COMMUNICATION

SYSTEM
PCC-ECE202-G

By
Amit Dalal
Assistant Professor
Department of Electronics & Communication
Engineering
Ganga Institute of Technology & Management
 Introduction
Angle modulation is the process by which the
angle (frequency or phase) of the carrier
signal is changed in accordance with the
instantaneous amplitude of modulating or
message signal.
 Cont’d…
classified into two types such as
 Frequency modulation (FM)
 Phase modulation (PM)
Used for :
Commercial radio broadcasting
Television sound transmission
Two way mobile radio
Cellular radio
Microwave and satellite communication system
Cont’d…
Advantages over AM:
 Freedom from interference: all natural and
external noise consist of amplitude variations,
thus receiver usually cannot distinguish
between amplitude of noise or desired signal.
AM is noisy than FM.
 Operate in very high frequency band (VHF):
88MHz-108MHz
 Can transmit musical programs with higher
degree of fidelity.
 FREQUENCY MODULATION PRINCIPLES

In FM the carrier amplitude remains

constant, the carrier frequency varies with
the amplitude of modulating signal.
The amount of change in carrier frequency
produced by the modulating signal is known
as frequency deviation.
Carrier Modulating signal

Resting fc
FM

Increasing fc

Decreasing fc

Increasing fc

Resting fc
 PHASE MODULATION(PM)
The process by which changing the phase of
carrier signal in accordance with the
instantaneous of message signal. The amplitude
remains constant after the modulation process.
Mathematical analysis:
Let message signal:
 m t  Vm cos mt
And carrier signal:
 c t  Vc cos[ c t   ]
 PM (cont’d)
 Where  = phase angle of carrier signal. It is changed in
accordance with the amplitude of the message signal;
i.e.  KVm (t ) KVm cos mt
 After phase modulation the instantaneous voltage will
be or
v pm ( t ) VC cos(C t  KVm cos m t )
v pm ( t ) VC cos(C t  m p cos m t )
 Where mp = Modulation index of phase modulation
 K is a constant and called deviation sensitivities of the
phase
 FREQUENCY MODULATION (FM)

A process where the frequency of the carrier

wave varies with the magnitude variations of
the modulating or audio signal.
The amplitude of the carrier wave is kept
constant.
FM(cont’d)
Mathematical analysis:
Let message signal:

 m t  Vm cos mt
And carrier signal:
 c t  Vc cos[ c t   ]
FM (cont’d)
During the process of frequency
modulations the frequency of carrier signal
is changed in accordance with the
instantaneous amplitude of message
signal .Therefore the frequency of carrier
after modulation is written as
i c  K1v m t  C  K1Vm cos m t

 To find the instantaneous phase angle of modulated

signal, integrate equation above w.r.t. t
K1Vm
 
i i dt C  K1Vm cos m t dt C t 
m
sin m t
FM(cont’d)
Thus, we get the FM wave as:
K1Vm
v FM ( t ) Vc cos 1 VC cos(C t  sin m t )
m

vFM (t ) VC cos(C t  m f sin mt )

Where modulation index for FM is
given by
K1Vm
mf 
m
FM(cont’d)
Frequency deviation: ∆f is the
relative placement of carrier
frequency (Hz) w.r.t its unmodulated
value. Given
max as:
C  K1Vm
min C  K1Vm
d max  C C  min K1Vm
d K1Vm
f  
2 2
FM(cont’d)
Therefore:

K1Vm
f  ;
2
f
mf 
fm
Tomasi Copyright ©2004 by Pearson Education, Inc.
Electronic Communications Systems, 5e Upper Saddle River, New Jersey 07458
All rights reserved.
Example (FM)
Determine the peak frequency
deviation (∆f) and modulation index
(m) for an FM modulator with a
deviation sensitivity K1 = 5 kHz/V and
a modulating
v m ( t ) signal,
2 cos(22000t )
Example (PM)
Determine the peak phase deviation
(m) for a PM modulator with a
deviation sensitivity K = 2.5 rad/V
v m ( t ) 2 cos(2signal,
and a modulating 2000t )
FM&PM (Bessel function)
Thus, for general equation:
vFM (t ) VC cos(C t  m f cos mt )


 n 
cos(  m cos )   J n (m) cos   n  
n   2 


 n 
m( t ) VC  J n (m) cos c t  nm t  
n   2 
 Bessel function

   
vt FM VC {J 0 (m f ) cos C t  J1 (m f ) cos  (C  m ) t    J1 (m f ) cos  (C  m ) t  
 2  2

 J 2 (m f ) cos(C  2m ) t   J 2 (m f ) cos(C  2m ) t   ...J n (m f )...}

B.F. (cont’d)
 It is seen that each pair of side band is preceded by J
coefficients. The order of the coefficient is denoted by
subscript m. The Bessel function can be written as

 mf 
n
 1 m f / 2 2 m f / 2 4 
J n m f        ....
 2   n 1!n  1! 2!n  2 ! 

