E5.1 commn unit 2 md

E5.2 COMMUNICTION
UNIT 2 MODULATION AND DE MODULATION

BY .MAIBOOB ALI K MULLA MSc ,MPhil.
Asst.Prof, in electronics,SSGFG College,
NARAGUND

The ionosphere is a shell of electrons and electrically charged
atoms and molecules that surrounds the Earth, stretching from a
height of about 50 km (31 mi) to more than 1,000 km (620 mi). It
exists primarily due to ultraviolet radiation from the Sun.
The lowest part of the Earth's atmosphere, the troposphere extends
from the surface to about 10 km (6.2 mi). Above that is the
stratosphere, followed by the mesosphere. In the stratosphere
incoming solar radiation creates the ozone layer. At heights of above
80 km (50 mi), in the thermosphere, the atmosphere is so thin that
free electrons can exist for short periods of time before they are
captured by a nearby positive ion. The number of these free electrons
is sufficient to affect radio propagation. This portion of the
atmosphere is partially ionized and contains a plasma which is

referred to as the ionosphere.Ultraviolet (UV), X-ray and shorter
wavelengths of solar radiation are ionizing, since photons at these
frequencies contain sufficient energy to dislodge an electron from a
neutral gas atom or molecule upon absorption. In this process the
light electron obtains a high velocity so that the temperature of the
created electronic gas is much higher (of the order of thousand K)
than the one of ions and neutrals. The reverse process to ionization is
recombination, in which a free electron is "captured" by a positive
ion. Recombination occurs spontaneously, and causes the emission of
a photon carrying away the energy produced upon recombination. As
gas density increases at lower altitudes, the recombination process
prevails, since the gas molecules and ions are closer together. The

The balance between these two processes determines the quantity of
ionization present.Ionization depends primarily on the Sun and its activity. The
amount of ionization in the ionosphere varies greatly with the amount of
radiation received from the Sun. Thus there is a diurnal (time of day) effect and
a seasonal effect. The local winter hemisphere is tipped away from the Sun,
thus there is less received solar radiation. The activity of the Sun modulates
following the solar cycle, with more radiation occurring with more sunspots,
with a periodicity of around 11 years. Radiation received also varies with
geographical location (polar, auroral zones, mid-latitudes, and equatorial
regions). There are also mechanisms that disturb the ionosphere and decrease
the ionization. There are disturbances such as solar flares and the associated
release of charged particles into the solar wind which reaches the Earth and
interacts with its geomagnetic field.

Layers of ionization
Ionosphere layers. At night the F layer is the only layer of significant ionization
present, while the ionization in the E and D layers is extremely low. During the
day, the D and E layers become much more heavily ionized, as does the F layer,
which develops an additional, weaker region of ionisation known as the F1
layer. The F2 layer persists by day and night and is the main region responsible
for the refraction and reflection of radio waves.
D layer
The D layer is the innermost layer, 60 km (37 mi) to 90 km (56 mi) above the
surface of the Earth. Ionization here is due to Lyman series-alpha hydrogen
radiation at a wavelength of 121.6 nanometre (nm) ionizing nitric oxide (NO).
In addition, high solar activity can generate hard X-rays (wavelength < 1 nm)
that ionize N2 and O2. Recombination rates are high in the D layer, so there are
many more neutral air molecules than ions.

In fact, absorption levels can increase by many tens of dB during intense events,
which is enough to absorb most (if not all) transpolar HF radio signal
transmissions. Such events typically last less than 24 to 48 hours.
E layer
The E layer is the middle layer, 90 km (56 mi) to 150 km (93 mi) above the
surface of the Earth. Ionization is due to soft X-ray (1–10 nm) and far ultraviolet
(UV) solar radiation ionization of molecular oxygen (O2). Normally, at oblique
incidence, this layer can only reflect radio waves having frequencies lower than
about 10 MHz and may contribute a bit to absorption on frequencies above.
However, during intense sporadic E events, the Es layer can reflect frequencies
up to 50 MHz and higher. The vertical structure of the E layer is primarily
determined by the competing effects of ionization and recombination. At night
the E layer weakens because the primary source of ionization is no longer
present. After sunset an increase in the height of the E layer maximum increases
the range to which radio waves can travel by reflection from the layer.

