ELE321

ANALOG
MODULATION

by
Enock Kachokola
BSC. ELECTRONICS
 Modulation
• For a signal to be transmitted to a distance, without the effect of any external
interferences or noise addition and without getting faded away, it has to
undergo a process called as Modulation.
• It improves the strength of the signal without disturbing the parameters of the
original signal.
• What is Modulation?
• A message carrying a signal has to get transmitted over a distance and for it
to establish a reliable communication, it needs to take the help of a high
frequency signal which should not affect the original characteristics of the
message signal.
• The characteristics of the message signal, if changed, the message
contained in it also alters. Hence, it is a must to take care of the message
signal. A high frequency signal can travel up to a longer distance, without
getting affected by external disturbances.
• We take the help of such high frequency signal which is called as a carrier
signal to transmit our message signal. Such a process is simply called as
Modulation.
• Modulation is the process of changing the parameters of the carrier signal, in
accordance with the instantaneous values of the modulating signal.
 Modulation (2)
• Need for Modulation
• 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.
 Advantages Modulation
•Reduction of antenna size
•No signal mixing
•Increased communication range
•Multiplexing of signals
•Possibility of bandwidth adjustments
•Improved reception quality
 Signals in the Modulation Process
• Following are the three types of signals in the modulation process:

• Message or Modulating Signal: The signal which contains a

message to be transmitted.
• It is a baseband signal, which has to undergo the process
of modulation, to get transmitted.

• Carrier Signal: The high frequency signal, which has a certain

amplitude, frequency and phase but contains no information.
• It is an empty signal and is used to carry the signal to the
receiver after modulation.

• Modulated Signal: The resultant signal after the process of

modulation.
• This signal is a combination of modulating signal and carrier signal.
 Types of Modulation
• Broadly classified into continuous-wave modulation and pulse modulation.
 Continuous-wave Modulation
• In continuous-wave modulation, a high frequency sine wave is used
as a carrier wave.
• Further divided into: amplitude and angle modulation.
1. Amplitude Modulation
Amplitude of the high frequency carrier wave is varied wrt.
instantaneous amplitude of the modulating signal.

2. Angle Modulation
Angle of the carrier wave is varied wrt. instantaneous value of the
modulating signal.

Angle modulation is further divided into:

a) Frequency Modulation: frequency of the carrier wave is varied
wrt. instantaneous value of the modulating signal.

b) Phase Modulation: phase of the high frequency carrier wave is

varied wrt. instantaneous value of the modulating signal.
 Pulse Modulation
• In Pulse modulation, a periodic sequence of rectangular pulses, is
used as a carrier wave.
• This is further divided into analog and digital modulation.

• 1. Analog Modulation technique: amplitude or duration or position

of a pulse is varied wrt. instantaneous values of the baseband
modulating signal.
• Types: Pulse Amplitude Modulation (PAM) or Pulse
Duration/Width Modulation (PDM/PWM), or Pulse Position
Modulation (PPM).

• 2. Digital Modulation: modulation technique used is Pulse Code

Modulation (PCM) where the analog signal is converted into digital
form of 1s and 0s.
• Resultant is a coded pulse train
• This is further developed as Delta Modulation (DM).
 Sideband Modulation
• In the process of AM or PM, the modulated wave consists of
carrier
wave and two sidebands.
• The modulated signal has the information in the whole band except
at the carrier frequency.
• A Sideband is a band of frequencies, containing power, which
are the lower and higher frequencies of the carrier frequency.
• Both the sidebands contain the same information.
• Figure shows an amplitude modulated wave in frequency domain:
 DSBFC vs DSBSC
• Both the sidebands in the figure contain the same information.
• Transmission of a signal which contains a carrier along with two
sidebands, is called Double Sideband Full Carrier system (DSBFC).
• However, such a transmission is inefficient: two-thirds of the power
is wasted in the carrier, which carries no information.
• If this carrier is suppressed and the power saved is distributed to
the two sidebands, such a process is called as Double Sideband
Suppressed Carrier system (DSBSC).
 SSBSC
• Now, we get an idea that, as the two sidebands carry the same
information twice, why can’t we suppress one sideband?
• The process of suppressing one of the sidebands, along with the
carrier and transmitting a single sideband is called as Single
Sideband Suppressed Carrier system (SSBSC) or SSB.

• This SSB-SC or SSB system, which transmits a single sideband has

high power, as the power allotted for both the carrier and the other
sideband is utilized in transmitting this Single Sideband (SSB).
• Modulation done using SSB technique is called SSB Modulation.
 Sideband Modulation Advantages
& Disadvantages
Advantages
• Bandwidth or spectrum space occupied is lesser than AM and
DSB signals.
• Transmission of more number of signals is allowed.
• Power is saved.
• High power signal can be transmitted.
• Less amount of noise is present.
• Signal fading is less likely to occur.

Disadvantages
• The generation and detection of SSB signal is a complex process.
• Quality of the signal gets affected unless the SSB transmitter
and receiver have an excellent frequency stability.
 Sideband Modulation Applications
• For power saving requirements and low bandwidth requirements.
• In land, air, and maritime mobile communications.
• In point-to-point communications.
• In radio communications.
• In television, telemetry, and radar communications.
• In military communications, such as amateur radio, etc.
 Vestigial Modulation
• In case of SSB modulation, when a sideband is passed through the
filters, the band pass filter may not work perfectly in practice. As a
result some information may be lost.
• To avoid this loss, a compromise between DSB-SC and SSB
is chosen, known as Vestigial Sideband (VSB) technique.
• The word vestige which means “a part” from which the name
is derived.
• Vestigial Sideband: Both of the sidebands are not required for the
transmission, as it is a waste. But a single band if transmitted, leads
to loss of information. Hence, this technique has evolved.
• Vestigial Sideband Modulation or VSB Modulation is the process
where a part of the signal called as vestige is modulated, along with
one sideband.
 Vestigial Modulation
• Figure shows a VSB signal

• Along with the USB, a part of the LSB is also being transmitted in this
technique.
• To avoid interferences, a guard band of very small width is laid on
either side of VSB.
 Transmission Bandwidth of VSB
Modulated Wave
BW = fm + fv

Where, fm = Message bandwidth

fv = Width of the vestigial sideband
 VSB Modulation Advantages
and Disadvantages
Advantages
• Highly efficient.
• Reduction in bandwidth.
• Filter design is easy as high accuracy is not needed.
• The transmission of low frequency components is possible, without
difficulty.
• Possesses good phase characteristics.

