Physics SME copy 1

There has been a move by the government of Rwanda to make her citizens to 
change from using analog devices to digital devices. Analog devices transmit 
and receive signals in analog form whereas digital devices transmit and 
receive signals digitally.
a) What are different forms of signals you know that you normally use in 
daily life communication?
b) Why do you think there is a need to change from analog to digital signal 
transmission?
c) Mutesi communicates to her brother Ndayisenga who studies abroad 
using Facebook. Is the flow of information analog or digital? Explain your 
argument.
d) Using information gained in above questions, discuss different signals 
shown in the illustration.

8.1.1. Classification of types of Information
Information is any entity or form that resolves uncertainty or provides the 
answer to a question of some kind. It is thus related to data and knowledge, 
as data represents values attributed to parameters, and knowledge signifies 
understanding of real things or abstract concepts.
Buck (1983) provides a useful classification of types of information that can 
be displayed to users. These are: Instructions, Command, Advisory, Answers, 
Historical, and Predictive.
Each of these types of information can, in theory, be provided on most types of 
displays. However, some lend themselves better to one form of display rather 
than another. The characteristics of each of these types can now be briefly 
discussed. 
1. Instructions: Refer to information that guides behavior in a particular way. 
In other words, it supports performance to carry out a task by prompting 
on what to do and when to do it. A simple sign telling people to enter or 
not enter a door would be one example. Other simple cases include the 
dialogue messages that are provided on automated cash machines (ACM). 
More complex instructions will appear in printed form on the packaging or 
the instructional manuals for pieces of equipment.
2. Command: Messages give a very straightforward statement on what 
is or what is not permitted. ‘Do not enter’, ‘do not smoke’, ‘do not eat or 
drink’, are examples of command messages. Sometimes they are similar to 
instructions, but are much more focused on simple statements that refer to 
high priority items.
3. Advisory: Messages are somewhat watered down versions of command 
messages. In some cases, these will be recommendations to avoid a situation, 
at other times they would be information allowing for the preparation or 
planning of particular activities. For example, we might be advised that 
our train is late by a spoken message and we might, possibly, be given an 
accurate time estimate for when the train will be available.
4. Answers: Information may be provided in response to a particular enquiry 
that has been made. This is typical of an interactive information-handling 
situation, where we have a particular question in mind or degree of 
uncertainty and we seek information from a source with regard to removing 
that uncertainty.
It turns out that most of the information that is sought from displays is of 
the answer kind. If we want to know what the time of day is, we look at our 
watches and clocks to find the answer.
5. Historical: Displays are used to look back at the state of a variable over a 
period of minutes, hours, days or even years. A graphical representation 
of road accidents over the last century would be a historical display of 
information. If we want to know what the temperature fluctuation has 
been in an office on a daily basis, then specialist devices can be brought in 
and placed in the office that will give a pen recording over a fixed period of 
time.
 It is much easier to see if there is a trend in information if it is displayed in 
this way; the alternative is to hold in memory a general impression of what
the temperature readings have been at a number of points during the day 
or record them manually on a chart. Gauging the temperature in an office 
concerns a relatively low risk situation.
 However, if the concern is with the temperature in a critical vessel in a 
chemical process, then the temperature trends exhibited over the time are 
quite important. 
6. Predictive: displays are much more specialized, but increasingly found 
in complex processes. In the same way that historical data support 
performance in making a judgment based on the current value, predictive 
information enables examination of the current value and indicates any 
likely change in the future.
Predictor displays enable better control over vehicles, typically at sea or 
airborne, and enable smoother transitions from one state to another. They 
are used in slow response systems where it is difficult to see the immediate 
effect of an action that has been carried out.
Predictive displays will enable a variable to be plotted into the future. 
The same graphs that are used as historical displays can also be used as 
predictive displays. 
Telecommunication in real life is the transmission of signals and other types 
of data of any nature by wire, radio, optical or other electromagnetic systems 
of communication. 
Telecommunication occurs when the exchange of information between 
communicating participants includes the use of signs or other technologically 
based materials such as telephone, TV set, radio receiver, radio emitter, computer, 
and so on. All can be done either mechanically, electrically or electronically.
Message: A message is a term standing for information put in an appropriate 
form for transmission. Each message contains information. A message can be 
either analog message (a physical time variable quantity usually in smooth 
and continuous form) or a digital message (an ordered sequence of symbols 
selected from finite set of elements) 
- Analog message: a physical time-variable quantity usually in smooth 
and continuous form. 
- Digital message: ordered sequence of symbols selected from finite set of 
elements. 
A signal is a mathematical function representing the time variation of a physical 
variable characterizing a physical process and which, by using various models, 