Medium frequency (MF) and lower high frequency (HF) radio waves are
significantly attenuated within the D layer, as the passing radio waves cause
electrons to move, which then collide with the neutral molecules, giving up their
energy. Lower frequencies experience greater absorption because they move the
electrons farther, leading to greater chance of collisions. This is the main reason
for absorption of HF radio waves, particularly at 10 MHz and below, with
progressively less absorption at higher frequencies. This effect peaks around
noon and is reduced at night due to a decrease in the D layer's thickness; only a
small part remains due to cosmic rays. A common example of the D layer in
action is the disappearance of distant AM broadcast band stations in the
daytime.During solar proton events, ionization can reach unusually high levels in
the D-region over high and polar latitudes. Such very rare events are known as
Polar Cap Absorption (or PCA) events, because the increased ionization
significantly enhances the absorption of radio signals passing through the
region.[13

Es layer
The Es layer (sporadic E-layer) is characterized by small, thin clouds of
intense ionization, which can support reflection of radio waves, rarely
up to 225 MHz. Sporadic-E events may last for just a few minutes to
several hours. Sporadic E propagation makes VHF-operating radio
amateurs very excited, as propagation paths that are generally
unreachable can open up. There are multiple causes of sporadic-E
that are still being pursued by researchers. This propagation occurs
most frequently during the summer months when high signal levels
may be reached. The skip distances are generally around 1,640 km
(1,020 mi). Distances for one hop propagation can be anywhere from
900 km (560 mi) to 2,500 km (1,600 mi). Double-hop reception over
3,500 km (2,200 mi) is possible.

F layer
Main article: F region
The F layer or region, also known as the Appleton–Barnett layer, extends from
about 150 km (93 mi) to more than 500 km (310 mi) above the surface of
Earth. It is the layer with the highest electron density, which implies signals
penetrating this layer will escape into space. Electron production is dominated
by extreme ultraviolet (UV, 10–100 nm) radiation ionizing atomic oxygen. The F
layer consists of one layer (F2) at night, but during the day, a secondary peak
(labelled F1) often forms in the electron density profile. Because the F2 layer
remains by day and night, it is responsible for most skywave propagation of
radio waves and long distance high frequency (HF, or shortwave) radio
communications.
Above the F layer, the number of oxygen ions decreases and lighter ions such
as hydrogen and helium become dominant. This region above the F layer peak
and below the plasmasphere is called the topside ionosphere.

The critical frequency is the limiting frequency at or below which a radio
wave is reflected by an ionospheric layer at vertical incidence. If the
transmitted frequency is higher than the plasma frequency of the ionosphere,
then the electrons cannot respond fast enough, and they are not able to re-
radiate the signal. It is calculated as shown below:
f critical =9 𝑵
where N = electron density per m3 and f critical is in Hz.
The Maximum Usable Frequency (MUF) is defined as the upper frequency limit
that can be used for transmission between two points at a specified time.
f muf = f critical/sinα
where α = angle of attack, the angle of the wave relative to the horizon, and sin
is the sine function.
The cutoff frequency is the frequency below which a radio wave fails to
penetrate a layer of the ionosphere at the incidence angle required for
transmission between two specified points by refraction from the layer.

Virtual Height
When a wave is refracted, it is bent down gradually, but not sharply. However, the
path of incident wave and reflected wave are same if it is reflected from a surface
located at a greater height of this layer. Such a greater height is termed as virtual
height.
Virtual Height
The figure clearly distinguishes the virtual height (height of wave,
supposed to be reflected) and actual height (the refracted height). If the
virtual height is known, the angle of incidence can be found.

