lesson 3 Amplitude Modulation Transmission.pdf

OD 12 Mining Engineering Lesson 3

Telecommunications Principles Page 1

1 Amplitude Modulation Transmission

Changing some property of a (high-frequency) sinusoid in accordance with another signal,

for example speech, is called modulation. It is possible to change the amplitude of the high-

frequency signal, called the carrier, in accordance with speech and or music. The modulation

is then called amplitude modulation or AM for short. It is also possible to change the phase

angle of the carrier, in which case we have phase modulation (PM), or the frequency, in

which case we have frequency modulation (FM).

In amplitude modulation, the amplitude of a carrier signal is varied by the intelligence signal

whose frequency is invariably lower than that of the carrier. In practice, the carrier may be

high frequency (HF) while the modulation is audio. AM is defined as a system of modulation

in which the amplitude of the carrier is made proportional to the instantaneous amplitude of

the intelligence signal (modulating voltage).

Figure 1: Amplitude modulated wave: the carrier frequency remains sinusoidal at c while

the envelope varies at frequency s.



Let the carrier voltage and the modulating voltage, Vc and Vm, respectively, be represented

by

Vc= VC sin wct

Vm= Vmsin wmt

Note that phase angle has been ignored in both expressions since it is unchanged by the

amplitude modulation process. Its inclusion here would merely complicate the proceedings,

without affecting the result. However, it will certainly not be possible to ignore phase angle

when we deal with frequency and phase modulation.

From the definition of AM, you can see that the (maximum) amplitude Vc of the unmodulated

carrier will have to be made proportional to the instantaneous modulating voltage Vmsin wmt

when the carrier is amplitude-modulated.

When a carrier is amplitude-modulated, the proportionality constant is made equal to unity,

and the instantaneous modulating voltage variations are superimposed onto the carrier

amplitude. Thus, when there is temporarily no modulation, the amplitude of the carrier is

equal to its unmodulated value. When modulation is present, the amplitude of the carrier is

varied by its instantaneous value.

The distortion will occur if Vm is greater than Vc, this distortion is a result of overdriving the

amplifier stage. This, and the fact that the ratio Vm/Vc often occurs, leads to the definition of

the modulation index.

Figure 2 Amplitude of AM wave.



The modulation index is a number lying between 0 and I, and it is very often expressed as a

percentage and called the percentage modulation

It has been shown that the carrier component of the modulated wave has the same amplitude

as the unmodulated carrier. That is, the amplitude of the carrier is unchanged; energy is

either added or subtracted. The modulated wave contains extra energy in the two sideband

components. Therefore, the modulated wave contains more power than the carrier had

before modulation took place. Since the amplitude of the sidebands depends on the

modulation index Vm/Vc is anticipated that the total power in the modulated wave will

depend on the modulation index also. This relation may now be derived.

The total power in the modulated wave will

Example:

A 400-watt(400-W) carrier is modulated to a depth of 75 percent. Calculate the total power

in the modulated wave.

Current calculations. The situation which very often arises in AM is that the modulated and

unmodulated currents are easily measurable, and it is then necessary to calculate the

modulation index from them. This occurs when the antenna current of the transmitter is

metered, and the problem may be resolved as follows. Let Ic be the unmodulated current and

It the total, or modulated, current of an AM transmitter, both being rms values. If R is the

resistance in which these currents flow, then



Example:

The antenna current of an AM transmitter is 8 amperes (8 A) when only the carrier is sent, but

it increases to 8.93 A when the carrier is modulated by a single sine wave. Find the

percentage modulation. Determine the antenna current when the percent of modulation

changes to 0.8.

The choice of carrier frequency for a radio transmitter is largely determined by government

regulations and international agreements.. In theory, every transmitter must have a unique

frequency of operation and sufficient bandwidth to ensure no interference with others.

However, bandwidth is limited by considerations such as cost and the sophistication of the

transmission technique to be used so that, in practice, two radio transmitters may operate on

frequencies which would normally cause interference so long as they propagate their signals



within specified limits of power and are located (geographically) sufficiently far apart. The

location as well as the power transmitted by each transmitter is monitored and controlled by

the government (TCRA).

