. Hence position of the pulses is
proportional to thr PWM pulses.
 Output of the PWM generator is given to a monostable multivibrator to generate PPM pulses.


Advantages of PPM
 As it has constant amplitude noise interference is less.
 We can easily separate signal from a noisy signal.
 Among all three types, it has the most power efficiency.
 Requires less power when compared to pulse amplitude modulation.
Disadvantages of PPM
 The system is highly complex.
 The system requires more bandwidth.
 The synchronization between the transmitter and the receiver is a must

Applications of PPM
 It is used in the air traffic control system and telecommunication systems.
 Remote controlled cars, planes, trains use pulse code modulations.
 It is used to compress data and hence it is used for storage.

 PULSE CODE MODULATION (PCM)

 In PCM analog information is converted in to a binary

sequence. i.e., sequence of 1’s and 0’s.
 In Pulse Code Modulation, the message signal is
represented by a sequence of coded pulses.
 The block diagram of PCM is shown below. It consists of the
following blocks.

Low Pass Filter

 Analog message signal is given to a low pass filter ( LPF ).
 This filter eliminates the high frequency components present in the input analog signal which is greater than
the highest frequency ( fm ) of the message signal.
 It avoids aliasing of the message signal.
Sampler
 It converts analog signal in to discrete form.
 The sampling frequency must be greater than or equal to twice of the highest frequency component of the
message signal for faithful reproduction.
𝒇𝒔 ≥ 𝟐𝒇𝒎
𝒇𝒔 - Sampling frequency 𝒇𝒎 - Maximum frequency of the message signal

Quantizer
 Sampled values are rounded to values of certain levels.
 This rounding off operation is called quantization.
 The sampled output when given to Quantizer, reduces the redundant bits and compresses the value.
Encoder
 It converts a quantized signal in to binary code sequence for transmission.
 The digitization of analog signal is done by the encoder.
 Encoding minimizes the bandwidth used.

PHYSICAL CONCEPTS OF RADIATION OF ELECTROMAGNETIC WAVE

ELECTRIC FIELD
It is the space around an electric charge, where an
electrostatic force is experienced by another
charge.

MAGNETIC FIELD
A region around a magnetic material or a moving electric
charge within which the force of magnetism acts.
 ELECTROMAGNETIC WAVES
EM waves are energy transported through space in the form
of periodic disturbances of electric and magnetic fields.
 Electromagnetic waves are waves that are created as a
resultof variations of electric field and a magnetic field.
 Electromagnetic waves are those waves in which electric and magnetic field vectors changes
sinusoidal and are perpendicular to each other. Also, both magnetic and electric field vectors
are atright angles to the direction of propagation of wave.
 Electromagnetic waves don't need a medium to propagate.

Source of Electromagnetic Waves:
Accelerating charges produce electromagnetic waves. (Varying electric and magnetic fields can
alsoproduce them)
Explanation:
 A time-space varying electric field produces a varying magnetic field at right angles to it
 The time-space varying magnetic field in turn produces an electric field at right angles to it
 This continues forming oscillations of electric and magnetic field components which constitute an
electromagnetic wave.
Wave model of Electromagnetic energy
 EM waves travel through space at the same speed, c = 2.99792458 x 108 m/s, commonly
known asthe speed of light.
 It consists of an electric field E and a magnetic field B.
 E varies in magnitude in the direction perpendicular to the travelling direction; B is
perpendicularto E.
 An EM wave is characterized by a frequency and a wavelength.
 These two quantities are related to the speed of light by the equation speed of light =
frequency Xwavelength