 N = number of the side frequency

 Mf = modulation index
B.F. (cont’d)
Representation of frequency spectrum
 Example
For an FM modulator with a modulation
index m = 1, a modulating signal v m(t) =
Vmsin(2π1000t), and an unmodulated carrier
vc(t) = 10sin(2π500kt). Determine the number
of sets of significant side frequencies and
their amplitudes. Then, draw the frequency
spectrum showing their relative amplitudes.
 FM Bandwidth
Power distribution of FM
Generation & Detection of FM
Application of FM
FM Bandwidth
 Theoretically, the generation and transmission of FM
requires infinite bandwidth. Practically, FM system have
finite bandwidth and they perform well.
 The value of modulation index determine the number of
sidebands that have the significant relative amplitudes
 If n is the number of sideband pairs, and line of
frequency spectrum are spaced by fm, thus, the
bandwidth is:
B fm 2nf m

 For n≥1
FM Bandwidth (cont’d)
 Estimation of transmission b/w;
 Assume m is large and n is approximate m + 2; thus
f f
 B =2(m + 2)f
fm f m

f
2(  2) f m
= fm

B fm 2(f  f m )........(1)

(1) is called Carson’s rule

Example
For an FM modulator with a peak
frequency deviation, Δf = 10 kHz, a
modulating-signal frequency fm = 10 kHz,
Vc = 10 V and a 500 kHz carrier,
determine
Actual minimum bandwidth from the Bessel
function table.
Approximate minimum bandwidth using
Carson’s rule.
Then
Plot the output frequency spectrum for the
Bessel approximation.
Deviation Ratio (DR)

 The worse case modulation index which produces the

widest output frequency spectrum.
f (max)
DR  f m (max)

 Where
 ∆f(max) = max. peak frequency deviation
 fm(max) = max. modulating signal frequency
Example
Determine the deviation ratio and
bandwidth for the worst-case (widest-
bandwidth) modulation index for an FM
broadcast-band transmitter with a maximum
frequency deviation of 75 kHz and a
maximum modulating-signal frequency of 15
kHz.
Determine the deviation ratio and maximum
bandwidth for an equal modulation index
with only half the peak frequency deviation
and modulating-signal frequency.
 FM Power Distribution
As seen in Bessel function table, it shows that as
the sideband relative amplitude increases, the
carrier amplitude,J0 decreases.

This is because, in FM, the total transmitted power

is always constant and the total average power is
equal to the unmodulated carrier power, that is
the amplitude of the FM remains constant whether
or not it is modulated.
 FM Power Distribution (cont’d)

In effect, in FM, the total power that is originally in

the carrier is redistributed between all
components of the spectrum, in an amount
determined by the modulation index, mf, and the
corresponding Bessel functions.
At certain value of modulation index, the carrier
component goes to zero, where in this condition,
the power is carried by the sidebands only.
 Average Power
Vc2
Pc 
 The average power in unmodulated carrier
2R
 The total instantaneous power in the angle modulated
carrier.
m( t ) 2 Vc2
Pt   cos 2 [c t  ( t )]
R R
Vc2  1 1  Vc
2
Pt    cos[2c t  2( t )] 
R 2 2  2R

 The total modulated power

2
Vc 2(V1 ) 2 2(V2 ) 2 2(Vn ) 2
Pt P0  P1  P2  ..  Pn     .. 
2R 2R 2R 2R
Example
For an FM modulator with a modulation
index m = 1, a modulating signal
vm(t) = Vmsin(2π1000t),
and an unmodulated carrier
vc(t) = 10sin(2π500kt).
Determine the unmodulated carrier power
for the FM modulator given with a load
resistance, RL = 50Ω. Determine also the
total power in the angle-modulated wave.
Quiz
 For an FM modulator with modulation index,
m = 2, modulating signal,
vm(t) = Vmcos(2π2000t),
and an unmodulated carrier,
vc(t) = 10 cos(2π800kt).

a) Determine the number of sets of significant

sidebands.
b) Determine their amplitudes.
c) Draw the frequency spectrum showing the relative
amplitudes of the side frequencies.
d) Determine the bandwidth.
e) Determine the total power of the modulated wave.
Generation of FM
 Two major FM generation:
i) Direct method:
i) straight forward, requires a VCO whose oscillation
frequency has linear dependence on applied voltage.
ii) Advantage: large frequency deviation
iii) Disadvantage: the carrier frequency tends to drift and
must be stabilized.
iv) Common methods:
i) FM Reactance modulators
ii) Varactor diode modulators
Generation of FM (cont’d)
1) Reactance
modulator
Generation of FM (cont’d)
2) Varactor diode
modulator
Generation of FM (cont’d)
ii) Indirect method:
i. Frequency-up conversion.
ii. Two ways:
a. Heterodyne method
b. Multiplication method
iii. One most popular indirect method is the Armstrong
modulator
Wideband Armstrong Modulator
A complete Armstrong modulator is supposed to
provide a 75kHz frequency deviation. It uses a
balanced modulator and 90o phase shifter to phase-
modulate a crystal oscillator. Required deviation is
obtained by combination of multipliers and mixing,
raise the signal from 400kHz 14.47Hz to 90.2MHz 75kHz
suitable for broadcasting.
 FM Detection/Demodulation
 FM demodulation

 is a process of getting back or regenerate the

original modulating signal from the modulated FM
signal.