can be mathematically represented.

In telecommunication, the message is also known as a signal and the signal is 
transmitted in an electrical or voltage form.
8.1.2. Elements of Communication
Communication is the process of sharing the message through continuous flow 
of Symbols. It is composed by the following elements:
Sender 
The sender is a party that plays the specific role of initiating communication. 
To communicate effectively, the sender must use effective verbal as well as 
nonverbal techniques such as:
- Speaking or writing clearly.
- Organizing your points to make them easy to follow and understand.
- Maintaining eye contact.
- Using proper grammar.
- Giving accurate information.
All the above components are essential in the effectiveness of your message. 
One will lose the audience if it becomes aware of obvious oversights on ones 
part. The sender should have some understanding of who the receiver is, in 
order to modify the message to make it more relevant. 
Receiver 
The receiver means the party to whom the sender transmits the message. 
A receiver can be one person or an entire audience of people. In the basic 
communication model, the receiver is directly connected with the speaker. 
The receiver can also communicate verbally and nonverbally. The best way to 
receive a message is:
- To listen carefully.
- Sitting up straight. 
- Making eye contact.
- Don’t get distracted or try to do something else while you’re listening.
- Nodding and smiling as you listen.
- Demonstrate that you understand the message. 
Message
The message is the most crucial element of effective communication which 
includes the content a sender conveys to the receiver. A message can come in
many different forms, such as an oral presentation, a written document, an 
advertisement or just a comment. 
In the basic communication model, the way from one point to another represents 
the sender’s message travelling to the receiver. The message isn’t necessarily 
what the receiver perceive it to be. Rather, the message is what the sender 
intends the message to be. The sender must not only compose the message 
carefully, but also evaluate the ways in which the message can be interpreted. 
Channel
The channel is a medium through which a message travels from the sender to 
the receiver. The message travels from one point to another via a channel of 
communication. The channel is a physical medium stands between the sender 
and receiver. 
Many channels or types of communication exist, such as 
- The spoken word.
- Radio or television.
- An Internet site.
- Something written, like a book, letter or magazine. 
Every channel of communication has its advantages and disadvantages. For 
example, one disadvantage of the written word, on a computer screen or in 
a book, is that the receiver cannot evaluate the tone of the message. For this 
reason, effective communicators should make written word communications 
clear so receivers don’t rely on a specific tone of voice to convey the message 
accurately. 
The advantages of television as a channel for communication include its 
expansive reach to a wide audience and the sender’s ability to further manipulate 
the message using editing and special effects. 
Feedback
This describes the receiver’s response or reaction to the sender’s message. The 
receiver can transmit feedback through asking questions, making comments or 
just supporting the message that was delivered. 
Feedback helps the sender to determine how the receiver interpreted the 
message and how it can be improved. The signal normally, must be raised at a 
level that will permit it to reach its destination. This operation is accomplished 

by amplifiers.

8.1.3. Modes of transmission
1) Simplex transmission
Simplex transmission is a single one-way base band transmission. Simplex 
transmission, as the name implies, is simple. It is also called unidirectional 
transmission because the signal travels in only one direction. An example 
of simplex transmission is the signal sent from the TV station to the home 
television. 
Data in a simplex channel is always one way. Simplex channels are not often 
used because it is not possible to send back error or control signals to the 
transmit end.
2) Half-duplex communications
Half-duplex transmission is an improvement over simplex transmission 
because the traffic can travel in both directions. Unfortunately, the road is 
not wide enough to accommodate bidirectional signals simultaneously. This 
means that only one side can transmit at a time. Two-way radios, such as 
police or emergency communications mobile radios, work with half-duplex 
transmissions. If people at both ends try to talk at the same time, none of the 

transmissions get through.