Disadvantages
• Bandwidth when compared to SSB is greater.
• Demodulation is complex.
 VSB Modulation Applications
• The most prominent and standard application of VSB is for
transmission of television signals.
• Also, it is a most convenient and efficient technique when bandwidth
usage is considered.
 Amplitude Modulation (AM)
• A continuous-wave goes on continuously without any intervals and it
is the baseband message signal, which contains the information.
• This wave has to be modulated.
• In AM, “The amplitude of the carrier signal varies in accordance with
the instantaneous amplitude of the modulating signal.”
• This means, the amplitude of the carrier signal containing no
information varies as per the amplitude of the signal containing
information, at each instant.
 Amplitude Modulation (AM) (2)

• The first figure shows the modulating wave (message signal).

• The next one is the carrier wave (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 that of the message signal.
 Time-domain Representation of the AM Waves
Let the modulating signal be,
𝑚(𝑡) = 𝐴𝑚cos(2п𝑓𝑚𝑡)
and the carrier signal be,
𝑐(𝑡) = 𝐴𝑐cos(2п𝑓𝑐𝑡)
Where,
𝐴𝑚and 𝐴𝑐 are amplitudes of modulating signal and carrier signal resp.
𝑓𝑚and 𝑓𝑐 are frequencies of modulating signal and carrier signal resp.

Then, Amplitude Modulated wave will be

𝑠(𝑡) = [𝐴𝑐+𝐴𝑚cos(2п𝑓𝑚𝑡)] cos(2п𝑓𝑐𝑡) (Equation 1)
 Modulation Index
• Modulation Index or Modulation Depth states the level of
modulation that a carrier wave undergoes.
• Rearranging eqn. 1,

Where μ is Modulation index, equal to the ratio of 𝐴𝑚 and 𝐴𝑐:

Note: Equation 3 can be used when the amplitudes of the message and
carrier signals are known
 Modulation Index (2)
• When the maximum and minimum amplitudes of the modulated wave
are known, Modulation Index can also be calculated.
• Let 𝐴𝑚𝑎𝑥 and 𝐴𝑚𝑖𝑛 be the maximum and minimum amplitudes of the
modulated wave:
• Maximum amplitude is when cos(2п𝑓𝑚𝑡) = 1.

• Minimum amplitude of the modulated wave, when cos(2п𝑓𝑚𝑡) = −1.

• Add Eqns. 4 and 5

 Modulation Index (3)
• Subtract Equation 5 from Equation 4

• The ratio of Eqn. 7 and Eqn. 6 is,

• Equation 8 can also be used to find Modulation index when

maximum and minimum amplitudes of the modulated wave are
known.
 Percentage of Modulation
• The modulation index or modulation depth is often denoted in percentage called as
Percentage of Modulation.
• Percentage of modulation is found by multiplying modulation index value with 100.
• For a perfect modulation, the value of modulation index should be 1, i.e. percentage of
modulation of 100%.
• If percentage of modulation is < 1, (modulation index is 0.5), then the modulated
output is called as Under-modulation.
• If modulation index is > 1, (eg. 1.5 or so), then the wave will be an Over-modulated
wave.

• As modulation index increases, the carrier experiences a 180o phase reversal, which
causes additional sidebands and hence, the wave gets distorted.
• Such an over-modulated wave causes interference, which cannot be eliminated.
 Bandwidth of AM Wave
• Bandwidth (BW) is the difference between the highest and lowest
frequencies of the signal.
• Mathematically, we can write it as
𝐵𝑊 = 𝑓𝑚𝑎𝑥 − 𝑓𝑚𝑖𝑛

• Consider the following equation of amplitude modulated wave.

• Hence, the amplitude modulated wave has three frequencies,

namely: carrier frequency 𝑓𝑐, upper sideband frequency 𝑓𝑐 + 𝑓𝑚 and
lower sideband frequency 𝑓𝑐 − 𝑓𝑚
• Here,
 Bandwidth of AM Wave (2)
• Substituting 𝑓𝑚𝑎𝑥 and 𝑓𝑚𝑖𝑛 values in the bandwidth formula,

• Thus, it can be said that the bandwidth required for amplitude

modulated wave is twice the frequency of the modulating signal.
 Power Calculations of AM Wave
• Consider the following equation of amplitude modulated wave

• Power of AM wave is equal to the sum of powers of carrier,

upper
sideband, and lower sideband frequency components

• We know that the standard formula for power of cos signal is

Where,
𝑣𝑟𝑚𝑠 is the rms value of cos signal.
𝑣𝑚 is the peak value of cos signal.
 Power Calculations of AM Wave (2)
• First, let us find the powers of the carrier, the upper and
lower sideband one by one.
• Carrier power

• Upper sideband power

• Similarly, we will get the lower sideband power same as that of

the upper side band power.
 Power Calculations of AM Wave (3)
• Now, let us add these three powers in order to get the power of
AM wave.

• 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, power of AM wave is1.5 times
the carrier power.
• So, the power required for transmitting an AM wave is 1.5 times
the
carrier power for a perfect modulation.
 Example 1
A modulating signal 𝑚(𝑡) = 10cos(2п×103𝑡) is amplitude modulated with
a carrier signal 𝑐(𝑡) = 50cos(2п×105𝑡). Find the modulation index, the
carrier power, and the power required for transmitting AM wave.

Solution
Given, modulating signal equation: 𝑚(𝑡) = 10cos(2п×103𝑡),
Known: standard equation of modulating signal: 𝑚(𝑡) = 𝐴𝑚cos(2п𝑓𝑚𝑡)
By comparison, we get 𝐴𝑚 = 10 volts, 𝑓𝑚 = 103 Hz = 1 KHz

Given: modulating signal equation as 𝑐(𝑡) = 50cos(2п×105𝑡) Known:

standard equation of carrier signal as 𝑐(𝑡) = 𝐴𝑐cos(2п𝑓𝑐𝑡) By
comparison, we get 𝐴𝑐 = 50 volts, 𝑓𝑐 = 105 Hz = 100 KHz

Therefore, modulation index is 0.2 and percentage of modulation is

20%.
 Example 1 (2)
Assume R = 1Ω, then Carrier power, 𝑃𝑐

Power for transmitting AM wave, 𝑃𝑡

 Example 2
The equation of amplitude wave is given by
𝑠(𝑡) = 20[1 + 0.8cos(2п×103𝑡)] cos(4п×105𝑡). Find the carrier power, the
total sideband power, and the band width of AM wave.