Multi-path
For the frequencies above 30 MHz, the sky wave propagation exists. Signal
multipath is the common problem for the propagation of electromagnetic waves
going through Sky wave. The wave, which is reflected from the ionosphere, can
be called as a hop or skip. There can be a number of hops for the signal as it may
move back and forth from the ionosphere and earth surface many times. Such a
movement of signal can be termed as multipath.

The above figure shows an example of multi-path propagation. Multipath
propagation is a term, which describes the multiple paths a signal travels to
reach the destination. These paths include a number of hops. The paths may be
the results of reflection, refraction or even diffraction. Finally, when the signal
from such different paths gets to the receiver, it carries propagation delay,
additional noise, phase differences etc., which decrease the quality of the
received output.
Fading
The decrease in the quality of the signal can be termed as fading. This happens
because of atmospheric effects or reflections due to multipath.
Fading refers to the variation of the signal strength with respect to
time/distance. It is widely prevalent in wireless transmissions. The most
common causes of fading in the wireless environment are multipath propagation
and mobility (of objects as well as the communicating devices).

Skip Distance: The measurable distance on the surface of the Earth from
transmitter to receiver, where the signal reflected from the ionosphere can
reach the receiver with minimum hops or skips, is known as skip distance.
Optimum Working Frequency (OWF): The frequency, which is being used
mostly for a particular transmission and which has been predicted to be used
over a particular period of time, over a path, is termed as Optimum Working
Frequency (OWF).
Duct Propagation : At a height of around 50 mts from the troposphere, a
phenomenon exists; the temperature increases with the height. In this region
of troposphere, the higher frequencies or microwave frequencies tend to
refract back into the Earth’s atmosphere, instead of shooting into ionosphere,
to reflect. These waves propagate around the curvature of the earth even up to
a distance of 1000km.This refraction goes on continuing in this region of
troposphere. This can be termed as Super refraction or Duct propagation.

The above image shows the process of Duct Propagation. The main
requirement for the duct formation is the temperature inversion. The increase
of temperature with height, rather than the decrease in the temperature is
known as the phenomenon of temperature inversion.
We have discussed the important parameters, which we come across in wave
propagation. The waves of higher frequencies are transmitted and received
using this wave propagation technique.

Modulation is the process of changing the parameters of the carrier signal, in
accordance with the instantaneous values of the modulating signal.
Need for Modulation
The baseband signals are incompatible for direct transmission. For such a signal,
to travel longer distances, its strength has to be increased by modulating with a
high frequency carrier wave, which doesn’t affect the parameters of the
modulating signal.
1. AF transmission is noisy.
2. Hight of antenna is impracticable at AF ie λ/4
3. AF signal power is low ,restricts for distance communication
Following are some of the advantages for implementing modulation in the
communication systems:•Antenna size gets reduced.•No signal mixing
occurs.•Communication range increases.•Multiplexing of signals
occur.•Adjustments in the bandwidth is allowed.•Reception quality improves.

Continuous-wave Modulation: This is further divided into amplitude and angle
modulation.
If the amplitude of the high frequency carrier wave is varied in accordance with
the instantaneous amplitude of the modulating signal, then such a technique is
called as Amplitude Modulation.
If the angle of the carrier wave is varied, in accordance with the instantaneous
value of the modulating signal, then such a technique is called as Angle
Modulation.
The angle modulation is further divided into frequency and phase modulation.
If the frequency of the carrier wave is varied, in accordance with the
instantaneous value of the modulating signal, then such a technique is called as
Frequency Modulation.
If the phase of the high frequency carrier wave is varied in accordance with the
instantaneous value of the modulating signal, then such a technique is called as
Phase Modulation.

AM: According to the standard definition, “The amplitude of the carrier signal
varies in accordance with the instantaneous amplitude of the modulating
signal.” Which means, the amplitude of the carrier signal which contains no
information varies as per the amplitude of the signal, at each instant, which
contains information. This can be well explained by the following figures.