3) Full-duplex communications
Full-duplex transmission operates like a two-way, two-lane street. Traffic can 
travel in both directions at the same time. A land-based telephone conversation 
is an example of full-duplex communication. Both parties can talk at the same 
time, and the person talking on the other end can still be heard by the other 
party while they are talking. Although when both parties are talking at the 

same time, it might be difficult to understand what is being said.

8.2.1. Analog signal system
Analog signals
Analog signal is a continuous signal that contains time varying quantities. An 
analog signal is a continuous wave denoted by a sine wave and may vary in signal 
strength (amplitude) or frequency (time). The sine wave’s amplitude value can 
be seen as the higher and lower points of the wave, while the frequency (time) 

value is measured in the sine wave’s physical length from left to right.

Analog signal can be used to measure changes in physical phenomenon such as 
light, sound, pressure, or temperature. For instance, microphone can convert 
sound waves into analog signal. Even in digital devices, there is typically some 
analog component that is used to take in information from the external world 
which will then get translated into digital form –using analog to digital converter.
A system is a physical set of components that take a signal and produces a 
signal. In terms of engineering, the input is generally some electrical signal and 
the output is another electrical signal.
Analog systems operate with values that vary continuously and have no abrupt 
transitions between levels. For a long time, almost all electronic systems were 
analog, as most things we measure in nature are analog. For example, your 
voice is analogous; it contains an infinite number of levels and frequencies. 
Therefore, if you wanted a circuit to amplify your voice, an analog circuit seems 
a likely choice. 
Example of analog electronic systems
A public address system
A public address system (PAS) is an electronic sound amplification and 
distribution system with a microphone, amplifier and loudspeakers, used to 
allow a person to address a large public, for example for announcements of 
movements at large and noisy air and rail terminals or a sports stadium.
Advantages of analog signals
- Uses less bandwidth than digital sounds.
- More accurate representation of sound.
- It is the natural form of sound.
- Because of editing limitations, there is little someone can do to tinker 
with the sound, so what you are hearing is the original sound.
Disadvantages
- There are limitations in editing.
- Recording analog sound on tape is expensive.
- It is harder to synchronize analogous sound.
- Quality is easily lost if the tape becomes ruined.
- A tape must always be wound and rewound in order to listen to specific 
part of sound which can damage it.
- Analog is susceptible to clipping where the highest and lowest notes of a 

sound are cut out during recording.

In Rwanda recently analog systems were replaced by digital systems that 
provide greater capacity of data transfer and increased reliability and security.
8.2.2. Digital Signal system
A digital signal refers to an electrical signal that is converted into a pattern of 
bits. Unlike an analog signal, which is a continuous signal that contains timevarying quantities, a digital signal has a discrete value at each sampling point. 
The precision of the signal is determined by how many samples are recorded 
per unit of time. For example, the illustration of fig.8.5 below shows an analog 
pattern (represented as the curve) alongside a digital pattern (represented as 

the discrete lines).

A digital signal is easily represented by a computer because each sample can 
be defined with a series of bits that are either in the state 1 (on) or 0 (off). 
Digital signals can be compressed and can include additional information for 
error correction. 
A radio signal, for example, will be either on or off. Digital signals can be sent 
for long distances and suffer less interference than analog signals.
Unlike analog technology which uses continuous signals, digital technology 
encodes the information into discrete signal states. When only two states are 
assigned per digital signal, these signals are termed binary signals. One single 

binary digit is termed a bit - a contraction for binary digit.