Solution
Given: the equation of AM wave as
𝑠 𝑡 = 20[1 + 0.8cos(2п×103𝑡)] cos 4п×105𝑡
Re-write the above equation as
𝑠 𝑡 = 20[1 + 0.8cos(2п×103𝑡)] cos 2п×2×105𝑡

Known: equation of AM wave is 𝑠(𝑡) = 𝐴𝑐[1 + μ cos(2п𝑓𝑚𝑡)] cos(2п𝑓𝑐𝑡)

By comparison, we get 𝐴𝑐 = 20 volts, μ = 0.8, 𝑓𝑚 = 103𝐻𝑧 = 1𝐾𝐻𝑧,

𝑓𝑐 = 2×105𝐻𝑧 = 200𝐾𝐻𝑧

Assume R = 1Ω, Carrier power, 𝑃𝑐

 Example 2 (2)
Total side band power
is

Bandwidth of the AM
wave,
 AM Modulators
• The following two modulators generate AM
wave.
i. Square law modulator
ii. Switching modulator
 Square Law Modulator
• Following is the block diagram of the square law modulator

• Let the modulating and carrier signals be denoted as 𝑚 𝑡 and

𝐴𝑐cos(2п𝑓𝑐𝑡) respectively.
• These two signals are applied as inputs to the summer (adder) block.
• This summer block produces an output, which is the addition of the
modulating and the carrier signal.
• Mathematically, we can write it as
𝑉1 𝑡 = 𝑚 𝑡 + 𝐴𝑐cos(2п𝑓𝑐𝑡)
 Square Law Modulator (2)
• This signal 𝑉1 𝑡 is applied as input to a nonlinear device like diode.
The characteristics of the diode are closely related to square law

Where, 𝑘1 and 𝑘2 are constants.

• Substitute 𝑉1 𝑡 in Equation 1,
 Square Law Modulator (3)
• The last term of the above equation represents the desired AM wave
and the first three terms of the above equation are unwanted.
• So, with the help of band pass filter, we can pass only AM wave
and eliminate the first three terms.

• Therefore, the output of square law modulator is

• The standard equation of AM wave is

Where 𝐾𝑎 is the amplitude sensitivity.

• By comparing the output of the square law modulator with the

standard equation of AM wave, we will get the scaling factor
as 𝑘1and the amplitude sensitivity 𝑘𝑎 as
 Switching Modulator
• Following is the block diagram of the switching modulator

• Switching modulator is similar to square law modulator.

• The only difference is that in the square law modulator, the diode is
operated in a non-linear mode, whereas, in the switching
modulator, the diode has to operate as an ideal switch.
• Let the modulating and carrier signals be denoted as 𝑚 𝑡 and
𝑐 𝑡 = 𝐴𝑐cos(2п𝑓𝑐𝑡) respectively.
• These two signals are applied as inputs to the summer (adder) block.
 Switching Modulator (2)
• Summer block produces an output, which is the additionof
modulating and carrier signals.
• Mathematically, we can write it as
𝑉1 𝑡 = 𝑚 𝑡 + 𝑐 𝑡= 𝑚 𝑡 + 𝐴𝑐cos(2п𝑓𝑐𝑡)

• This signal 𝑉1 𝑡 is applied as input of diode.

• Assume, the magnitude of the modulating signal is very small when
compared to the amplitude of carrier signal 𝐴𝑐. So, the diode’s ON
and OFF action is controlled by carrier signal 𝑐 𝑡 . This means, the
diode will be forward biased when 𝑐 𝑡 > 0 and it will be reverse
biased when 𝑐 𝑡 < 0 .
• Therefore, the output of the diode is
 Switching Modulator (3)
• We can approximate this as

• Where, 𝑥 𝑡 is a periodic pulse train with time period 𝑇 = 1/𝑓𝑐

• The Fourier series representation of this periodic pulse train is

 Switching Modulator (4)
• Substitute, 𝑉1 𝑡 and 𝑥 𝑡 values in Equation
2.

• The 1st term of the above equation represents the desired AM wave
and the remaining terms are unwanted terms.
• Thus, with the help of band pass filter, we can pass only AM wave
and eliminate the remaining terms.
 Switching Modulator (4)
• Therefore, the output of switching modulator is

• We know the standard equation of AM wave is

Where 𝐾𝑎 is the amplitude sensitivity.

• By comparing the output of the switching modulator with the

standard equation of AM wave, we will get the scaling
factor as 0.5 and the amplitude sensitivity 𝑘𝑎 as
 AM Demodulators
• The process of extracting an original message signal from the
modulated wave is known as detection or demodulation.
• The circuit, which demodulates the modulated wave is known as the
demodulator.
• The following demodulators (detectors) are used for demodulating
AM wave:
i. Square Law Demodulator
ii. Envelope Detector
 Square Law Demodulator
• Square law demodulator is used to demodulate low level AM
wave.
• Following is the block diagram of the square law demodulator.

• This demodulator contains a square law device and low pass filter.
• The AM wave 𝑉1 𝑡 is applied as an input to this demodulator.
• The standard form of AM wave is

• We know that the mathematical relationship between the input and the
output of square law device is

Where, 𝑉1 𝑡 is the input of the square law device, which is

nothing but the AM wave
𝑉2 𝑡 is the output of the square law device
𝑘1 and 𝑘2 are constants.
 Square Law Demodulator (2)
• Substitute 𝑉1 𝑡 in Equation 1

• In the above equation, the term 𝑘2𝐴𝑐2𝑘𝑎𝑚(𝑡) is the scaled version of the
message signal. It can be extracted by passing the above signal through a
2
𝑘2𝐴𝑐
low pass filter and the DC component 2 can be eliminated with the help
a coupling
of
capacitor.
 Envelope Detector
• Envelope detector is used to detect (demodulate) high level
AM wave.
• Following is the block diagram of the envelope detector.