Once the carrier frequency is assigned to a radio station, it is very important that it maintains

that frequency as constant as possible. There are two reasons for this: (1) if the carrier

frequency were allowed to drift then the listeners would have to re-tune their radios from time

to time to keep listening to that station, which would be unacceptable to most listeners; (2) if

a station drifts (in frequency) towards the next station, their sidebands would overlap and

cause interference. The carrier signal is usually generated by an oscillator, but to meet the

required precision of the frequency it is common practice to use a crystal-controlled oscillator.

At the heart of the crystal-controlled oscillator is a quartz crystal cut and polished to very tight

specifications which maintains the frequency of oscillation to within a few hertz of its nominal

value.

Figure 3: Block diagram showing the components which make up the AM transmitter

Crystal-Controlled Oscillator

The purpose of the crystal oscillator is to generate the carrier signal. To minimize interference

with other transmitters, this signal must have extremely low levels of distortion so that the

transmitter operates at only one frequency.



Frequency Multiplier

The purpose of the frequency multiplier is to accept an incoming signal of frequency fc=n,

where n is an integer, and to produce an output at a frequency fc. A frequency multiplier can

have a single stage of multiplication or it can have several stages. The output of the

frequency multiplier goes to the carrier input of the amplitude modulator.

Amplitude Modulator

The amplitude modulator has two inputs, the first being the carrier signal generated by the

crystal oscillator and multiplied by a suitable factor, and the second being the modulating

signal (voice or music) which is represented by the single frequency fs. In reality, the

frequencies present in the modulating signal are in the audio range 2020,000 Hz. The

output from the amplitude modulator consists of the carrier, the lower and upper sidebands.

Audio Amplifier

The audio amplifier accepts its input from a microphone and supplies the necessary gain to

bring the signal level to that required by the amplitude modulator.

Radio-Frequency Power Amplifier

The power level at the output of the modulator is usually in the range of watts and the power

required to broadcast the signal effectively is in the range of tens of kilowatts. The radio-

frequency amplifier provides the power gain as well as the necessary impedance matching to

the antenna.

Antenna

The antenna is the circuit element that is responsible for converting the output power from the

transmitter amplifier into an electromagnetic wave suitable for efficient radiation in free

space. Antennae take many different physical forms determined by the frequency of operation

and the radiation pattern desired.



2 The Amplitude Modulated Radio Receiver

The electromagnetic disturbance created by the transmitter is propagated by the transmitter

antenna and travels at the speed of light. It is evident that, if the electromagnetic wave

encounters a conductor, a current will be induced in the conductor. How much current is

induced will depend on the strength of the electromagnetic field, the size and shape of the

conductor and its orientation to the direction of propagation of the wave. The conductor will

then capture some of the power present in the wave and hence it will be acting as a receiver

antenna. However, other electromagnetic waves emanating from all other radio transmitters

will also induce some current in the antenna. The two basic functions of the radio receiver

are:

(1) to separate the signal induced in the antenna by the transmission which we wish to

receive from all the other signals present,

(2) to recover the message signal which was used to modulate the transmitter carrier.

The superheterodyne receiver takes the incoming radio-frequency signal whose frequency

varies from station to station and transforms it to a fixed frequency called the intermediate

frequency (IF). It is then easier to do the necessary filtering to eliminate interference and, at

the same time, to provide some power gain or amplification to the desired signal.

Receiver Antenna

The AM receiver antenna can take many different forms such as the ferrite bar found in most

portable receivers, the whip antenna found on automobiles, and the outdoor wire type

consisting of several metres of wire strung between two towers.



Low-Power Radio-Frequency Amplifier

Since the input voltage of the amplifier is of the order of microvolts and the signal to be

delivered to the demodulator is usually in volts, the amplifier must have a high gain. A multi-

stage amplifier has to be used to realize the necessary gain. Some of the stages of gain can

be placed before the frequency changer, in which case they are referred to as the radio-

frequency amplifier stage, or after the frequency changer, in which case they are called the

intermediate-frequency amplifier stage.