In electronic signal and information processing and transmission, digital 
technology is increasingly being used because, in various applications, digital 
signal transmission has many advantages over analog signal transmission. 
Numerous and very successful applications of digital technology include the 
continuously growing number of Personal Computers, the communication 
network ISDN as well as the increasing use of digital control stations (Direct 

Digital Control: DDC)

Advantages of digital signals 
- More capacity from the same number of frequencies; that is, they 
provide superior Spectral Efficiency. This is a result of the modulation 
methods used, and the fact that, in many cases more than one ‘conversation’ 
can be accommodated within a single radio channel.
- Consistent voice clarity at low received signal levels near the edge 
of coverage. The general consensus is that digital radios provide better 
audio quality than analog ones. With analog FM radios, the audio quality 
steadily declines as the received signal strength gets weaker.
Digital radios however, will have a consistent audio quality throughout the 
full service area. The edges of the coverage area in a digital radio system 
are similar to those experienced with cellular telephones.
- Data is defined in the standard. This means data implementations are 
no longer proprietary, there are a wide variety of data mechanisms and 
inter operability can extend into the data domain. With the accepted 
increase of efficiency by using data communications over voice, this will 
further increase the usability and effectiveness of digital radio systems.
- Secure transmissions: In digital technologies, data and voice can be 
secured using encryption without impacting voice quality using industry 

standard encryption techniques.

8.2.3. Principle of digital signal systems
Digital systems process digital signals which can take only a limited number 
of values (discrete steps), usually just two values are used: the positive supply 
voltage (+Vs) and zero volts (0V).
Digital systems contain devices such as logic gates, flip-flops, shift registers 
and counters. A computer is an example of a digital system.
A logic gate is a building block of a digital circuit. Most logic gates have two 
inputs and one output and are based on Boolean algebra. At any given moment, 
every terminal is in one of the two binary conditions false (high) or true (low). 
False represents 0, and true represents 1. Depending on the type of logic gate 
being used and the combination of inputs, the binary output will differ. A logic 
gate can be thought of like a light switch, wherein one position the output is off 
(0), and in another, it is on (1). Logic gates are commonly used in integrated 

circuits (IC).

Boolean functions may be practically implemented by using electronic gates. 
The following points are important to understand.
- Electronic gates require a power supply.
- Gate INPUTS are driven by voltages having two nominal values, e.g. 0 V 
and 5 V representing logic 0 and logic 1 respectively. 
- The OUTPUT of a gate provides two nominal values of voltage only, e.g. 0 
V and 5 V representing logic 0 and logic 1 respectively. In general, there is 
only one output to a logic gate except in some special cases.
- There is always a time delay between an input being applied and the 
output responding. 
Truth tables are used to help to show the function of a logic gate. Digital systems 
are said to be constructed by using logic gates. These gates are the AND, OR, 
NOT, NAND, NOR, EXOR and EXNOR gates. The basic operations are described 
below with the aid of truth tables.
AND gate and Truth Tables
The AND gate is called the “all or nothing” gate. The graph of fig.8.8 shows the 
idea of the AND gate. The lamp (Y) will light only when both input switches (A 
and B) are closed. The truth table shows that the output (Y) is enabled (lit) only 

when both inputs are closed.

The AND gate is an electronic circuit that gives a high output (1) only if all its 
inputs are high. A dot (.) is used to show the AND operation i.e. A.B. Bear in 
mind that this dot is sometimes omitted we write AB.
OR gate and truth tables
The OR gate is called the “any or all” gate. The schematic Fig.8.10 shows the 
idea of the OR gate. The lamp ( Y ) will glow when either switch A or switch B 
is closed. The lamp will also glow when both switches A and B are closed. The 
lamp (Y) will not glow when both switches ( Aand B ) are open. The truth table 
details the OR function of the switch and lamp circuit are shown in fig. 8.10. 
The output of the OR circuit will be enabled (lamp lit) when any or all input 

switches are closed.

