COMMUNICATION
1. Introduction
2. AM and FM
3. Analogue versus
Digital transmission.
4. Channels of
Communication.
5. Satellite
communication.
Welcome
Enjoy Physics
lesson.
1st objective
understand the
term modulation
and be able to
distinguish
between amplitude
modulation (AM)
and frequency
modulation (FM).
2nd objective
Welcome
know and
describe the
relative
advantages of
AM and FM
transmissions
Our lesson begins:
Slide 1
Slide 1
MODULATION

Modulation is the process where
audio signal is added onto a
carrier signal.

In our case, we use the radio
waves and microwaves as the
carrier.

The carrier is of sinusoidal form
and hence carries the function,
x = x0sint
where x0 is its amplitude and
 its angular frequency
both of which can be altered.
1 Shorter aerial required.
2 Less distortion
3
4
5
Longer transmission range.
Less attenuation(signal power loss).
Allows more than one station in a region.
Types of Modulation
The signals that we
receive
are either
amplitude
modulated (AM)
Or
frequency modulated
(FM)
Amplitude Modulation (AM)
(i) The amplitude of the carrier wave is
made to vary in synchrony with the
displacement of the information signal.
(ii) The amplitude variations of the carrier
wave is at the signal frequency fs.
(iii) The frequency of the amplitude
modulated wave remains the same i.e.
carrier frequency fc.
Sideband Frequencies in AM Wave
1. The AM wave contains three frequencies viz
fc, fc + fs and fc − fs.
2. The first frequency fc is the carrier frequency.
The process of modulation does not change the
original carrier frequency but produces two
new frequencies (fc + fs) and (fc − fs) which are
called sideband frequencies.
3. The sum of carrier frequency and signal
frequency i.e. (fc + fs) is called upper sideband
frequency.
4. The lower sideband frequency is fc − fs i.e.
the difference between carrier and signal
frequencies.
5. The signal frequency fs is contained in the
sideband frequencies.
Bandwidth
In an AM wave, the bandwidth
is from (fc − fs) to (fc + fs) i.e., 2
fs.
Bandwidth
.
Example 1.
An audio signal of 1 kHz is used
to modulate a carrier of 500
kHz. Determine: (i) sideband
frequencies (ii) bandwidth
required.
Solution.
Carrier frequency, fc = 500 kHz
Signal frequency, fs = 1 kHz
The AM wave has sideband frequencies of ( fc + fs)
and ( fc − fs ).
(i) Sideband frequencies = (500 + 1) kHz and (500 −
1) kHz
= 501 kHz and 499 kHz
(ii) Bandwidth required = 499 kHz to 501 kHz = 2
kHz
Frequency Modulation
POINTS TO NOTE:
1. Definition: In frequency modulation (FM) frequency of the carrier wave is
made to vary in synchrony with the displacement of the information signal.
2. the amplitude of the modulated wave remains the same i.e. carrier
wave amplitude.
3.The rate at which the carrier wave frequency is made to vary is equal
to the frequency of the information signal.
Comparison of AM and FM
Amplitude Modulation AM
Advantages
Disadvantages
1. Smaller bandwidth so more stations available 1. Requires a high power transmitter.
in frequency range.
2. Susceptible to noise and intereference.
2. Greater area covered by one transmitter.
3. Cheaper radio sets.
Frequency Modulation FM
Advantages
Disadvantages
1. Less electrical noise and
interference.
2. Greater bandwidth produces
better quality sound.
1. Shorter range
2. More complex circuitry.
3. More expensive