Frequency Changer or Mixer

Two distinct approaches can be used in the design of a mixer. The first is based on an analog

multiplication of the radio-frequency and the local oscillator signals. The second uses the

local oscillator signal to switch segments of the radio-frequency signal positive and negative.

Intermediate-Frequency Stage

The output of the mixer contains a multitude of frequencies made up of the sums and

differences of the local oscillator frequency and the radio-frequency signal and their various

harmonics.

Automatic Gain Control

The function of the automatic gain control is to ensure that the signal reaching the

demodulator is sufficiently high and within the limits for efficient demodulation. It does this by

sensing the level of the signal at the input to the modulator and adjusting the gain of a

variable gain amplifier to keep the level constant.

Demodulator

The input to the demodulator is a carrier of frequency 455 kHz (intermediate frequency) with

an amplitude envelope determined by the audio signal.

Audio-Frequency Amplifier

A few minor changes may be necessary in the design of the final stage of the amplifier so that

it can drive a loudspeaker efficiently.



Loudspeaker

So far, the audio-frequency signal is in an electrical form. To convert it to an acoustic signal,

an electrical-to-acoustic transducer is required. This can take two basic forms: the

loudspeaker and the headphone.

3 Frequency Modulated Radio Transmitter

In frequency modulated (FM) radio, the frequency of the carrier is varied about a fixed value

in accordance with the amplitude of the audio frequency. The amplitude of the carrier is kept

constant.. In FM systems the information to be transmitted is contained in the variation of the

frequency of the carrier about a pre-set value. The amplitude of the FM signal is kept

constant and, indeed, if there are changes in the amplitude of the FM signal, they are

removed by clipping before demodulation. By comparison, FM systems are less susceptible to

degradation by noise.

The Amplitude Limiter.



An FM signal, by definition, must have constant amplitude. In practice, circuit non-linearities

cause variations in the envelope of the FM signal related to the modulating signal, that is,

some amplitude modulation takes place. It is evident that any AM present in an FM system

will interfere with the signal during the demodulation process since most FM demodulators

convert the variation of frequency to a variation of amplitude before detection. A second

reason for limiting the amplitude of the FM signal is that the noise present in the

communication channel generally rides on the envelope of the signal and therefore by

clipping the amplitude of the signal some of the noise can be removed.

Bandpass Filter.

It is reasonable to assume that the output of the clipper is a square wave. The frequency of

the square wave varies but it is centered at the carrier frequency.

Discriminator.

The purpose of the discriminator is to convert the variation of frequency to a variation of

amplitude. A frequency-to-amplitude convertor followed by an envelope detector is used to

recover the message contained in the modulating signal.

Envelope Detector.

The purpose of this envelope detector is to correct the long-term frequency deviation of the

main oscillator while allowing the short-term frequency deviation caused by the modulating

signal.

4 The Frequency Modulated Radio Receiver

In amplitude modulation, the frequency of the carrier is kept constant while its amplitude is

changed in accordance with the amplitude of the modulating signal. In frequency

modulation, the amplitude of the carrier is kept constant and its frequency is changed in

accordance with the amplitude of the modulating signal. It is evident that, if a circuit could be

found which will convert changes in frequency to changes in amplitude, the techniques used

for detecting AM can be used for FM as well.



The superheterodyne technique is used in FM for the same reasons it is used in AM; it

translates all incoming frequencies to a fixed intermediate frequency at which the filtering

process can be carried out effectively.

Although the structures of AM and FM receivers are similar, there are very important

differences which require different design and construction approaches.

Antenna

An important point to remember is that an antenna is a reciprocal device, that is, it can be

used both for transmitting signals as well as for receiving them. An antenna structure that

produces a good ground wave radiation pattern will have a good response to the same

ground wave radiation when used in the receiving mode.

