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FREQUENCY MODULATION
(FM)

PRESENTED BY

DIPIKA KORDAY
fybsc it student
Mumbai
Email: dipika.sweetfox4@gmail.in

F.M.

Before studing FM.. Lets study AM, and the faults in AM and
how FM conquers those problems giving us the best service.

AM TRANSMITTER
ANTENNA

OSC RF AMP

Modulated
SOUND WAVES Radio freq radio
frequency

MODULATOR

Audio freq

FM TRANSMITTER

MODU
AUDIO LATOR
OCS

FM RECEIVER

LIMITTER DISCRIMINA AF
TOR AMP

DIFFRENCE BETWEEN FM AND AM
• The power transmission of FM waves is better
than that of the AM signals.
• In FM wave’s noise can be controlled by
increasing the deviation up to some amount. This
is impossible in case of AM waves.
• AM is amplitude modulation while FM is
frequency modulation. But what is modulation? It
is the act of modifying a certain aspect of the
carrier frequency in accordance to the
information being sent. It is then clear that AM
modifies the amplitude of the carrier frequency
while FM modifies its frequency.

• FM is a development over AM broadcasting and it provides
a lot of substantial benefits thus it is much more complex
compared to AM. The first and most substantial benefit is
its ability to send out two channels of information at the
same time with the use of advanced algorithms. This allows
the station to broadcast left and right audio channels for
full stereo sound. Since most environmental factors that
distort radio waves only affect its amplitude and not the
frequency where FM stores the actual voice signal, the data
in the FM signal doesn’t degrade as easily as AM. This also
means that FM signal quality doesn’t degrade linearly as
you get further from the transmitting station.

• Range wise, AM takes the cake with its much greater
distance travelled. FM signals usually drop of at around 50
miles from the station, but AM waves can be refracted in
the atmosphere resulting in greater range. The range is also
one of the reasons why talk radio prefers AM even if the
sound quality is not very high.

.FM.
(FREQUENCY MODULATION)
FM broadcasting is a broadcasting
technology pioneered by Edwin
Howard Armstrong which uses
frequency modulation (FM) to provide
high-fidelity sound over broadcast
radio. The term "FM band" describes
the "frequency band in which FM is
used for broadcasting".

Frequency modulation
• Frequency modulation is the process by which
frequency of the carrier signal c(t) changes
with respect to the modulating signal m(t).
• FM modulated signal s(t) is a nonlinear
function of the modulating signal m(t), thus it
is known as nonlinear modulation process.
• More difficult than amplitude modulation
(AM).

Carrier
Signal

Modulating
Signal

Modulated
Signal

If the modulating signal is m(t) and

Then the modulated signal s(t) is

Where, B is the modulation index.

Depending to this modulation index FM can ne classified into
two types –

1. Narrowband FM : modulation index is smaller than one
radian
2. Wideband FM : modulation index is larger than 1 radian

Modulation index is less than 1
Maximum modulating frequency is usually
3kHz & maximum frequency deviation is =75
kHz.
A narrowband FM wave consist of a
carrier, an upper side-frequency component
& a lower side component.
The modulated narrowband signal differs
from the ideal response in two fundamental
respects :

1.The envelope contains a residual AM, so it
varies with time.
2.For sinusoidal modulating wave, the angle
θi(t) contains harmonic distortion in the
form of 3rd & higher order harmonics of
modulation frequency, fm.

By restricting β≤0.3 radians, the effects of
residual AM & harmonic PM are limited to
negligible labels.

 A narrow band signal may be
represented by phasor diagram as shown
in below –

Fig: Phasor diagram of a narrowband FM

Uses :
Narrowband FM is used by mobile
communication services such as police
wireless, ambulances, taxicabs, short
range very high frequency(VHF) ship to
shore sources and defence.

 Modulation index normally exceeds unity.
 Modulation frequencies extend from 30
Hz to 15 kHz.
 Maximum permissible deviation is=75
kHz.
 Wideband FM system need large
bandwidth, typically 15 times that of
narrowband FM system.
 Wideband FM systems are used in
entertainment broadcasting.

 For large values of β compared to 1
radian, the FM wave contains a carrier &
an infinite number of side- frequency
components located symmetrically
around the carrier.
 The amplitude of the carrier component
contained in a wideband FM wave varies
with the modulation index β in
accordance with Bessel function Јn(β).

 We can plot Bessel function Јn(β) versus
modulation index β for different positive
integer values of n.

 In practice, FM wave is limited to finite number of
significant side-frequencies compatible with a
specified amount of distortion. Thus an effective
bandwidth is required for the transmission of an FM
wave.
 Approximate rule for transmission bandwidth of
an FM wave generated by a single-tone
modulating wave of frequency f is m

This relation is known as Carson’s rule.

 According to international regulations of
FM broadcast –

Maximum frequency deviation, fd=±75 kHz
Allowable bandwidth per channel=200 kHz
Frequency stability of carrier =±2 kHz

Two basic methods of generating FM waves
 Indirect :
 By integrating message signal a phase-
modulated signal generates.
 This is used to modulate a VCO & the result is
passed through a frequency multiplier.
 Direct :
 The message is directly feeding into the input
of a VCO.

 Modulating wave first used to produce a
narrowband FM wave, then frequency
multiplication is used to increase the
frequency deviation to the desired level.

 Here instantaneous frequency of the
carrier wave is varied directly in
accordance with the message signal by
means of VCO.

Baseband Wide-band
signal
Frequenc Frequenc FM
Mixer Bandpas
VCO y y
s Filter
Multiplier Multiplier

Fixed
Oscillator

Fig: Block diagram of direct method of generating wide-band FM

 Amplitude of the frequency modulated
wave remains unaffected.
 Large decrease in noise, hence increase
in S/N ratio.
 Noise may reduce by increasing deviation
 Frequency allocation allows for a guard
band which reduces adjacent channel
interference.
 FM permits several independent
transmitters on the same frequency with
negligible interference.

FM has too much advantages besides it also
has some disadvantages -
 FM wave can’t cover large area.

 Transmitting & receiving equipments for
FM are complex & costly.
 Reception using conventional method is
limited to line of side.
 A much wider channel, typically 200 kHz,
is needed for FM.

FM has so many applications such as –
 Magnetic tape storage:
FM is used at intermediate frequencies by analog VCR systems
(including VHS) to record both the luminance(black and white)
portions of the video signal.
 Sound :
FM is also used at audio frequencies to synthesize sound, which
is known as FM synthesis.

 Radio :
FM is used in many radio stations (FM Radio Stations) all
over the world. Normally the frequency range of FM
channels are between 88.0 MHz - 108.0 MHz.

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