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Frequency Modulation and
1. Flow diagram
2. What is frequency modulation?
3. Frequency modulation index
4. Significant-sidebands Spectrum
5. Types of FM
6. Generation of FM using PM
7. Advantages and disadvantages
8. Comparison with FM and PM
What is frequency modulation?
When the frequency of carrier wave is changed in
accordance with the message signal, The process is called
In FM the carrier amplitude remain constant the carrier
It is a type of Angle modulation
Why Frequency modulation is called nonlinear-modulation?
FM modulation index
• FM modulation index is equal to the ratio of the
frequency deviation to the modulating frequency.
• Thus the formula for the modulation index for FM
is simply given by that shown below:
Significant Sidebands – Spectrum
• The table below shows the number of significant
sidebands for various modulation
No of sidebands 1% of
0.1 2 2fm
0.3 4 4fm
0.5 4 4fm
1.0 6 6fm
2.0 8 8fm
5.0 16 16fm
10.0 28 28fm
For = 5,
Types of FM
Narrow band FM
1. Narrow band FM is defined as the situation where the
modulation index is small.
2. From the table of Bessel functions it may be seen that for
small , ( 0.3) there is only the carrier and significant
sidebands, i.e. BW = 2fm.
FM with 0.3 is referred to as narrowband FM (NBFM).
3. Maximum modulating frequency is usually 3kHz
4. maximum frequency deviation is =75 kHz.
Types of FM
Wide band FM
1. Wideband FM is defined as the situation where the
modulation index is larger.
2. For > 0.3 there are more than 2 significant sidebands. As
increases the number of sidebands increases. This is referred
to as wideband FM (WBFM).
3. Modulation frequencies extend from 30 Hz to 15 kHz.
4. Maximum permissible deviation is=75 kHz.
5. Wideband FM system need large bandwidth, typically 15 times
that of narrowband FM system.
Generation of FM using PM
Modulating Wave x(t) FM Wave
dtt x ) (
1. Amplitude of the frequency modulated wave remains
2. Large decrease in noise, hence increase in S/N ratio.
3. Noise may reduce by increasing deviation.
4. Frequency allocation allows for a guard band which
reduces adjacent channel interference.
5. Operate In Very high frequency (VHF).
• FM has too much advantages besides it also has some
1. FM wave can’t cover large area.
2. Transmitting & receiving equipments for FM are
complex & costly.
3. A much wider channel, typically 200 kHz, is needed for
Comparison of FM with PM
1 Frequency deviation is proportional to
Phase deviation is proportional
to the modulating voltage
2 Noise immunity is better than AM and PM Noise immunity is better than
AM but worse than FM
3 SNR is better than PM SNR is worse than FM
4 FM is widely used for radio broadcasting PM is only used in some mobile
5 It is possible to receive FM on PM receive It is possible to receive PM on
6 Modulation index is proportional to
modulating voltage as well as the modulating
Modulation index is
proportional to modulating
Comparison of FM and AM
1 FM receivers are immune to noise AM receivers are not immune
2 It is possible to decrease noise by increasing
This feature is absent in AM
3 Bandwidth is higher and depends on modulation
Bandwidth is lower
compared to AM but
independent of modulation
4 FM transmission and reception equipment are more
FM transmission and
reception equipment are less
5 All transmitted power is useful Carrier power and one
sideband power is useless
FM radio uses a modulation index, m > 1, and this is called wideband FM. As its name
suggests the bandwidth is much larger than AM.
In national radio broadcasts using FM, the frequency deviation of the carrier fc , is
chosen to be 75kHz, and the information baseband is the high fidelity range 20Hz to
BW of FM radio=2(75k+15k)
In terrestrial TV broadcasts, the video information is transmitted using AM .
However the sound information is transmitted using FM, in order to reduce possible
interference between the video and sound signals. In this case, the maximum deviation of
the carrier, fc , is chosen to be 50kHz, and the information baseband is again the high
fidelity range 20Hz to 15kHz. Therefore the bandwidth required for TV Sound is:
BW Of TV Sound=2(50k+15k)
Some satellite TV transmissions broadcast an analogue video signal using FM. This
helps to obtain an acceptable signal at the receiving station In this case, the maximum deviation
of the carrier fc , is chosen to be about 10 MHz, with a video baseband of around 5MHz.
Therefore the bandwidth required for Satellite TV is:
BW of satellite TV =2(10+5)
• Analog and digital communication system by Sanjay Sharma.
• Modern digital and analog communication systems by B.P. Lathi