Radio-Frequency Amplifier

The purpose of the radio-frequency amplifier is to boost the power of the incoming signal

relative to all the other signals picked up by the antenna to a level which can be used in the

frequency changer.

Local Oscillator



The local oscillator can take any of the usual oscillator forms with a bipolar transistor as the

active element. It must produce enough power to drive the mixer.

Frequency Changer

For this application the dual-gate FET mixer has the advantage of low leakage of the local

oscillator signal to the antenna via the radio-frequency amplifier. Such a leakage and its

subsequent radiation can cause interference with other communication and radio navigation

equipment.

Amplitude Limiter

The radio-frequency amplifier, the mixer, and the intermediate-frequency amplifier, in theory,

should have a flat amplitude response in their pass bands. In practice, this is not so. The

result is that the signal emerging from the intermediate-frequency amplifier has some

variation of amplitude with respect to frequency. This is a form of AM and it must be removed

if distortion is to be avoided.

Frequency Discriminator

The purpose of the frequency discriminator is to convert relatively small changes of frequency

(in a very high-frequency signal) to relatively large changes in amplitude with respect to time.

5 The Television Transmitter The transmission of video images depends on a scanning device that can break up the image

into a grid and measure the brightness of each element of the grid. This information can be

sent serially or in parallel to a distant point and used to reproduce the image. It is evident that

the smaller the size of the grid element, the better the definition of the image.

One of the simplest devices which can measure the brightness of light is the phototube. It

consists of a cathode which is coated with a material which gives off electrons when light is

shone on it and an anode which can collect the emitted electrons when a suitable voltage is

applied to it. The cathode and anode are enclosed in an evacuated glass envelope. The

number of electrons emitted by the cathode is proportional to the intensity of the light

impinging on it.



A system of lenses focus the image onto a camera tube which collects and codes the

information about the brightness and position of each element of the matrix forming the

picture by scanning the matrix. A pulse generator supplies pulses to the camera to control the

scanning process. The output from the camera goes to a video amplifier for amplification and

the addition of extra pulses to be used at the receiver for decoding purposes.

A microphone picks up the sound associated with the picture and after amplification the

signal is fed to the audio terminal of a frequency modulator. The carrier signal supplied to the

modulator is a 4.5 MHz signal, generated by a crystal controlled oscillator at a lower

frequency and multiplied by an appropriate factor.

The FM signal carrying the audio information is added to the video signal. The output of the

video amplifier consisting of the video signal, receiver control pulses, and the frequency

modulated signal is fed to the amplitude modulator.

Scanning System

A series of pulses are generated which control the initiation of the horizontal sweep of the

electron beam across the target (horizontal trace) from the left-hand side to the right. A

blanking pulse is used to cut off the beam while it is returned to the left-hand side ready for

the next sweep (harmonic retrace). During this period, the vertical trace circuit moves the



beam down just the right distance for the second line to be swept. When the whole frame has

been scanned, a reset pulse returns the beam to the top left-hand corner ready to repeat the

process.

Video Amplifier

Ideally, a video signal consists of frequencies from zero (dc) to some high frequency. The dc

response is required when large areas of black, white or other intermediate shades have to

be transmitted. The limit of the high-frequency response is determined by the resolution

required for a single vertical black line on a white background or vice versa.

Radio-Frequency Circuits

The composite video signal from the output of the video amplifier is used to amplitude

modulate the radio-frequency carrier obtained at a lower frequency from a crystal oscillator

and multiplied by a suitable factor by a frequency multiplier.

Vestigial Sideband Filter

Single-sideband (SSB) transmission could be used but the filter needed to remove one of the

sidebands is fairly sophisticated and the demodulation equipment is complex and difficult to

maintain.

Antenna

Television broadcast frequencies are either in the very-high-frequency (VHF) band which is

from 30 to 300MHz or in the ultra-high-frequency (UHF) band which is from 300 to

3000MHz. At these frequencies, antennas have highly directional properties. To get the

circular radiation pattern in the horizontal plane normally used for broadcasting television

signals, several arrangements of antenna arrays can be used.