• This envelope detector consists of a diode and low pass filter. Here,
diode is the main detecting element.
• Hence, envelope detector is also called diode detector.
• The low pass filter contains a parallel combination of the resistor and
the capacitor.
 Envelope Detector (2)
• The AM wave s(t) is applied as an input to this
detector.
• We know the standard form of AM wave is

• In the positive half cycle of AM wave, the diode conducts and

the capacitor charges to the peak value of AM wave.
• When the value of AM wave is less than this value, the diode will
be reverse biased.
• Thus, the capacitor will discharge through resistor R till the next
positive half cycle of AM wave.
• When the value of AM wave is greater than the capacitor voltage,
the diode conducts and the process will be repeated.
• We should select the component values in such a way that
the capacitor charges very quickly and discharges very slowly.
• As a result, we will get the capacitor voltage waveform same as that
of the envelope of AM wave, which is almost similar to the
modulating signal.
 Double Sideband Full Carrier (DSBFC)
• In the process of Amplitude Modulation, the modulated wave consists
of the carrier wave and two sidebands.
• The modulated wave has the information only in the sidebands.
• Sideband is nothing but a band of frequencies, containing power,
which are the lower and higher frequencies of the carrier frequency.
• The transmission of a signal, which contains a carrier along with two
sidebands is termed as Double Sideband Full Carrier system
(DSBFC).

• However, such a transmission is inefficient. Because, two-thirds of

the power is being wasted in the carrier, which carries no information.
Double Sideband Suppressed Carrier (DSBSC)
• If this carrier is suppressed and the saved power is distributed to the
two sidebands, then such a process is called as Double Sideband
Suppressed Carrier system (DSBSC).
 Mathematical Expressions for DSBSC
• Let us consider the same mathematical expressions for modulating
and carrier signals as considered earlier chapters
i.e., Modulating signal,
𝑚(𝑡) = 𝐴𝑚cos(2п𝑓𝑚𝑡)
and carrier signal,
𝑐(𝑡) = 𝐴𝑐cos(2п𝑓𝑐𝑡)

• Mathematically, we can represent a DSBSC wave as product

of modulating and carrier signals.
𝑠(𝑡) = 𝑚(𝑡)𝑐(𝑡)

𝑠(𝑡) = 𝐴𝑚𝐴𝑐cos(2п𝑓𝑚𝑡) cos(2п𝑓𝑐𝑡)

 Bandwidth of DSBSC Wave
• We know 𝐵𝑊 = 𝑓𝑚𝑎𝑥 − 𝑓𝑚𝑖𝑛

• Consider the equation of DSBSC modulated wave.

• The DSBSC modulated wave has only two frequencies. So, the
maximum and minimum frequencies are 𝑓𝑐 + 𝑓𝑚 and 𝑓𝑐 − 𝑓𝑚
respectively.

• Substitute, 𝑓𝑚𝑎𝑥 and 𝑓𝑚𝑖𝑛 values in the bandwidth formula,

• Thus, BW of DSBSC wave is same as that of AM wave and it

is
equal to twice the frequency of the modulating signal.
 Power Calculations of DSBSC Wave
• Consider the equation of DSBSC modulated wave.

• Power of DSBSC wave is equal to sum of powers of upper sideband

and lower sideband frequency components.

• We know that the standard formula for power of cos signal is

Where,
𝑣𝑟𝑚𝑠 is the rms value of cos signal.
𝑣𝑚 is the peak value of cos signal.
 Power Calculations of DSBSC Wave (2)
• First, let’s find powers of upper and lower sideband one by one.
• USB power

• Similarly, we will get the LSB power same as that of the USB
power.

• Adding these two sideband powers gives power of DSBSC wave,

• Therefore, power required for transmitting DSBSC wave is equal to

the power of both the sidebands.
 DSBSC Modulators
• The following two modulators generate DSBSC wave
i. Balanced modulator
ii. Ring modulator
 Balanced Modulator
• Following is the block diagram of the balanced modulator
 Balanced Modulator (2)
• Balanced modulator consists of two identical AM modulators.
• These two modulators are arranged in a balanced configuration in
order to suppress the carrier signal. Hence, the name.

• The same carrier signal 𝑐 𝑡 = 𝐴𝑐cos(2п𝑓𝑐𝑡) is applied as one of the

inputs to these two AM modulators.
• The modulating signal 𝑚 𝑡 is applied as another input to the
upper AM modulator.
• Whereas, the modulating signal 𝑚 𝑡 with opposite polarity, i.e.,
− 𝑚 𝑡 is applied as another input to the lower AM modulator.

• Output of the upper AM modulator is

• Output of the lower AM modulator is

 Balanced Modulator (3)
• We get the DSBSC wave 𝑠 𝑡 by subtracting 𝑠2 𝑡 from 𝑠1 𝑡
using the summer block.
• 𝑠1 𝑡 with +ve sign and 𝑠2 𝑡 with -ve sign are applied as inputs to
the summer

• The standard equation of DSBSC wave is

• By comparing the output of summer block with the standard equation

of DSBSC wave, we will get the scaling factor as
 Ring Modulator
• Following is the block diagram of the ring modulator
 Ring Modulator (2)
• In this diagram, the four diodes D1,D2,D3 and D4 are connected in
the ring structure. Hence, the name.
• Two center tapped transformers are used in this diagram.
• The message signal m(t) is applied to the input transformer.
• Whereas, the carrier signals c(t) is applied between the two center
tapped transformers.

• For positive half cycle of the carrier signal, the diodes D1 and D3 are
switched ON and the other two diodes D2 and D4are switched OFF.
• In this case, the message signal is multiplied by +1.

• For negative half cycle of the carrier signal, the diodes D2 and D4
are switched ON and the other two diodes D1 and D3 are switched
OFF.
• In this case, the message signal is multiplied by -1.
• This results in 180 degrees phase shift in the resulting DSBSC
wave.
 Ring Modulator (3)
• From the above analysis, we can say that the four diodes D1, D2, D3
and D4 are controlled by the carrier signal. If the carrier is a square
wave, then the Fourier series representation of c(t) is represented as

• We will get DSBSC wave s(t), which is just the product of the carrier
signal c(t) and the message signal m(t) i.e.,

• The above equation represents DSBSC wave, which is obtained at

the output transformer of the ring modulator.
• DSBSC modulators are also called as product modulators as they
produce the output, which is the product of two input signals.
 DSBSC Demodulators
• The following demodulators (detectors) are used for demodulating
DSBSC wave:
i. Coherent Detector
ii. Costas Loop
 Coherent Detector
• Here, the same carrier signal (which is used for generating DSBSC
signal) is used to detect the message signal.
• Hence, this process of detection is called as coherent or
synchronous detection.
• Following is the block diagram of the coherent detector.
 Coherent Detector (2)
• In this process, the message signal can be extracted from DSBSC
wave by multiplying it with a carrier, having the same frequency
and the phase of the carrier used in DSBSC modulation.
• The resulting signal is then passed through a Low Pass Filter.
• Output of this filter is the desired message signal.