The modulating wave which is shown first is the message signal. The next one
is the carrier wave, which is just a high frequency signal and contains no
information. While the last one is the resultant modulated wave.
It can be observed that the positive and negative peaks of the carrier wave,
are interconnected with an imaginary line. This line helps recreating the exact
shape of the modulating signal. This imaginary line on the carrier wave is
called as Envelope. It is the same as the message signal.

Amplitude Modulation Derivation: The amplitude modulation derivation is
provided here. Amplitude modulation is modulation technique commonly used
for transmission of information via a radio carrier wave. This is the earliest
modulation used in radio to transmit voice. It was developed by Landell de
Moura and Reginald Fessenden’s in the year 1900 with the experiments of a
radiotelephone. It finds applications in two-way radios, computer modems in
the form of QAM, VHF aircraft radio and citizens band radio.
Derivation of Amplitude Modulation
Mathematical representation of Amplitude Modulated waves in time domain
m(t)=Amcos(2πfmt) (modulating signal)
c(t)=Accos(2πfct) (carrier signal)
s(t)=⌊Ac+Amcos(2πfmt)⌋cos(2πfct) (equation of Amplitude Modulated wave)
Where, Am: amplitude of modulating signal
Ac: amplitude of carrier signal , fm: frequency of modulating signal
fc: frequency of carrier signal.Therefore, above is the derivation of AM

Modulation index derivation: Modulation index is also known as modulation
depth is defined for the carrier wave to describe the modulated variable of
carrier signal varying with respect to its unmodulated level. It is represented as
follows: μ=Am/Ac
Consider maximum and minimum amplitudes of the wave as Amax and Amin
Depending upon cos(2𝜋fmt) following two equations are derived with
maximum and minimum amplitude of the modulated waves.
Amax=Ac + Am , Amin=Ac−Am
Amax+Amin=Ac+Am+Ac−Am=2Ac
⇒Ac=Amax+Amin/2
Amax−Amin=Ac + Am−(Ac−Am)=2Am
⇒Am=Amax−Amin/2
μ=Am/Ac or Am= μ Ac
Therefore, this is the derivation of the modulation index.

Ie. S(t)=Ac cosWct + Am cosWmt cosWct = Ac cosWct +( Am/2) 2
cosWmt cosWct
By using formula 2 cos A cos B = cos (A+B) + cos (A-B)
S(t) = Ac cos Wct + Am/2 cos (Wc+Wm)t + Am/2 cos (Wc-Wm)t
Since µ=Am/Ac Implies Am=µAc
S(t)=Ac cos ωct+µAc/2 cos (ωc+ωm)t+ µAc/2 cos (ωc-ωm)t
Is the final AM modulated signal.It has original carrier frequency fc
and amplitude µAc ,
And two side band frequencies (fc+fm) called upper side band , (fc-
fm) called lower side band with each amplitude µAc/2
⇒s(t)=Accos(2πfct)+Acμ2cos[2π(fc+fm)t]+Acμ2cos[2π(fc−fm)t]

Power Calculations of AM Wave :
Consider the following equation of amplitude modulated wave.
S(t)=Accos(2πfct)+Acμ2cos[2π(fc+fm)t]+Acμ2cos[2π(fc−fm)t]
Power of AM wave is equal to the sum of powers of carrier, upper sideband,
and lower sideband frequency components.
Pt = Pc + PUSB+PLSB
We know that the standard formula for power of cos signal is
P=𝑽𝒓𝒎𝒔𝟐/R = (𝑽𝒎/ 𝟐)
𝟐
/2
Where,
vrms is the rms value of cos signal.
vm is the peak value of cos signal.
First, let us find the powers of the carrier, the upper and lower sideband one by
one.
Carrier power:
Pc=(Ac/ 𝟐)𝟐
/R = 𝑨𝒄𝟐
/2R