 ANTENNAS

 An Antenna (or an Aerial), is an electrical device that converts electric power into
electromagnetic waves (or simply radio waves) and vice-versa.
 An antenna can be used as a transmitting antenna or a receiving antenna.
 A transmitting antenna is one, which converts electrical signals into electromagnetic waves and
radiates them.
 A receiving antenna is one, which converts electromagnetic waves from the received beam into
electrical signals.
 In two-way communication, the same antenna can be used for both transmission and reception.
 When electric charges undergo acceleration or deceleration, electromagnetic radiation will be
produced.
 Hence it is the motion of charges, that is currents, is the source of radiation.
 A conductor, which is designed to carry current over large distances with minimum losses, is termed
as a transmission line.
 For a transmission line, to radiate power,
 If the current conduction is with uniform velocity, the wire or transmission line should be
curved, bent, truncated or terminated.
 If transmission line has current, which accelerates or decelerates with time, then it radiates the
power even though the wire is straight.
 The device or tube, if bent or terminated to radiate energy, then it is called as waveguide. These
are especially used for the microwave transmission or reception.
Radiation from a Single Wire:
 For a single wire antenna, there is radiation if the wire is curved, bent, discontinuous, terminated,
or truncated, as shown in Figure.
 If charge is oscillating in a time-motion, it
radiates even if the wire is straight.
Radiation from a Two Wire:

 A voltage across the two-conductor

transmission line creates an electric field
between theconductors.
 The electric field is radiated as free-space waves
from the open ends of the electric conductor.
 The radiation moves faster than the speed of
light but approaches the speed of light at
points faraway from the antenna

Radiation from a Dipole:

 The conducting wires are split into two
separate conductors in a dipole.
 The ac signal flowing to these conductors produce varying charge distribution and hence EM
radiation.
ANTENNA PARAMETERS
1. DIRECTIVITY
 It is the ratio of energy transmitted ( or received ) by the antenna to the energy transmitted by
the isotropic antenna. Hence, a high directivity antenna will have a beam pattern that is
very strong in a singledirection.

𝐸𝑛𝑒𝑟𝑔𝑦 𝑡𝑟𝑎𝑛𝑠𝑚𝑖𝑡𝑡𝑒𝑑 ( 𝑜𝑟 𝑟𝑒𝑐𝑒𝑖𝑣𝑒𝑑 )𝑏𝑦 𝑎𝑛𝑡𝑒𝑛𝑛𝑎

𝐃𝐢𝐫𝐞𝐜𝐭𝐢𝐯𝐢𝐭𝐲 =
𝐸𝑛𝑒𝑟𝑔𝑦 𝑡𝑟𝑎𝑛𝑠𝑚𝑖𝑡𝑡𝑒𝑑 ( 𝑜𝑟 𝑟𝑒𝑐𝑒𝑖𝑣𝑒𝑑 )𝑏𝑦 𝑖𝑠𝑜𝑡𝑟𝑜𝑝𝑖𝑐 𝑎𝑛𝑡𝑒𝑛𝑛𝑎

2. ANTENNA GAIN (DBI/NUMERIC)

 It is the ratio of radiated power output to input power of antenna.
𝑹𝒂𝒅𝒊𝒂𝒕𝒆𝒅 𝒐𝒖𝒕𝒑𝒖𝒕 𝒑𝒐𝒘𝒆𝒓
Antenna Gain = 𝒊𝒏𝒑𝒖𝒕 𝒑𝒐𝒘𝒆𝒓

 It is expressed in dB.

3. APERTURE EFFICIENCY
 It is the ratio of effective radiating area to physical area of the aperture.

𝑬𝒇𝒇𝒆𝒄𝒕𝒊𝒗𝒆 𝒓𝒂𝒅𝒊𝒂𝒕𝒊𝒏𝒈 𝒂𝒓𝒆𝒂

Aperture Efficiency = 𝒑𝒉𝒚𝒔𝒊𝒄𝒂𝒍 𝒂𝒓𝒆𝒂 𝒐𝒇 𝒂𝒑𝒆𝒓𝒕𝒖𝒓𝒆
4. ANTENNA EFFICIENCY
 It is the ratio of power output to input power to antenna.