• Let the DSBSC wave be

• The output of the local oscillator is

Where, ϕ is the phase difference between the local oscillator signal

and the carrier signal, which is used for DSBSC modulation.

• From the figure, we can write the output of product modulator as

 Coherent Detector (3)
• Substitute, 𝑠 𝑡 and 𝑐 𝑡 values in the above
equation

• In the above equation, the first term is the scaled version of the
message signal. It can be extracted by passing the above signal
through a low pass filter.
• Therefore, the output of low pass filter is
 Coherent Detector (4)
• The demodulated signal amplitude will be maximum, when ϕ = 00.
That’s why the local oscillator signal and the carrier signal should be
in phase, i.e., there should not be any phase difference between
these two signals.
• The demodulated signal amplitude will be zero, when ϕ = ±900.
• This effect is called as quadrature null effect.
 Costas Loop
• Costas loop is used to make both the carrier signal (used for
DSBSC modulation) and the locally generated signal in phase.
• Following is the block diagram of Costas loop.
 Costas Loop (2)
• Costas loop consists of two product modulators with common input
s(t), which is DSBSC wave.
• The other input for both product modulators is taken from
Voltage Controlled Oscillator (VCO) with −900 phase shift to
one of the product modulator as shown in figure.

• We know that the equation of DSBSC wave is

• Let the output of VCO be

• This output of VCO is applied as the carrier input of the upper

product modulator.
• Hence, the output of the upper product modulator is

• Substitute, 𝑠 𝑡 and 𝑐1 𝑡 values in the above equation.

 Costas Loop (3)
• After simplifying, we get 𝑣1 𝑡 as

• This signal is applied as an input of the upper LPF.

• The output of this LPF is

• Therefore, the output of this low pass filter is the scaled version of
the modulating signal.
• The output of −900 phase shifter is

• This signal is applied as the carrier input of the lower product

modulator.
• The output of the lower product modulator is

• Substitute, 𝑠 𝑡 and 𝑐2 𝑡 values in the above equation.

 Costas Loop (4)
• After simplifying, we get 𝑣2 𝑡 as

• This signal is applied as an input of the upper LPF.

• The output of this LPF is

• The output of this low pass filter has −900 phase difference with the
output of the upper low pass filter.
• The outputs of these two low pass filters are applied as inputs of the
phase discriminator.
• Based on the phase difference between these two signals, the
phase discriminator produces a DC control signal.
• This signal is applied as an input of VCO to correct the phase error
in VCO output. Therefore, the carrier signal (used for DSBSC
modulation) and the locally generated signal (VCO output) are in
phase.
 Single Sideband Suppressed Carrier (SSBSC)
• The DSBSC modulated signal has two sidebands. Since, the two
sidebands carry the same information, there is no need to transmit
both sidebands. We can eliminate one sideband.
• The process of suppressing one of the sidebands along with the
carrier and transmitting a single sideband is called as Single
Sideband Suppressed Carrier system (SSBSC).
• It is plotted as shown in the following figure.
 Single Sideband Suppressed Carrier (SSBSC)
• (2)
In the above figure, the carrier and the lower sideband are
suppressed. Hence, the upper sideband is used for transmission.
• Similarly, we can suppress the carrier and the upper sideband while
transmitting the lower sideband.
• This SSBSC system, which transmits a single sideband has high
power, as the power allotted for both the carrier and the other
sideband is utilized in transmitting this Single Sideband.
 Mathematical Expressions for SSBSC
Let us consider the same mathematical expressions for modulating and
carrier signals as considered earlier chapters
i.e., Modulating signal,
𝑚(𝑡) = 𝐴𝑚cos(2п𝑓𝑚𝑡)
and carrier signal,
𝑐(𝑡) = 𝐴𝑐cos(2п𝑓𝑐𝑡)

• Mathematically, we can represent equation of SSBSC wave as mmm

for the upper sideband

OR for the lower sideband

 Bandwidth of SSBSC Wave
• We know that the DSBSC modulated wave contains two sidebands
and its bandwidth is 2𝑓𝑚.
• Since the SSBSC modulated wave contains only one sideband, its
bandwidth is half of the bandwidth of DSBSC modulated wave.

i.e., Bandwidth of SSBSC modulated wave:

• Therefore, the bandwidth of SSBSC modulated wave is 𝑓𝑚 and it is

equal to the frequency of the modulating signal.
 Power Calculations of SSBSC Wave
• Consider equation of SSBSC modulated wave,

for the upper sideband

OR for the lower sideband

• Power of SSBSC wave is equal to the power of any one sideband

frequency components

• We know that the standard formula for power of cos signal is

 Power Calculations of SSBSC Wave (2)
• In this case, the power of the upper sideband is

• Similarly, we will get the lower sideband power same as that of

the
upper side band power.

• Therefore, the power of SSBSC wave is

 SSBSC
Advantages
• Bandwidth or spectrum space occupied is lesser than AM
and DSBSC waves.
• Transmission of more number of signals is allowed.
• Power is saved.
• High power signal can be transmitted.
• Less amount of noise is present.
• Signal fading is less likely to occur.

Disadvantages
• The generation and detection of SSBSC wave is a complex process.
• The quality of the signal gets affected unless the SSB transmitter
and receiver have an excellent frequency stability.
 SSBSC
Applications
• For power saving requirements and low bandwidth requirements.
• In land, air, and maritime mobile communications.
• In point-to-point communications.
• In radio communications.
• In television, telemetry, and radar communications.
• In military communications, such as amateur radio, etc.
 SSBSC Modulators
• We can generate SSBSC wave using the following two methods
i. Frequency discrimination method
ii. Phase discrimination method
 Frequency Discrimination Method
• The following figure shows the block diagram of SSBSC modulator
using frequency discrimination method.
 Frequency Discrimination Method (2)
• In this method, first we will generate DSBSC wave with the help of
the product modulator.
• Then, apply this DSBSC wave as an input of band pass filter.
• This band pass filter produces an output, which is SSBSC wave.