Upper sideband power:
PUSB=
(Acμ/2 2)
2
𝑅
= 𝑨𝒄𝟐
µ𝟐
/8R Similarly, we will get the lower sideband power
same as that of the upper side band power.
PLSB= 𝑨𝒄𝟐µ𝟐/8R
Now, let us add these three powers in order to get the power of AM wave.
Pt=(𝑨𝒄𝟐/2R)+(𝑨𝒄𝟐µ𝟐/8R) + (𝑨𝒄𝟐µ𝟐/8R)
⇒Pt= (𝐴𝑐2/2R)[1+(µ2/4)+(µ2/4)]
⇒Pt=Pc(1+µ𝟐/2)
We can use the above formula to calculate the power of AM wave, when the
carrier power and the modulation index are known.
If the modulation index μ=1(for 100% mod.) then the power of AM wave is
equal to 1.5 times the carrier power. So, the power required for transmitting an
AM wave is 1.5 times the carrier power for a perfect modulation.

Problem 1
A modulating signal m(t)=10cos(2π×103t) is amplitude modulated with a carrier
signal c(t)=50cos(2π×105t). Find the modulation index, the carrier power, and
the power required for transmitting AM wave.
Solution
Given, the equation of modulating signal as
m(t)=10cos(2π×103t)
We know the standard equation of modulating signal as
m(t)=Amcos(2πfmt)
By comparing the above two equations, we will get
Amplitude of modulating signal as Am=10volts
and Frequency of modulating signal as
fm=103Hz=1KHz
Given, the equation of carrier signal is
c(t)=50cos(2π×105t)

The standard equation of carrier signal is
c(t)=Accos(2πfct)
By comparing these two equations, we will get
Amplitude of carrier signal as Ac=50volts
and Frequency of carrier signal as fc=105Hz=100KHz
We know the formula for modulation index as
μ=AmAc
Substitute, Am and Ac values in the above formula.
μ=1050=0.2
Therefore, the value of modulation index is 0.2 and percentage of modulation
is 20%.
The formula for Carrier power, Pc= is
Pc=Ac22R
Assume R=1Ω and substitute Ac value in the above formula.
Pc=(50)22(1)=1250W

Therefore, the Carrier power, Pc is 1250 watts.
We know the formula for power required for transmitting AM wave is
⇒Pt=Pc(1+μ22)
Substitute Pc and μ values in the above formula.
Pt=1250(1+(0.2)22)=1275W
Therefore, the power required for transmitting AM wave is 1275 watts.
Problem 2:
The equation of amplitude wave is given by
s(t)=20[1+0.8cos(2π×103t)]cos(4π×105t). Find the carrier power, the total
sideband power, and the band width of AM wave.
Solution
Given, the equation of Amplitude modulated wave is
s(t)=20[1+0.8cos(2π×103t)]cos(4π×105t)
Re-write the above equation as
s(t)=20[1+0.8cos(2π×103t)]cos(2π×2×105t)

We know the equation of Amplitude modulated wave is
s(t)=Ac[1+μcos(2πfmt)]cos(2πfct)
By comparing the above two equations, we will get
Amplitude of carrier signal as Ac=20volts
Modulation index as μ=0.8
Frequency of modulating signal as fm=103Hz=1KHz
Frequency of carrier signal as fc=2×105Hz=200KHz
The formula for Carrier power, Pcis
Pc=Ae22R
Assume R=1Ω and substitute Ac value in the above formula.
Pc=(20)22(1)=200W
Therefore, the Carrier power, Pc is 200watts.
We know the formula for total side band power is
PSB=Pcμ22
Substitute Pc and μ values in the above formula.

PSB=200×(0.8)22=64W
Therefore, the total side band power is 64 watts.
We know the formula for bandwidth of AM wave is
BW=2fm
Substitute fm value in the above formula.
BW=2(1K)=2KHz
Therefore, the bandwidth of AM wave is 2 KHz.
Transisor AM Modulator : A better AM modulation circuit uses a
transistor. In this circuit, the carrier-wave generated by an oscillator
that isn't shown in the circuit is applied to the base of a transistor.
Then, the audio input is applied to the transistor's emitter through a
transformer.