The standard logic symbol for an OR gate is drawn in Fig.8.11. Note the different 
shape of the OR gate. The OR gate has two inputs labeled A and B. The output 
is labeled Y. The OR gate is an electronic circuit that gives a high output (1) if 

one or more of its inputs are high. A plus (+) is used to show the OR operation. 

A NOT gate is also called an inverter. A NOT gate, or inverter, is an unusual gate. 
The NOT gate has only one input and one output as shwn in fig.8.12. If the input 
variable is A, the inverted output is known as NOT A. This is also shown as A’, or 

A with a bar over the top, as shown at the outputs.

The diagrams below show two ways that the NAND logic gate can be configured 
to produce a NOT gate. It can not also be done using NOR logic gates in the same 
way



This is a NOT-AND gate which is equal to an AND gate followed by a NOT gate.
The outputs of all NAND gates are high if any of the inputs are low. The symbol 
is an AND gate with a small circle on the output. The small circle represents 

inversion.

The ‘Exclusive-OR’ gate is a circuit which will give a high output if either, but not 
both, of its two inputs are high. An encircled plus sign is used to show the 
EOR operation.



The ‘Exclusive-NOR’ gate circuit does the opposite to the EOR gate. It will give 
a low output if either, but not both, of its two inputs are high. The symbol is 
an EXOR gate with a small circle on the output. The small circle represents 
inversion.
The NAND and NOR gates are called universal functions since with either one 
the AND and OR functions and NOT can be generated.
Note:
A function in sum of products form can be implemented using NAND gates by 
replacing all AND and OR gates by NAND gates.
A neither function in product of sums form can be implemented using NOR gates 

by replacing all AND and OR gates by NOR gates. 

Table 8.18 is a summary truth table of the input/output combinations for the 
NOT gate together with all possible input/output combinations for the other 
gate functions. Also note that a truth table with ‘n’ inputs has 2n
 rows. 

You can compare the outputs of different gates.

Who invented the idea?
This logical way of comparing numbers to make decisions that produce either 
a yes or no, 1 or 0, true or false is called Boolean algebra after its discoverer, 
English mathematician George Boole (1815–1864), who set out the idea in 
an 1854 book titled An Investigation of the Laws of Thought, on Which Are 
Founded the Mathematical Theories of Logic and Probabilities. His objective 
was to show how complex human reasoning could be represented in a logical, 

mathematical form.

The figure above shows how network for a certain telecommunications 
company in Rwanda. Study it carefully and answer the following 
questions.
a) How many cells are shown on the figure above? Give their 

respective names.

b) Id8.3.1. Structure of cellular network
An overall cellular network contains a number of different elements from the 
base transceiver station (BTS) itself with its antenna back through a base 
station controller (BSC), and a mobile switching centre(MSC) to the location 
registers (HLR and VLR) and the link to the public switched telephone network 
(PSTN). 
Of the units within the cellular network, the BTS provides the direct 
communication with the mobile phones. There may be a small number of base 
stations linked to a base station controller. This unit acts as a small centre to 
route calls to the required base station, and it also makes some decisions about 
which base station is the best suited for a particular call. 
The links between the BTS and the BSC may use either land lines of even 
microwave links. Often the BTS antenna towers also support a small microwave 
dish antenna used for the link to the BSC. The BSC is often co-located with a 
BTS. 
The BSC interfaces with the mobile switching centre. This makes more 

widespread choices about the routing of calls and interfaces to the land line 

based PSTN as well as the location registers. entify different masts shown on the figure.
c) In regard to the figure, what is the importance of masts in those 
different cells?
d) Why do you think in transmission of network, the targeted area is 
divided into small portions?
e) Compare the number of cells that should be allocated for urban 

areas to those for rural areas.