• Select the frequency range of band pass filter as the spectrum of the
desired SSBSC wave.
• This means the band pass filter can be tuned to either upper
sideband or lower sideband frequencies to get the respective
SSBSC wave having upper sideband or lower sideband.
 Phase Discrimination Method
• Figure shows block diagram of SSBSC modulator using phase
discrimination method.
 Phase Discrimination Method (2)
• This block diagram consists of two product modulators, two
−900
phase shifters, one local oscillator and one summer block.
• The product modulator produces an output, which is the product of
two inputs.
• The −900 phase shifter produces an output, which has a phase lag of
−900 with respect to the input.
• The local oscillator is used to generate the carrier signal.
• Summer block produces an output, which is either the sum of two
inputs or the difference of two inputs based on the polarity of inputs.
• The modulating signal 𝐴𝑚cos(2п𝑓𝑚𝑡) and the carrier signal
𝐴𝑐cos(2п𝑓𝑐𝑡) are directly applied as inputs to the upper product
modulator.
• So, the upper product modulator produces an output, which is the
product of these two inputs.
 Phase Discrimination Method (3)
• The output of upper product modulator is

• The modulating signal 𝐴𝑚cos(2п𝑓𝑚𝑡) and the carrier signal

𝐴𝑐cos(2п𝑓𝑐𝑡) are phase shifted by −900 before applying as inputs to the
lower product modulator.
• So, the lower product modulator produces an output, which is the
product of these two inputs.
• The output of lower product modulator is
 Phase Discrimination Method (4)
• Add 𝑠1 𝑡 and 𝑠2 𝑡 in order to get the SSBSC modulated wave) 𝑠 𝑡
having a lower sideband.

• Subtract 𝑠2 𝑡 and 𝑠1 𝑡 in order to get the SSBSC modulated wave

𝑠 𝑡 having a upper sideband.

• Hence, by properly choosing polarities of inputs at summer block, we

will get SSBSC wave having a upper sideband or a lower sideband.
 Coherent Detector
• Here, the same carrier signal (which is used for generating
SSBSC signal) is used to detect the message signal.
• Hence, this process of detection is called as coherent or
synchronous detection.
• Following is the block diagram of the coherent detector.
 Coherent Detector (2)
• In this process, the message signal can be extracted from SSBSC
wave by multiplying it with a carrier, having the same frequency
and the phase of the carrier used in SSBSC modulation.
• The resulting signal is then passed through a Low Pass Filter.
• Output of this filter is the desired message signal.

• Consider the following SSBSC wave having a lower sideband.

• The output of the local oscillator is

• From the figure, we can write the output of product modulator as

 Coherent Detector (3)
• Substitute, 𝑠 𝑡 and 𝑐 𝑡 values in the above
equation

• In the above equation, the first term is the scaled version of the
message signal. It can be extracted by passing the above signal
through a low pass filter.
 Coherent Detector (4)
• Therefore, the output of low pass filter is

• Here, the scaling factor is

 Coherent Detector (5)
• We can use the same block diagram for demodulating SSBSC wave
having an upper sideband.
• Consider the following SSBSC wave having an upper sideband.

• The output of the local oscillator is

• We can write the output of the product modulator

as

• Substitute, 𝑠 𝑡 and 𝑐 𝑡 values in the above equation

 Coherent Detector (6)

• In the above equation, the first term is the scaled version of the
message signal. It can be extracted by passing the above signal
through a LPF.
 Coherent Detector (7)
• Therefore, the output of the LPF is

• Here too the scaling factor is

• Therefore, we get the same demodulated output in both the cases

by using coherent detector.
 Angle Modulation
• Angle Modulation is the process in which the frequency or the
phase of the carrier signal varies according to the message signal.
• The standard equation of the angle modulated wave is

Where, 𝐴𝑐 is the amplitude of the modulated wave, which is the same as

the amplitude of the carrier signal
𝜃𝑖 𝑡 is the angle of the modulated wave

• Angle modulation is further divided into:

i. Frequency Modulation: process of varying the frequency of
the carrier signal linearly with the message signal.
ii. Phase Modulation: process of varying the phase of the carrier
signal linearly with the message signal.
 Frequency Modulation (FM)
• In Frequency Modulation, 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.
 Frequency Modulation (FM) (2)
• 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 of FM
• The equation for instantaneous frequency 𝑓𝑖 in FM modulation is

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

• Substitute, 𝜃𝑖 𝑡 value in the standard equation of angle modulated

wave

• This is the equation of FM wave.

 FM Contd/…
• If the modulating signal is 𝑚(𝑡) = 𝐴𝑚cos(2п𝑓𝑚𝑡), then equation of FM
wave will be

Where

• 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.
 Narrowband FM vs Wideband FM
• FM can be divided into Narrowband FM and Wideband FM based
on the values of modulation index β.

Narrowband FM
•Narrowband FM has a small bandwidth compared to wideband FM.
•Modulation index β is small, i.e., less than 1.
• Its spectrum consists of the carrier, the upper sideband and the
lower sideband.
• It is used in mobile communications such as police
wireless, ambulances, taxicabs, etc.

Wideband FM
•Wideband FM has infinite bandwidth.
•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.
• It is used in entertainment, broadcasting applications such as FM
radio, TV, etc.
 Phase Modulation (PM)
• In Phase Modulation (PM), the phase of the carrier signal varies in
accordance with the instantaneous amplitude of the modulating
signal.
• Here, the amplitude and the frequency of the carrier signal remains
constant.
• This can be better understood by observing the following figures.
 Phase Modulation (PM) (2)
• The phase of the modulated wave has got infinite points, where the
phase shift in a wave can take place.
• The instantaneous amplitude of the modulating signal changes the
phase of the carrier signal.
• When the amplitude is positive, the phase changes in one direction
and if the amplitude is negative, the phase changes in the
opposite direction.
 Mathematical Representation For PM
• The equation for instantaneous phase 𝜙𝑖 in phase modulation is

Where, 𝑘𝑝 is the phase sensitivity, 𝑚(𝑡) is the message signal

• The standard equation of angle modulated wave is

• Substitute, 𝜃𝑖 𝑡 value in the above equation

• This is the equation of PM wave.