The transistor amplifies the input from the oscillator through the
emitter-collector circuit. However, as the audio input varies, it
induces a small current in the secondary coil of the transformer. This,
in turn, affects the amount of current that flows through the
collector-emitter circuit. In this way, the intensity of the output
varies with the audio input.
the carrier wave is a constant frequency and amplitude. In other
words, each cycle of the sine wave is of the same intensity. The
current of the audio wave varies, however.
When the two are combined by the modulator circuit, the result is a
signal with a steady frequency, but the intensity of each cycle of the
sine wave varies depending on the intensity of the audio signal.

The envelope of the modulating wave has the same shape as the
base band signal provided the following two requirements are
satisfied
1. The carrier frequency fc must be much greater then the highest
frequency components fm of the message signal m
(t) i.e. fc >> fm
2. The modulation index must be less than unity. if the modulation
index is greater than unity, the carrier wave becomes over
modulated.

Frequency Modulation : In Frequency Modulation (FM), the
frequency of the carrier signal varies in accordance with the
instantaneous amplitude of the modulating signal.
Hence, in frequency modulation, the amplitude and the phase of the
carrier signal remains constant. This can be better understood by
observing the following figures.

The frequency of the modulated wave increases, when the
amplitude of the modulating or message signal increases. Similarly,
the frequency of the modulated wave decreases, when the
amplitude of the modulating signal decreases. Note that, the
frequency of the modulated wave remains constant and it is equal to
the frequency of the carrier signal, when the amplitude of the
modulating signal is zero.
Mathematical Representation
The equation for instantaneous frequency fi in FM modulation is
fi=fc + kfm(t)
Where,
fc is the carrier frequency
kt is the frequency sensitivity

m(t) is the message signal
We know the relationship between angular frequency ωi and angle
θi(t) as
ωi=dθi(t)dt
⇒2πfi=dθi(t)/dt
⇒θi(t)=2π∫fidt
Substitute, fi value in the above equation.
θi(t)=2π∫(fc + kfm(t))dt
⇒θi(t)=2πfct+2πkf∫m(t)dt
Substitute, θi(t) value in the standard equation of angle modulated
wave.
s(t)=Ac cos(2πfct+2πkf∫ m(t)dt)

This is the equation of FM wave.
If the modulating signal is m(t)=Amcos(2πfmt), then the equation of
FM wave will be
s(t)=Ac cos(2πfct+βsin(2πfm t))
Where,
β = modulation index =Δf/fm=kf Am/fm
The difference between FM modulated frequency (instantaneous
frequency) and normal carrier frequency is termed as Frequency
Deviation. It is denoted by Δf, which is equal to the product of kf and
Am.
FM can be divided into Narrowband FM and Wideband FM based on
the values of modulation index β.

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

Narrowband FM
Following are the features of Narrowband FM.
This frequency modulation has a small bandwidth when compared to
wideband FM.
The modulation index β is small, i.e., less than 1.
Its spectrum consists of the carrier, the upper sideband and the lower
sideband.
This is used in mobile communications such as police wireless,
ambulances, taxicabs, etc.
Wideband FM
Following are the features of Wideband FM.
This frequency modulation has infinite bandwidth.
The modulation index β is large, i.e., higher than 1.

Its spectrum consists of a carrier and infinite number of sidebands,
which are located around it.
This is used in entertainment, broadcasting applications such as FM
radio, TV, etc.
The Instataneous value of FM modulating signal is given by:

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.

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 ]

some of the highlight points about frequency modulation sidebands,
FM spectrum & bandwidth.
The bandwidth of a frequency modulated signal varies with both
deviation and modulating frequency.
Increasing modulating frequency increases the frequency separation
between sidebands.
Increasing modulating frequency for a given level of deviation
reduces modulation index. As a result, it reduces the number of
sidebands with significant amplitude. This has the result of reducing
the bandwidth.
The frequency modulation bandwidth increases with modulation
frequency but it is not directly proportional to it.