8.3.1. Structure of cellular network
An overall cellular network contains a number of different elements from the 
base transceiver station (BTS) itself with its antenna back through a base 
station controller (BSC), and a mobile switching centre(MSC) to the location 
registers (HLR and VLR) and the link to the public switched telephone network 
(PSTN). 
Of the units within the cellular network, the BTS provides the direct 
communication with the mobile phones. There may be a small number of base 
stations linked to a base station controller. This unit acts as a small centre to 
route calls to the required base station, and it also makes some decisions about 
which base station is the best suited for a particular call. 
The links between the BTS and the BSC may use either land lines of even 
microwave links. Often the BTS antenna towers also support a small microwave 
dish antenna used for the link to the BSC. The BSC is often co-located with a 
BTS. 
The BSC interfaces with the mobile switching centre. This makes more 
widespread choices about the routing of calls and interfaces to the land line 
based PSTN as well as the location registers. 


8.3.2. Principle of cellular network
The increase in demand and the poor quality of existing service led mobile 
service providers to research ways to improve the quality of service and 
to support more users in their systems. Because the amount of frequency 
spectrum available for mobile cellular use was limited, efficient use of the 
required frequencies was needed for mobile cellular coverage.
In modern cellular telephony, rural and urban regions are divided into areas 
according to specific provisioning guidelines.
Deployment parameters, such as amount of cell-splitting and cell sizes, 
are determined by engineers experienced in cellular system architecture. 
Provisioning for each region is planned according to an engineering plan that 
includes cells, clusters, frequency reuse, and handovers.
Cells
A cell is the basic geographic unit of a cellular system. The term cellular comes 
from the honeycomb shape of the areas into which a coverage region is divided. 
Cells are base stations transmitting over small geographic areas that are 
represented as hexagons. Each cell size varies depending on the landscape. 
Because of constraints imposed by natural terrain and man-made structures, 
the true shape of cells is not a perfect hexagon. 
Clusters 
A cluster is a group of cells. No channels are reused within a cluster. 
Fig. 8.23 illustrates a seven-cell cluster. In clustering, all the available frequencies 
are used once and only once. As shown on fig.8.24, each cell has a base station 
and any mobile user moving remains connected due to hand-offs between the 

stations.

Frequency Reuse 
Because only a small number of radio channel frequencies were available 
for mobile systems, engineers had to find a way to reuse radio channels in 
order to carry more than one conversation at a time. The solution was called 
frequency planning or frequency reuse. Frequency reuse was implemented 
by restructuring the mobile telephone system architecture into the cellular 
concept. 
The concept of frequency reuse is based on assigning to each cell a group of 
radio channels used within a small geographic area. Cells are assigned a group 
of channels that is completely different from neighboring cells.
 The coverage areas of cells are called the footprint. This footprint is limited by 
a boundary so that the same group of channels can be used in different cells that 

are far enough away from each other so that their frequencies do not interfere.

Cells with the same number have the same set of frequencies. Here, because the 
number of available frequencies is 7, the frequency reuse factor is 1/7. That is, 
each cell is using 1/7 of available cellular channels.
Cell Splitting
Unfortunately, economic considerations made the concept of creating full 
systems with many small areas impractical. To overcome this difficulty, system 
operators developed the idea of cell splitting.

As a service area becomes full of users, this approach is used to split a single area 
into smaller ones. In this way, urban centers can be split into as many areas as 
necessary in order to provide acceptable service levels in heavy-traffic regions, 
while larger, less expensive cells can be used to cover remote rural regions. 
Handoff 
The final obstacle in the development of the cellular network involved the 
problem created when a mobile subscriber travelled from one cell to another 
during a call. As adjacent areas do not use the same radio channels, a call must 
either be dropped or transferred from one radio channel to another when a 
user crosses the line between adjacent cells. 

Because dropping the call is unacceptable, the process of handoff was created. 
Handoff occurs when the mobile telephone network automatically transfers a 

call from radio channel to radio channel as mobile crosses adjacent cells.