 It can be achieved by converting the frequency

deviation of FM signal to the variation of equivalent
voltage.

 The demodulator will produce an output where its

instantaneous amplitude is proportional to the
instantaneous frequency of the input FM signal.
FM detection (cont’d)
To detect an FM signal, it is necessary to
have a circuit whose output voltage varies
linearly with the frequency of the input
signal.

The most commonly used demodulator is

the PLL demodulator. Can be use to detect
either NBFM or WBFM.
PLL Demodulator

V0(t)
FM input
fi
Phase Low pass
Amplifier
detector filter

fvco
Vc(t)
VCO
 PLL Demodulator
 The phase detector produces an average output voltage
that is linear function of the phase difference between the
two input signals. Then low frequency component is pass
through the LPF to get a small dc average voltage to the
amplifier.

 After amplification, part of the signal is fed back through

VCO where it results in frequency modulation of the VCO
frequency. When the loop is in lock, the VCO frequency
follows or tracks the incoming frequency.
 PLL Demodulator
Let instantaneous freq of FM Input,
fi(t)=fc +k1vm(t),
and the VCO output frequency,
f VCO(t)=f0 + k2Vc(t);
f0 is the free running frequency.
For the VCO frequency to track the
instantaneous incoming frequency,
fvco = fi; or
PLL Demodulator
f + k V (t)= f +k v (t), so,
0 2 c c 1 m

Vc (t )  f c  f 0  k1vm (t )

If VCO can be tuned so that f =f , then

c 0

Vc (t )  k1vm (t )
Where Vc(t) is also taken as the output
voltage, which therefore is the demodulated
output
Comparison AM and FM
 Its the SNR can be increased without increasing
transmitted power about 25dB higher than in AM
 Certain forms of interference at the receiver are more
easily to suppressed, as FM receiver has a limiter which
eliminates the amplitude variations and fluctuations.
 The modulation process can take place at a low level
power stage in the transmitter, thus a low modulating
power is needed.
 Power content is constant and fixed, and there is no
waste of power transmitted
 There are guard bands in FM systems allocated by the
standardization body, which can reduce interference
between the adjacent channels.
 Application of FM
FM is commonly used at VHF radio frequencies
for high-fidelity broadcasts of music and speech
(FM broadcasting). Normal (analog) TV sound is
also broadcast using FM. The type of FM used in
broadcast is generally called wide-FM, or W-FM
A narrowband form is used for voice
communications in commercial and amateur
radio settings. In two-way radio, narrowband
narrow-fm (N-FM) is used to conserve bandwidth.
In addition, it is used to send signals into space.
Summary of angle modulation
-what you need to be familiar with
Summary (cont’d)
Summary (cont’d)
 Bandwidth:
a) Actual minimum bandwidth from
Bessel table:
B 2(n  f m )

b) Approximate minimum bandwidth

using Carson’s rule:
B 2(f  f m )
 Summary (cont’d)
 Multitone modulation (equation in
general):
i c  Kvm1  Kvm 2

i c  2f1 cos 1t  2f 2 cos 2t....

f1 f 2
i C t  sin 1t  sin 2t......
f1 f2
Summary (cont’d)
v fm t  VC cos i
f1 f 2
v fm t  VC cos[C t  sin 1t  sin 2t ]
f1 f2
VC cos[C t  m f 1 sin 1t  m f 2 sin 2t ]...........
Summary (cont’d)-
Comparison NBFM&WBFM
 Advantages

 Wideband FM gives significant improvement in the SNR at

the output of the RX which proportional to the square of
modulation index.
 Angle modulation is resistant to propagation-induced
selective fading since amplitude variations are unimportant
and are removed at the receiver using a limiting circuit.
 Angle modulation is very effective in rejecting interference.
(minimizes the effect of noise).
 Angle modulation allows the use of more efficient transmitter
power in information.
 Angle modulation is capable of handing a greater dynamic
range of modulating signal without distortion than AM.
 Disadvantages
Angle modulation requires a transmission
bandwidth much larger than the message
signal bandwidth.
Angle modulation requires more complex and
expensive circuits than AM.