Color Television

The transmission of video signals in color is a subject which can take up several volumes.

However, because color television is so common, a simplified explanation of how it works is

now offered. The first step is to discuss some of the properties of color and the results of

mixing them. There are three primary colors: red, blue and green, and by using appropriate

proportions of these, all other colors perceived by the human eye can be obtained.



6 The Television Receiver

The television receiver is almost identical to the AM radio receiver in its use of the

superheterodyne principle. There are a few differences in the details of the signal processing

due to the greater complexity of the system.

The antenna picks up the electromagnetic radiation from the transmitter and feeds it to the

radio-frequency amplifier. After amplification and filtering to attenuate other incoming signals

from other transmitters, the signal goes to the mixer where it is mixed with the output from the

local oscillator. As before, the local oscillator and the radio-frequency amplifier are tuned to

track each other with a constant frequency difference equal to the intermediate frequency.

The intermediate frequency for the television receiver is usually 45.75 MHz. The signal is

subjected to further filtering before it proceeds to the video demodulator for the recovery of

the baseband information in the signal. The next stage is to separate the composite video

signal into its three components, namely the video proper, the FM sound subcarrier and its

sidebands, and the vertical and horizontal control pulses. The video signal is amplified to the

level required to drive the picture tube by the video amplifier and the vertical and horizontal

control pulses are suitably conditioned and used in the deflection systems of the receiver to

synchronize it to the transmitter a condition that must be met for proper reproduction of the

images sent. The FM signal is amplified, amplitude limited and detected and after some

amplification it is used to drive the loudspeaker.

Antenna

Frequencies for commercial FM (88108 MHz) occupy the spectrum between channels 6 and

7 of the VHF television frequencies (5488MHz and 174216 MHz, respectively). Except for

slight differences in the physical dimensions, the antennas tend to take the same form. These



are frequency ranges in which a half-wavelength dipole antenna has reasonable physical

dimensions (0.72.7 m).

Superheterodyne Section

The radio-frequency amplifier is tunable over the VHF frequency range. This is accomplished

with a variable capacitor which is mechanically ganged to the variable capacitor which tunes

the local oscillator. The objective is to generate a local oscillator frequency which is equal to

the radio-frequency amplifier center frequency plus the intermediate frequency.

Intermediate-Frequency Amplifier

Like all superheterodyne receiver systems, the detailed selection of the desirable and the

rejection of the undesirable frequencies take place at the intermediate-frequency stage. At the

same time, some parts of the spectrum may be emphasized to equalize the quality of the low-

frequency video (large uniform areas) and the high-frequency video (areas with fine details).

The exact frequency response of the video intermediate- frequency amplifier is of no

importance at this point except to point out that it is designed to compensate for the

frequency response of the vestigial side band filter in the transmitter.

Video Detector

It will be recalled that the video signal is amplitude modulated and, in theory, it requires a

simple envelope detector to demodulate it. However, the situation is complicated somewhat

by the fact that the input signal to the detector is a vestigial sideband signal.

The Video Amplifier

In the television receiver, the load of the video amplifier is the grid of the cathode ray tube

(usually called the picture tube). This requires voltages between approximately 50V and

100Vand, in theory, no current flows in the grid circuit. However, the grid represents a

capacitive load and a capacitance requires the movement of charge (current) to change the

voltage across it. The output stage of the video amplifier must be capable of providing the

necessary current and hence power. Another way of saying the same thing is that the output

stage of the video amplifier must have a low output resistance so that the grid capacitance

can be charged much faster than the fastest change in voltage present in the video signal.

The Audio Channel

From the output of the video detector, a bandpass amplifier selects and boosts the FM signal

centered at 4.5 MHz. Electron Beam Control Subsystem In the television transmitter, the



pulse generator output was used to control the vertical and horizontal sweeps of the electron

beam which scanned the mosaic in the camera tube. The same pulses were added to the

video signal together with the 4.5MHz FM voice carrier to make up the composite video.

Figure

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lesson 3 Amplitude Modulation Transmission.pdf