During a call, two parties are on one voice channel. When the mobile unit moves 
out of the coverage area of a given cell site, the reception becomes weak. At 
this point, the cell site in use requests a handoff. The system switches the call 
to a stronger-frequency channel in a new site without interrupting the call or 
alerting the user. The call continues as long as the user is talking, and the user 
does not notice the handoff at all.
Conclusion
We can say that mobile communication system is a high capacity communication 
system arranged to establish and maintain continuity of communication paths 
to mobile stations passing from the coverage of one radio transmitter into the 
coverage of another radio transmitter.
A control center determines mobile station locations and enables a switching 
center to control dual access trunk circuitry to transfer an existing mobile 
station communication path from a formerly occupied cell to a new cell location. 
The switching center subsequently enables the dual access trunk to release the 

call connection to the formerly occupied cell.

While listening to radio on one of the evening, Mukamisha heard that the 
tuned channel was on FM at 100.7 MHz But her radio works efficiently 
when she pulls up the antenna.
a) What do you think is the significance of the antenna on her radio?
b) Hoping you has ever used/played a radio. Where do you think the 
information/sound from the radio come from?
c) Explain the mode of transmission of information as suggested in b) 
above to the receiving radio.
d) While going to sleep, her radio fell down and the speaker got 
problems. Do you think she was able to listen to late night programs 
on the same channel?

e) As indicated on the radio, what does FM, MW, and SW mean?

8.4.1. Simple radio transmitter
A radio transmitter consists of several elements that work together to generate 
radio waves that contain useful information such as audio, video, or digital 
data. The process by which a radio station transmits information is outlined in 

Fig. 8.29.

- Power supply: Provides the necessary electrical power to operate the 
transmitter.
- The audio (sound) information is changed into an electrical signal of the 
same frequencies by, say, a microphone, a laser, or a magnetic read write 
head. This electrical signal is called an audio frequency (AF) signal, 
because the frequencies are in the audio range (20 Hz to 20,000Hz). 
- The signal is amplified electronically in AF amplifier and is then mixed 
with a radio-frequency (RF) signal called its carrier frequency, which 
represents that station. AM radio stations have carrier frequencies from 
about 530 kHz to 1700 kHz. Today’s digital broadcasting uses the same 
frequencies as the pre-2009 analog transmission.
- The Modulator or Mixer adds useful information to the carrier wave. 
The mixing of the audio and carrier frequencies is done in two ways. 
In amplitude modulation (AM), the amplitude of the high-frequency carrier 
wave is made to vary in proportion to the amplitude of the audio signal, as 
shown in Fig.8.30. It is called “amplitude modulation” because the amplitude of 

the carrier is altered (“modulate” means to change or alter).


In frequency modulation (FM), the frequency of the carrier wave is made 
to change in proportion to the audio signal’s amplitude, as shown in Fig.8.31. 
The mixed signal is amplified further and sent to the transmitting antenna of 
fig.8.29 where the complex mixture of frequencies is sent out in the form of 

electromagnetic waves.

Phase modulation (PM) 
Phase modulation is a form of modulation that encodes information as 
variations in the instantaneous phase of the carrier wave. It is widely used for 
transmitting radio waves and is an integral part of many digital transmission 
coding schemes that underlie a wide range of technologies like Wi-Fi, GSM and 
satellite television. In this type of modulation, the amplitude and frequency of 

the carrier signal remains unchanged after P

The modulating signal is mapped to the carrier signal in the form of variations 
in the instantaneous phase of the carrier signal. Phase modulation is closely 
related to frequency modulation and is often used as intermediate step to 
achieve FM.
Amplifier: Amplifies the modulated carrier wave to increase its power. The 
more powerful the amplifier, the more powerful the broadcast.
In digital communication, the signal is put into digital form which modulates the 
carrier. A television transmitter works in a similar way, using FM for audio and 
AM for video; both audio and video signals are mixed with carrier frequencies.
8.4.2. Simple radio receiver
A radio receiver is the opposite of a radio transmitter. It uses an antenna to 
capture radio waves, processes those waves to extract only those waves that are 
vibrating at the desired frequency, extracts the audio signals that were added 
to those waves, amplifies the audio signals, and finally plays them on a speaker.
Now let us look at the other end of the process, the reception of radio and TV 
programs at home. A simple radio receiver is graphed in Fig. 8.30. The EM 

waves sent out by all stations are received by the antenna. 