𝑶𝒖𝒕𝒑𝒖𝒕 𝒑𝒐𝒘𝒆𝒓
𝑬𝒇𝒇𝒊𝒄𝒊𝒆𝒏𝒄𝒚 =
𝑰𝒏𝒑𝒖𝒕 𝒑𝒐𝒘𝒆𝒓

5. RADIATION INTENSITY

 It is the power radiated from an antenna per unit solid angle.

 Radiation intensity = Radiation density x d2
Where d = distance

6. RADIATION RESISTANCE
 It is the total resistance of the radiating antenna

7. ANTENNA BEAM WIDTH

 It is the distance between two half power points on power density radiation pattern.
RADIATION PATTERN

 It is the graphical representation of radiation

properties of the antenna as a function of space.

 The radiation patterns can be field patterns or

power patterns.
o The field patterns are plotted as a
function of electric and magnetic fields.
They are plottedon logarithmic scale.
o The power patterns are plotted as a
function of square of the magnitude of
electric and magnetic fields. They are
plotted on logarithmic or commonly on
dB scale.

 The radiation pattern has main lobe, side lobes and

back lobe.

 The major part of the radiated field, which covers a larger area, is the main lobe or major lobe. This is
the portion where maximum radiated energy exists. The direction of this lobe indicates the directivity of
the antenna.

 The other parts of the pattern where the radiation is distributed side wards are known as side lobes or minor
lobes. These are the areas where the power is wasted.

 There is other lobe, which is exactly opposite to the direction of main lobe. It isknown as back lobe, which is also a
minor lobe. A considerable amount of energy is wasted here also.

MICROSTRIP ANTENNAS

 Micro strip antennas are low-profile antennas. A metal patch mounted at a ground level
with a di-electric material in-between constitutes a Micro strip or Patch Antenna.
 These are very low size antennas having low radiation.
Frequency Range
 The patch antennas are popular for low profile applications at frequencies above 100MHz.
Construction & Working of Micro strip Antennas
 Micro strip antenna consists of a very thin metallic strip placed on a ground plane with a di-
electric material in-between.
 The radiating element and feed lines are placed by the process of photo-etching on the di-electric
material.
 Usually, the patch or micro-strip is chosen to be square, circular or rectangular in shape for the
ease of analysis and fabrication. The following figure shows a micro-strip or patch antenna.
 The length of the metal patch is λ/2.
 When the antenna is excited, the waves generated within the di-electric undergo reflections and
the energyis radiated from the edges of the metal patch, which is very low
Advantages
The following are the advantages of Micro strip antenna −
 Lighteweight
 Low cost
 Ease of installation
Disadvantages
The following are the disadvantages of Micro
strip antenna −
 Inefficient radiation
 Narrow frequency bandwidth
Applications
The following are the applications of Micro strip
antenna −
 Used in Space craft applications
 Used in Air craft applications
 Used in Low profile antenna
applications

HALF-WAVE DIPOLE ANTENNA

 A half-wave dipole antenna consists of two, quarter-wavelength conductors placed end to end
for a total length of approximately L = λ/2.
 This is the most widely used antenna because of its advantages.
 It is also known as Hertz antenna.
 Frequency range: The range of frequency in which half-wave dipole operates is around 3KHz to
300GHz.
 This is mostly used in radio receivers.
Construction & Working of Half-wave Dipole
 It is a dipole antenna, where the frequency of its operation
is half of its wavelength and the total length of dipole is also
half the wavelength (λ/2). Hence, it is called as half-wave dipole
antenna.
 The dipole in positive half cycle induces positive charges and the
electrons tend to move towards the dipole. During negative cycle
the electrons here tend to move away from the dipole. And the
cycle repeats.
 The cumulative effect of this produces a varying field effect which
gets radiated in the same pattern produced on it.
 Hence, the output would be an effective radiation following the
cycles of the output voltage pattern.
 Thus, a half-wave dipole radiates effectively. An antenna works
effectively at its resonant frequency, which occurs at its resonant
length.
Radiation Pattern:
The radiation pattern of this half-wave dipole is Omni-directional in the H-plane. It is desirable for many
applications such as mobile communications, radio receivers etc.