 PM Contd/..
• If the modulating signal is 𝑚(𝑡) = 𝐴𝑚cos(2п𝑓𝑚𝑡), then equation of
PM wave will be

Where

Δ𝜙, is phase deviation

• Phase modulation is used in mobile communication systems, while

frequency modulation is used mainly for FM broadcasting.
 Example 3
A sinusoidal modulating waveform of amplitude 5 V and a frequency of
2 KHz is applied to FM generator, which has a frequency sensitivity of
40 Hz/volt. Calculate the frequency deviation, modulation index,
and bandwidth.

Solution
Given, 𝐴𝑚 = 5V, 𝑓𝑚 = 2 KHz, 𝑘𝑓 = 40 Hz/volt,
Frequency deviation (∆𝒇):

Modulation index (β):

β of 0.1 is less than one. Therefore, it is Narrow Band FM.

Bandwidth (BW):
The formula for BW of Narrow Band FM is the same as that of AM wave
 Example 4
An FM wave is given by 𝑠 𝑡 = 20 cos(8 п×106𝑡 + 9 𝑠𝑖𝑛 2п×103𝑡 ).
Calculate the frequency deviation, bandwidth, and power of FM wave.

Solution
Given, eqn of an FM wave: 𝑠 𝑡 = 20 cos(8 п×106𝑡 + 9 𝑠𝑖𝑛 2п×103𝑡 )
Known, std eqn of FM wave

By comparison:

Here, β >1. Hence, it is Wide Band FM.

Frequency Deviation (∆𝒇):

Rearranging formula for modulation index to
 Example 4 (2)
BW of Wideband FM:

Power of FM wave: (Assumed, R =1Ω)

 FM Modulators
• In this section we discuss modulators which generate NBFM and
WBFM waves.
 Generation of NBFM
• We know that the standard equation of FM wave is

• For NBFM,

• We know that cos θ ≈ 1 and sin θ ≈ 1 when θ is very small.

• By using the above relations, we will get the NBFM equation as
 Generation of NBFM (2)
• The block diagram of NBFM modulator is shown in figure
 Generation of NBFM (3)
• Here, the integrator is used to integrate the modulating signal 𝑚 𝑡 .
• The carrier signal 𝐴𝑐cos(2п𝑓𝑐𝑡) is the phase shifted by −900 to get
𝐴𝑐 𝑠𝑖𝑛(2п𝑓𝑐𝑡) with the help of −900 phase shifter.
• The product modulator has two inputs ∫ 𝑚 𝑡 𝑑𝑡 and 𝐴𝑐 𝑠𝑖𝑛(2п𝑓𝑐𝑡) .
• It produces an output, which is the product of these two inputs.
• This is further multiplied with 2п𝑘𝑓 by placing a block 2п𝑘𝑓 in the
forward path.
• The summer block has two inputs, which are nothing but the two
terms of NBFM equation.
• Positive and negative signs are assigned for the carrier signal and
the other term at the input of the summer block.
• Finally, the summer block produces NBFM wave.
 Generation of WBFM
• The following two methods generate WBFM wave.
i. Direct method
ii. Indirect method
 Direct Method
• Named so because we are generating a wide band FM wave directly.
• In this method, Voltage Controlled Oscillator (VCO) is
usedto generate WBFM.
• VCO produces an output signal, whose frequency is proportional to
the input signal voltage.
• This is similar to the definition of FM wave.
• Block diagram of generation of WBFM wave is shown in figure.
 Direct Method (2)
• Here, the modulating signal 𝑚 𝑡 , is applied as an input of Voltage
Controlled Oscillator (VCO). VCO produces an output, which is
nothing but the WBFM.

Where, 𝑓𝑖 is the instantaneous frequency of WBFM wave.

 Indirect Method
• In Indirect Method because we generate a wide band
FM wave
indirectly.
• This means, first we will generate NBFM wave and then with the help
of frequency multipliers we will get WBFM wave.
• The block diagram of generation of WBFM wave is shown:
 Indirect Method (2)
• The block diagram contains mainly two stages.
• In first stage, the NBFM wave will be generated using NBFM modulator.
• We have seen the block diagram of NBFM modulator at the beginning of this
chapter. We know that the modulation index of NBFM wave is less than one.
• Hence, in order to get the required modulation index (greater than one) of
FM wave, choose the frequency multiplier value properly.

• Frequency multiplier is a non-linear device, which produces an output

signal whose frequency is ‘n’ times the input signal frequency. Where, ‘n’ is
the multiplication factor.
• If NBFM wave whose modulation index β is less than 1 is applied as the
input of frequency multiplier, then the frequency multiplier produces an
output signal, whose modulation index is ‘n’ times β and the frequency also
‘n’ times the frequency of WBFM wave.

• Sometimes, we may require multiple stages of frequency multiplier and

mixers in order to increase the frequency deviation and modulation index of
FM wave.
 FM Demodulators
• The following two methods demodulate FM wave.
i. Frequency discrimination method
ii. Phase discrimination method
 Frequency Discrimination Method
• We know equation of FM wave:

• Differentiate the above equation with respect to 't’.

• We can write, −sin θ as sin(θ − 1800)

• In above equation, the amplitude term resembles the envelope of

AM wave and the angle term resembles the angle of FM wave. Here,
our requirement is the modulating signal m(t). Hence, we can
recover it from the envelope of AM wave.
 Frequency Discrimination Method (2)
• Figure shows block diagram of FM demodulator using frequency
discrimination method.

• Block diagram consists of a differentiator and an envelope

detector.
• Differentiator is used to convert the FM wave into a combination
of AM wave and FM wave. This means, it converts the frequency
variations of FM wave into the corresponding voltage (amplitude)
variations of AM wave.
• Envelope detector produces the demodulated output of AM wave,
which is nothing but the modulating signal.
 Phase Discrimination Method
• The following figure shows block diagram of FM demodulator using
phase discrimination method.
 Phase Discrimination Method (2)
• Block diagram consists of the multiplier, the low pass filter, and the
Voltage Controlled Oscillator (VCO).
• VCO produces an output signal v(t), whose frequency is proportional
to the input signal voltage d(t).
• Initially, when the signal d(t) is zero, adjust the VCO to produce
an output signal v(t), having a carrier frequency and −900 phase
shift with respect to the carrier signal.