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.
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

Demodulation is extracting the original information-bearing signal
from a carrier wave. A demodulator is an electronic circuit (or
computer program in a software-defined radio) that is used to
recover the information content from the modulated carrier wave.
Diode detector: The simplest form of envelope detector is the diode
detector which is shown above. A diode detector is simply a diode
between the input and output of a circuit, connected to a resistor and
capacitor in parallel from the output of the circuit to the ground. If
the resistor and capacitor are correctly chosen, the output of this
circuit should approximate a voltage-shifted version of the original
(baseband) signal. A simple filter can then be applied to filter out the
DC component.

Drawbacks
The envelope detector has several drawbacks:
The input to the detector must be band-pass filtered around the
desired signal, or else the detector will simultaneously demodulate
several signals. The filtering can be done with a tunable filter or,
more practically, a superheterodyne receiver
It is more susceptible to noise than a product detector
If the signal is overmodulated, distortion will occur

Transistor AM detector eliminates noise due to non linear behavior
of diode and also amplifies weak signal.This detects good quality of
weak signal at the out put of transistor amplifier circuit
FM detectors are circuits that instantaneously convert the
frequency changes from the carrier signal to its output voltage
counterpart. They are also known as frequency demodulators or
discriminators

Balanced FM Slope Detector (Balanced Frequency Discriminator)
The circuit diagram of the balanced slope detector is shown in
Figure. 2.

As shown in the circuit diagram, the balanced slope detector
consists of two slope detector circuits.
The input transformer has a center tapped secondary. Hence, the
input voltages to the two slope detectors are 180° out of phase.
There are three tuned circuits.Out of them, the primary is tuned to
IF i.e., fc .The upper tuned circuit of the secondary (T1) is tuned
above fc by Δf i.e., its resonant frequency is (fc+ Δf).
The lower tuned circuit of the secondary is tuned below fc by Δf i.e.,
at (fc – Δf).R1C1 and R2C2 are the filters used to bypass the RF
ripple.Vo1 and Vo2 are the output voltages of the two slope
detectors.The final output voltage Vo is obtained by taking the
subtraction of the individual output voltages, Vo1 and Vo2, i.e.,

Working Operation of the Circuit :The circuit operation can be
explained by dividing the input frequency into three ranges as follows:
(i) fin = fc: When the input frequency is instantaneously equal to fc,
the induced voltage in the T1 winding of secondary is exactly equal to
that induced in the winding T2.Thus, the input voltages to both the
diodes D1 and D2 will be the same.Therefore, their dc output voltages
Vo1 and Vo2 will also be identical but they have opposite polarities.
Hence, the net output voltage Vo = 0.
(ii) fc < fin < (fc + Δf): In this range of input frequency, the induced
voltage in the winding T1 is higher than that induced in T2.
Therefore, the input to D1 is higher than D2.
Hence, the positive output Vo1 of D1 is higher than the negative
output Vo2 of D2.

Therefore, the output voltage Vo is positive.
As the input frequency increases towards (fc + Δf), the positive output voltage
increases as shown in 3.

If the output frequency goes outside the range of (fc – Δf) to (fc + Δf),
the output voltage will fall due to the reduction in tuned circuit
response.
Advantages
(i) This circuit is more efficient than simple slope detector.
(ii) It has better linearity than the simple slope detector.
Drawbacks
(i) Even though linearity is good, it is not good enough.
(ii) This circuit is difficult to tune since the three tuned circuits are to
be tuned at different frequencies i.e., fc, (fc+Δf) and (fc – Δf).
(iii) Amplitude limiting is not provided.

Foster-Seeley FM discriminator :
The Foster Seeley detector or as it is sometimes described the Foster
Seeley discriminator is quite similar to the ratio detector at a first look.
It has an RF transformer and a pair of diodes, but there is no third
winding - instead a choke is used.