Advantages
 Input impedance is not sensitive.
 Matches well with transmission line impedance.
 Has reasonable length.
 Length of the antenna matches with size and directivity.
Disadvantages
 Not much effective due to single element.
 It can work better only with a combination.
Applications
 Used in radio receivers.
 Used in television receivers.
FOLDED DIPOLE ANTENNA

Figure: Half wave Folded Dipole

 A folded dipole is an antenna, with two conductors connected on both sides, and folded to
form a cylindrical closed shape, to which feed is given at the center.
 A folded dipole is a half-wave dipole with an additional parallel wire connecting its two
ends.
 As the ends appear to be folded back, the antenna is called a folded dipole antenna.
 If the additional wire has the same diameter and cross-section as the dipole, the emission
pattern is nearly identical to half wave dipole, but at resonance, its feed point impedance is four
times the radiation resistance of a single-wire dipole.
 The length of the dipole is half of the wavelength. Hence, it is called as half wave folded dipole
antenna.
  Frequency range: The range of frequency in which half wave folded dipole operates is around
3 KHz to 300 GHz.
 This is mostly used in television receivers
Advantages
 Reception of balanced signals.
 Receives a particular signal from a band of frequencies without losing the quality.
 A folded dipole maximizes the signal strength.
Disadvantages
 Displacement and adjustment of antenna is difficult.
 Outdoor management can be difficult when antenna size increases.
Applications
 Mainly used as a feeder element in Yagi antenna, Parabolic antenna, turnstile antenna, log
periodic antenna, phased and reflector
arrays, etc.
Radiation Pattern of Half wave folded
dipole:
 The radiation pattern of half-wave
folded dipoles is the same as that of the
half-wave dipole antennas.
 The following figure shows the
radiation pattern of half-wave folded
dipole antenna, which is Omni-
directional pattern.
Yagi-Uda antenna
 Half-wave folded dipole antennas are used where optimum power transfer is needed and
where large impedances are needed.
 The Half-wave folded dipole is the main element in Yagi-Uda antenna.

 The figure shows a Yagi-Uda antenna.

 From the figure, it is seen that there are many directors placed to increase the directivity of
the antenna.
 The feeder is the folded dipole.
 The reflector is the lengthy element,
which is at the end of the structure.
Radiation Pattern:
 PRINCIPLES OF ELECTRONIC COMMUNICATION MODULE 2

PARABOLIC ANTENNA
 A parabolic antenna is an antenna that uses a parabolic reflector, which is a curved
surfacewith the cross-sectional shape of a parabola, to direct the radio waves.
 Parabolic Reflectors are Microwave antennas. The frequency range used for the application of
parabolic reflector antennas is above 1MHz.
 These antennas are widely used for radio and wireless applications.
Principle of Operation
 The following figure shows the geometry of parabolic reflector.
 The point F is the focus (feed is given) and V is the vertex. The line joining F and V is the axis of
symmetry. P,Q are the reflected rays where L represents the line directrix on which the reflected
points lie (to say that they are being collinear).

 The shape of the parabola is helpful for building an

antenna,using the waves reflected.
 Properties of Parabola
 All the waves originating from focus, reflects back to the parabolic axis. Hence, all the waves
reaching the aperture are in phase.
 As the waves are in phase, the beam of radiation along the parabolic axis will be strong and
concentrated.
Advantages
 Reduction of minor lobes
 Wastage of power is reduced
 Equivalent focal length is achieved
 Feed can be placed in any location, according to our convenience
Disadvantage
 Some of the power that gets reflected from the parabolic reflector is obstructed. This becomes
aproblem with small dimension paraboloid.
Applications
 The parabolic reflector is mainly used in satellite communications.
Also used in wireless telecommunication systems.

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