The signal antenna detects and sends the radio waves, to the receiver is very 
small and contains frequencies from many different stations. The receiver uses 
a resonant LC circuit to select out a particular RF frequency (actually a narrow 
range of frequencies) corresponding to a particular station. 
A simple way of tuning a station is shown in Fig.8.31. When the wire of antenna 
is exposed to radio waves, the waves induce a very small alternating current in 

the antenna.

A particular station is “tuned in” by adjusting the capacitance C and/or 
inductance L so that the resonant frequency of the circuit equals that of the 
station’s carrier frequency.
R.F. Amplifier: A sensitive amplifier that amplifies the very weak radio 
frequency (RF) signal from the antenna so that the signal can be processed by 
the tuner.
R.F. Tuner: A circuit that can extract signals of a particular frequency from a 
mix of signals of different frequencies. On its own, the antenna captures radio 
waves of all frequencies and sends them to the RF amplifier, which dutifully 
amplifies them all. Unless you want to listen to every radio channel at the same 
time, you need a circuit that can pick out just the signals for the channel you 

want to hear. That’s the role of the tuner.

The tuner usually employs the combination of an inductor (for example, a coil) 
and a capacitor to form a circuit that resonates at a particular frequency. This 
frequency, called the resonant frequency, is determined by the values chosen 
for the coil and the capacitor. This type of circuit tends to block any AC signals 
at a frequency above or below the resonant frequency.The fig.8.35 shows a 
combination of a radio transmitter and aradio receiver.
You can adjust the resonant frequency by varying the amount of inductance 
in the coil or the capacitance of the capacitor. In simple radio receiver circuits, 
the tuning is adjusted by varying the number of turns of wire in the coil. More 
sophisticated tuners use a variable capacitor (also called a tuning capacitor) to 

vary the frequency. 

The tuner usually employs the combination of an inductor (for example, a coil) 
and a capacitor to form a circuit that resonates at a particular frequency. This 
frequency, called the resonant frequency, is determined by the values chosen 
for the coil and the capacitor. This type of circuit tends to block any AC signals 
at a frequency above or below the resonant frequency.The fig.8.35 shows a 
combination of a radio transmitter and aradio receiver.
You can adjust the resonant frequency by varying the amount of inductance 
in the coil or the capacitance of the capacitor. In simple radio receiver circuits, 
the tuning is adjusted by varying the number of turns of wire in the coil. More 
sophisticated tuners use a variable capacitor (also called a tuning capacitor) to 

vary the frequency. 

8.4.3. Wireless Radio Communication
Let us now discuss the basic principles of wireless radio communications. 
We shall mainly concentrate on the principle of amplitude modulation and 
demodulation.
The simplest scheme of wireless communication would be to convert the speech 
or music to be transmitted to electric signals using a microphone, boost up the 
power of the signal using amplifiers and radiate the signal in space with the aid 
of an antenna. This would constitute the transmitter. At the receiver end, one 
could have a pick-up antenna feeding the speech or music signal to an amplifier 

and a loud speaker.

The above scheme suffers from the following drawbacks:

i) Electromagnetic waves in the frequency range of 20 Hz to 20 kHz (audiofrequency range) cannot be efficiently radiated and do not propagate well 
in space.
ii) Simultaneous transmission of different signals by different transmitters 
would lead to confusion at the receiver.
In order to solve these problems; we need to devise methods to convert or 
translate the audio signals to the radio-frequency range before transmission and 
recover the audio-frequency signals back at the receiver. Different transmitting 
stations can then be allotted slots in the radio-frequency range and a single 
receiver can then tune into these transmitters without confusion. 
The frequency range 500 kHz to 20 MHz is reserved for amplitude-modulated 
broadcast, which is the range covered by most three band transistor radios. The 
process of frequency translation at the transmitter is called modulation. The 
process of recovering the audio-signal at the receiver is called demodulation. 

A simplified block diagram of such a system is shown in the below figure.