The
FM or Frequency modulation has been available approximately since AM (Amplitude
Modulation) although it has only some issues. FM itself didn’t have a problem
apart from we couldn’t recognize the FM transmitter potential. In the earlier
time of wireless communication, it was measured that the required bandwidth of
this was narrower, and necessary to decrease noise as well as
interference. Under such a measure,
frequency modulation was suffered whereas AM increased. After that, an American
Engineer- “Edwin Armstrong” finished the conscious attempt to discover the
intensity of FM transmitters. Edwin initiated the design of using FM intended
for transmitting which was not in favor of the trend at that moment in time.
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What is a Frequency Modulation?
The
frequency modulation can be defined as; the frequency of the carrier signal is
varied proportional to (in accordance with) the Amplitude of the input
modulating signal. The input is a single tone sine wave. The carrier and the FM
waveforms also are shown in the following figure.
Frequency Modulation Generation
The
frequency of a carrier (fc) will increase as the amplitude of modulating
(input) signal increases. The carrier frequency will be maximum (fc max) when
the input signal is at its peak. The carrier deviates maximum from its normal
value. The frequency of a carrier will decrease as the amplitude of the
modulating (input) signal decreases. The carrier frequency will be minimum (fc
min) when the input signal is at its lowest. The carrier deviates minimum from
its normal value. The frequency of the carrier will be at its normal value
(free running) fc when the input signal value is 0V. There is no deviation in
the carrier. The figure shows the frequency of the FM wave when the input is at
its max, 0V and at its min.
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Frequency Deviation
The
amount of change in the carrier frequency produced, by the amplitude of the
input modulating signal, is called frequency deviation.
The
Carrier frequency swings between fmax and fmin as the input varries in its
amplitude.
The
difference between fmax and fc is known as frequency deviation. fd = fmax – fc
Similarly,
the difference between fc and fmin also is known as frequency deviation. fd = fc
–fmin
It
is denoted by Δf. Therefore Δf = fmax – fc = fc – fmin
Therefore
fd = fmax – fc = fc – fmin
Freq
deviation = 105 -100 = 5 MHz (or) Freq deviation = 95-100 = -5 MHz
Frequency Modulation Equation
The
FM equation include the following
v
= A sin [ wct + (Δf / fm) sin wmt ]
=
A sin [ wct + mf sin wmt ]
A
= Amplitude of the FM signal. Δf = Frequency deviation
mf
= Modulation Index of FM
mf
= ∆f/fm
mf
is called the modulation index of frequency modulation.
wm
= 2π fm wc = 2π fc
What is Modulation Index of Frequency
Modulation?
The
modulation index of FM is defined as the ratio of the frequency deviation of
the carrier to the frequency of the modulating signal
mf
= Modulation Index of FM = ∆f/fm
The bandwidth of Frequency Modulation
Signal
Recall,
the bandwidth of a complex signal like FM is the difference between its highest
and lowest frequency components, and is expressed in Hertz (Hz). Bandwidth
deals with only frequencies. AM has only two sidebands (USB and LSB) and the
bandwidth was found to be 2 fm.
In
FM it is not so simple. FM signal spectrum is quite complex and will have an
infinite number of sidebands as shown in the figure. This figure gives an idea,
how the spectrum expands as the modulation index increases. Sidebands are
separated from the carrier by fc ± fm, fc ± 2fm, fc ± 3fm and so on.
Bandwidth of FM Signal
Only
the first few sidebands will contain the major share of the power (98% of the
total power) and therefore only these few bands are considered to be
significant sidebands.
As
a rule of thumb, often termed as Carson’s Rule, 98% of the signal power in FM
is contained within a bandwidth equal to the deviation frequency, plus the
modulation frequency doubled.
Carson’s
rule: Bandwidth of FM BWFM = 2 [ Δf + fm ].
=
2 fm [ mf + 1 ]
FM is known as Constant Bandwidth
System. Why?
The
frequency modulation is known as a constant bandwidth system and an example of
this system is given below.
Δf = 75 KHz fm = 500 Hz BWFM = 2 [75 + (500/1000)] KHz =
151.0 KHz
Δf = 75 KHz fm = 5000 Hz BWFM = 2 [75 + (5000/1000)] KHz =
160.0 KHz
Δf = 75 KHz fm = 10000 Hz BWFM = 2 [75 + (10000/1000)] KHz
= 170.0 KHz
Although
modulating frequency increased 20 times (50 Hz to 5000 Hz), deviation increased
only marginally (151 KHz to 170 KHz). Hence FM is known as constant bandwidth
system.
Commercial
FM (Carson’s Rule.)
Max
freq deviation = 75 KHz
Max
Modulating freq = 15 KHz
BWFM
= 2 [ 75 + 15 ] = 180.0 KHz
Difference
between AM and FM
The main difference between AM and FM
include the following.
Equation
for FM: V= A sin [ wct +Δf / fm sin wmt ] = A sin [ wct + mf sin wmt ]
Equation
for AM = Vc ( 1 + m sin ωmt ) sin ωct where m is given by m = Vm / Vc
In
FM, the Modulation Index can have any value greater than 1 or less than one
In
AM, the Modulation Index will be between 0 and 1
In
FM, carrier amplitude is constant.
Therefore
transmitted power is constant.
Transmitted
power does not depend on the modulation index
Transmitted
power depends on the modulation index
PTotal
= Pc [ 1+ (m2/2) ]
The
number of significant sidebands in FM is large.
Only
two sidebands in AM
A
bandwidth of FM depends on the modulation index of FM
Bandwidth
does not depend on the modulation index of AM. Always 2 sidebands. BW of AM is
2 fm
FM
has better noise immunity.FM is rugged/robust against noise. The quality of FM
will be good even in the presence of noise.
In
AM, quality is affected seriously by noise
The
bandwidth required by FM is quite high.FM bandwidth = 2 [Δf + fm].
The
bandwidth required by AM is less (2 fm)
Circuits
for FM transmitter and receiver are very complex and very expensive.
Circuits
for AM transmitter and receiver are simple and less expensive
Thus,
this is all about frequency modulation. The applications of frequency
modulation include in FM radio broadcasting, radar, seismic prospecting,
telemetry, & observing infants for seizure through EEG, music synthesis,
two-way radio systems, magnetic tape recording systems, video broadcast
systems, etc. From the above information, finally, we can conclude that, in
frequency modulation, both efficiencies as well as bandwidth depends on the
maximum the modulation index and modulating frequency. Contrasted to amplitude
modulation, the frequency modulation signal has a larger bandwidth, superior
efficiency, & improved immunity toward the noise. What are the different
types of modulation techniques in communication system?
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CONTACT
US
Dingshengwei
Electronics Co., Ltd
Company
Address: Buliding A,the first industry park of Guanlong,Xili
Town,Nanshan,Shenzhen,Guangdong,China
Tel
: +86 0755 26909863
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Source：elprocus
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