In many respects the Foster Seeley FM demodulator resembles the
circuit of a full wave bridge rectifier - the format that uses a centre
tapped transformer, but additional components are added to give it
a frequency sensitive aspect.
The basic operation of the circuit can be explained by looking at the
instances when the instantaneous input equals the carrier frequency,
the two halves of the tuned transformer circuit produce the same
rectified voltage and the output is zero. If the frequency of the input
changes, the balance between the two halves of the transformer
secondary changes, and the result is a voltage proportional to the
frequency deviation of the carrier.
Looking in more detail at the circuit, the Foster-Seeley circuit
operates using a phase difference between signals.

To obtain the different phased signals a connection is made to the
primary side of the transformer using a capacitor, and this is taken to
the centre tap of the transformer. This gives a signal that is 90° out of
phase.
When an un-modulated carrier is applied at the centre frequency,
both diodes conduct, to produce equal and opposite voltages across
their respective load resistors. These voltages cancel each one
another out at the output so that no voltage is present. As the carrier
moves off to one side of the centre frequency the balance condition
is destroyed, and one diode conducts more than the other. This
results in the voltage across one of the resistors being larger than the
other, and a resulting voltage at the output corresponding to the
modulation on the incoming signal.

The choke is required in the circuit to ensure that no RF signals
appear at the output. The capacitors C1 and C2 provide a similar
filtering function.
Both the ratio detector and Foster-Seeley detectors are expensive to
manufacture. Any wound components like the RF transformers are
Foster-Seeley detector advantages & disadvantages
As with any circuit there are a number of advantages and
disadvantages to be considered when choosing between the various
techniques available for FM demodulation.
Advantages of Foster-Seeley FM discriminator:
Offers good level of performance and reasonable linearity.
Simple to construct using discrete components.
Provides higher output and lower distortion than the ratio detector

Disadvantages of Foster-Seeley FM discriminator:
Does not easily lend itself to being incorporated within an integrated
circuit.
High cost of transformer.
Narrower bandwidth than the ratio detector
The circuit is sensitive to both frequency and amplitude and
therefore needs a limiter before it to remove amplitude variations
and hence amplitude noise.
Ratio Detector Circuit
In the Foster-Seeley discriminator, changes in the magnitude of the
input signal will give rise to
amplitude changes in the resulting output voltage. This makes prior
limiting necessary. It is possible to

It is possible to
modify the discriminator circuit to provide limiting, so that the
amplitude limiter may be dispensed with.
A circuit so modified is called a Ratio Detector Circuit.
As we now, the sum Vao + Vbo remains constant, although the
difference varies because of changes in
input frequency. This assumption is not completely true. Deviation
from this ideal does not result in
undue distortion in the Ratio Detector Circuit, although some
distortion is undoubtedly introduced. It
follows that any variations in the magnitude of this sum voltage can
be considered spurious here. Their
suppression will lead to a discriminator which is unaffected by the

amplitude of the incoming signal. Itwill therefore not react to noise
amplitude or spurious amplitude modulation.

Operation: With diode D2 reversed, o is now positive with respect to
b’, so that Va′b′ is now a sum voltage, rather than the difference it
was in the discriminator. It is now possible to connect a large
capacitor between a’ and b’ to keep this sum voltage constant. Once
C5 has been connected, it is obvious that Va′b′ is no
longer the output voltage; thus the output voltage is now taken
between o and o′. It is now necessary to ground one of these two
points, and o happens to be the more convenient, as will be seen
when dealing with practical Ratio Detector Circuit. Bearing in mind
that in practice R5 = R6, Vo is calculated as follows:

Equation (6-21) shows that the ratio detector output voltage is equal
to half the difference between the
output voltages from the individual diodes. Thus (as in the phase
discriminator) the output voltage is
proportional to the difference between the individual output
voltages. The Ratio Detector Circuit
therefore behaves identically to the discriminator for input frequency
changes. The S curve of Figure
6-40 applies equally to both circuits.
Limitations: Large time constant ,slow response.
Slow response for amplitude variations
AGC is required.
Interference due to aircrafts, noisy.

E5.1 commn unit 2 md

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