• FM wave s(t) and the VCO output v(t) are applied as inputs of
the multiplier.
• The multiplier produces an output, having a high frequency
component and a low frequency component.
• Low pass filter eliminates the high frequency component and
produces only the low frequency component as its output.
• This low frequency component contains only the term-related phase
difference.
• Hence, we get the modulating signal m(t) from this output of the low
pass filter.
 Comparison of AM and
FM
AMPLITUDE MODULATION FREQUENCY MODULATION
1. Band width is very small which is one of the It requires much wider channel ( 7 to 15 times )
biggest advantage as compared to AM.
2. The amplitude of AM signal varies depending on The amplitude of FM signal is constant and
modulation index. independent of depth of the modulation.
3. Area of reception is large The are of reception is small since it is limited
to line of sight.
4. Transmitters are relatively simple & cheap. Transmitters are complex and hence
expensive.
5. The average power in modulated wave is The average power in frequency modulated
greater than carrier power. This added power wave is same as contained in un-modulated
is provided by modulating source. wave.
6. More susceptible to noise interference and has Noise can be easily minimized amplitude
low signal to noise ratio, it is more difficult to variations can be eliminated by using limiter.
eliminate effects of noise.
7. it is not possible to operate without interference. it is possible to operate several
independent transmitters on
same frequency.
8. The maximum value of modulation index = 1, No restriction is placed on modulation index.
other wise over-modulation would
result in distortions.
 AM/FM Transmitters and Receivers
• The antenna present at the end of transmitter section, transmits
the modulated wave.

• The antenna present at the beginning of the receiver section,

receives the modulated wave.

• In this section, let us discuss about AM, FM transmitters and

receivers.
 AM Transmitter
• AM transmitter takes the audio signal as an input and delivers amplitude
modulated wave to the antenna as an output to be transmitted. Figure below
shows block diagram of an AM transmitter

• Working of AM transmitter:
The audio signal from the output of the microphone is sent to the pre-
amplifier, which boosts the level of the modulating signal.
The RF oscillator generates the carrier signal.
Both the modulating and the carrier signal is sent to AM modulator.
Power amplifier is used to increase the power levels of AM wave.
This
wave is finally passed to the antenna to be transmitted.
 AM Receiver
• The AM super heterodyne receiver takes an amplitude modulated
wave as input and produces the original audio signal as an output.
• Figure shows block diagram of AM receiver
 FM Transmitter
• FM transmitter is the whole unit, which takes the audio signal as an input and
delivers FM wave to the antenna as an output to be transmitted. Block
diagram of FM transmitter is shown

• Working of FM transmitter:
Audio signal from microphone output is sent to pre-amplifier, which boosts
the level of the modulating signal.
This signal is then passed to HPF, which acts as a pre-emphasis network to filter
out the noise and improve the signal to noise ratio.
This signal is further passed to the FM modulator circuit.
Oscillator circuit generates a high frequency carrier, which is sent to
the modulator along with the modulating signal.
Several stages of frequency multiplier are used to increase the operating
frequency. Even then, the power of the signal is not enough to transmit. Hence,
a RF power amplifier is used at the end to increase the power of the modulated
signal. This FM modulated output is finally passed to antenna to be transmitted.
 FM Receiver
• Figure shows block diagram of AM
receiver

• This block diagram of FM receiver is similar to that of AM

receiver.
• The two blocks Amplitude limiter and De-emphasis network
are included before and after FM demodulator.
• The operation of the remaining blocks is the same as that of AM
receiver.
 Requirements of a Receiver
• Following are the requirements of both AM and FM receiver.
It should be cost-effective.
It should receive the corresponding modulated waves.
The receiver should be able to tune and amplify the desired
station.
It should have an ability to reject the unwanted stations.
Demodulation has to be done to all the station signals,
irrespective of the carrier signal frequency.

• For these requirements to be fulfilled, the tuner circuit and the

mixer circuit should be very effective. The procedure of RF mixing
is an interesting phenomenon.
 Practical Concept: FM Transmitter
The FM transmitter is a single transistor circuit. In telecommunication, the frequency
modulation (FM) transfers the information by varying the frequency of carrier wave
according to the message signal. Generally, the FM transmitter uses VHF radio
frequencies of 87.5 to 108.0 MHz to transmit & receive the FM signal. This transmitter
accomplishes the most excellent range with less power. The performance and working
of the wireless audio transmitter circuit is depends on the induction coil & variable
capacitor.

The FM transmitter is a low power transmitter and it uses FM waves for transmitting the
sound, this transmitter transmits the audio signals through the carrier wave by the
difference of frequency. The carrier wave frequency is equivalent to the audio signal of
the amplitude and the FM transmitter produce VHF band of 88 to 108 MHZ.

Block diagram of FM
Transmitter
 Working of FM Transmitter Circuit
The following circuit diagram shows the FM transmitter circuit and the required
electrical and electronic components for this circuit is the power supply of 9V, resistor,
capacitor, trimmer capacitor, inductor, mic, transmitter, and antenna. Let us consider the
microphone to understand the sound signals and inside the mic there is a presence of
capacitive sensor. It produces according to the vibration to the change of air pressure
and the AC signal.

FM Transmitter
circuit
 Working of FM Transmitter Circuit (2)
The formation of the oscillating tank circuit can be done through the transistor of
2N3904 by using the inductor and variable capacitor. The transistor used in this circuit is
an NPN transistor used for general purpose amplification. If the current is passed at the
inductor L1 and variable capacitor then the tank circuit will oscillate at the resonant
carrier frequency of the FM modulation. The negative feedback will be the capacitor C2
to the oscillating tank circuit.

To generate the radio frequency carrier waves the FM transmitter circuit requires an
oscillator. The tank circuit is derived from the LC circuit to store the energy for
oscillations.

The input audio signal from the mic penetrated to the base of the transistor, which
modulates the LC tank circuit carrier frequency in FM format. The variable capacitor is
used to change the resonant frequency for fine modification to the FM frequency band.
The modulated signal from the antenna is radiated as radio waves at the FM frequency
band and the antenna is nothing but copper wire of 20cm long and 24 gauge. In this
circuit the length of the antenna should be significant and here you can use the 25-27
inches long copper wire of the antenna.
 Working of FM Transmitter Circuit (2)
Advantages of the FM Transmitters
• The FM transmitters are easy to use and the price is low
• The efficiency of the transmitter is very high
• It has a large operating range
• This transmitter will reject the noise signal from an amplitude variation.

Application of FM Transmitter
• The FM transmitters are used in the homes like sound systems in halls to fill the
sound with the audio source.
• These are also used in the cars and fitness centers.
• The correctional facilities have used in the FM transmitters to reduce the prison
noise in common areas.