Chapter 2: Analog Modulation Techniques

2.1 Introduction to Modulation

Modulation is the process of encoding information from a message source in a manner

suitable for transmission. It involves modifying a high-frequency carrier signal using the
information from a lower-frequency message signal. This allows the information to be
transmitted efficiently over long distances and through different transmission media.

In communication systems, the original information signal, such as audio, video, or sensor
data is typically a low-frequency signal, referred to as a baseband signal. Baseband
signals are not well-suited for direct transmission over most physical media, especially
wireless channels.

To make transmission feasible, the baseband signal is used to modify a higher frequency
carrier signal, resulting in a modulated signal known as a passband signal. The
carrier itself carries no information but serves as a vehicle for transporting the baseband
message over long distances. The resulting passband signal occupies a band of frequencies
centered around the carrier frequency.

2.1.1 Why Modulation is Necessary

The following are key reasons why modulation is essential in communication systems:

• Efficient Radiation and Reception: Higher frequency (passband) signals

can be transmitted and received more effectively using practical antenna sizes† .
Low-frequency baseband signals would require physically large antennas, which are
often impractical. Low-frequency baseband signals would require physically large
antennas, which are often impractical.

• Multiplexing: Modulation enables multiple signals to share the same transmission

medium by assigning different carrier frequencies. This allows for simultaneous
communication channels, as in radio and television broadcasting.

• Noise Immunity: Passband signals are generally more resistant to certain types
of noise and distortion, particularly in wireless environments.
†
The physical size of an antenna is typically related to the signal’s wavelength, given by λ = fc . A
common approximation for antenna size is a fraction of the wavelength, such as a half-wave dipole with
length L = λ2 = 2f
c
. For example, at f = 100 MHz (FM radio), λ = 3 m, so a half-wave antenna would
be L = 1.5 m.

1
 • Frequency Translation: Modulation shifts the baseband signal to a higher
frequency band suitable for the channel characteristics and spectrum regulations.

2.1.2 Types of Modulation

There are various types of modulation, depending on the nature of the message signal
and how the carrier is modified. In this chapter, we focus on analog modulation, where
the message signal is analog and varies continuously in amplitude and time.

The three main analog modulation techniques are:

1. Amplitude Modulation (AM): The amplitude of the carrier is varied in

proportion to the message signal.
2. Frequency Modulation (FM): The frequency of the carrier is varied in
accordance with the message signal.
3. Phase Modulation (PM): The phase of the carrier is changed based on the
message signal.

These analog modulation techniques are fundamental in understanding traditional

broadcast systems and serve as a foundation for learning more advanced digital
communication methods.

2.2 Amplitude Modulation (AM)

Amplitude Modulation (AM) is a technique in which the amplitude of a high-frequency

carrier signal is varied in direct proportion to the instantaneous value of the message
signal. It is one of the simplest and earliest forms of modulation and is widely used in
broadcasting applications, particularly in the AM radio band.

Let the message (modulating) signal be

m(t) = Am cos(2πfm t), (2.1)

where:

• Am is the amplitude of the message signal,

• fm is the frequency of the message signal.

Let the carrier signal be

c(t) = Ac cos(2πfc t), (2.2)

where:

2
 • Ac is the amplitude of the carrier,

• fc is the frequency of the carrier signal, with fc ≫ fm .

In AM, the instantaneous amplitude of the carrier is varied according to the message
signal. The total AM signal is formed by replacing the carrier amplitude Ac in (2.2) with
a time-varying envelope:
h i
SAM (t) = Ac + Am cos(2πfm t) cos(2πfc t). (2.3)

Factoring out Ac , we get:

Am
 
SAM (t) = Ac 1 + cos(2πfm t) cos(2πfc t). (2.4)
Ac

Define the modulation index k as:

Am
k= .
Ac

Substituting into the expression:

SAM (t) = Ac [1 + k cos(2πfm t)] cos(2πfc t). (2.5)

The modulation index k determines the extent of amplitude variation. For effective AM
without distortion, the condition 0 ≤ k ≤ 1 must be satisfied. If k > 1, overmodulation
occurs, which leads to distortion of the transmitted signal.

The expression in (2.5) describes a standard AM wave, where:

• The term Ac cos(2πfc t) represents the unmodulated carrier,

• The term kAc cos(2πfm t) cos(2πfc t) contains the information-carrying sidebands.

Figure 2.1 shows a sinusoidal modulating signal and the corresponding AM signal. For
the case shown, Am = 0.5Ac , and the signal is said to be 50% modulated. A percentage of
modulation greater than 100% will distort the message signal if detected by an envelope
detector.

Figure 2.2 shows the message signal, carrier signal, and AM signals for k =
0.5, 1.0, and 1.5, illustrating the effects of under-modulation, 100% modulation,
and over-modulation, respectively. In the under-modulated case (k < 1), the envelope
does not reach zero, resulting in reduced transmission efficiency. At 100% modulation
(k = 1), the envelope just touches zero at its minima, ensuring maximum efficiency
without distortion. In the over-modulated case (k > 1), the envelope crosses zero,
causing phase reversals that distort the demodulated signal.

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 (b) Corresponding AM signal with modulation index
(a) A sinusoidal modulating signal. 0.5.

Figure 2.1: (a) and (b) show the modulating signal and its corresponding AM waveform.

Figure 2.2: Message Signal, Carrier Signal and AM Signals for k = 0.5, 1 and 1.5.

2.2.1 Spectrum of an AM Signal

We consider the expression of the AM signal in (2.5):

sAM (t) = Ac [1 + k cos(2πfm t)] cos(2πfc t)

Expanding the product:

sAM (t) = Ac cos(2πfc t) + Ac k cos(2πfm t) cos(2πfc t)

Using the identity cos A cos B = 12 [cos(A + B) + cos(A − B)]:

Ac k Ac k
sAM (t) = Ac cos(2πfc t) + cos 2π(fc + fm )t + cos 2π(fc − fm )t (2.6)
2 2

Equation (2.6) shows three components: a carrier at fc , and sidebands at fc ± fm .

For a general message signal m(t), the AM signal is:

sAM (t) = Ac [1 + km(t)] cos(2πfc t) (2.7)

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Its spectrum is:
Ac Ac k
SAM (f ) = [δ(f − fc ) + δ(f + fc )] + [M (f − fc ) + M (f + fc )] (2.8)
2 2

The first term is the carrier; the second replicates the message spectrum at fc and −fc ,
forming the upper and lower sidebands.

If m(t) is bandlimited to fm , the AM signal spans from fc − fm to fc + fm . Thus, the

total bandwidth is:
B = 2fm

(a) (b)

Figure 2.3: Figure 5.2 (a) Spectrum of a message signal. (b) Spectrum of the corresponding
AM signal.

2.2.2 AM Signal Power

The total power in an AM signal is the sum of the power in the carrier and in the two
sidebands. Given the AM signal:

kAc kAc
sAM (t) = Ac cos(2πfc t) + cos[2π(fc − fm )t] + cos[2π(fc + fm )t] (2.9)
2 2

Assuming a load resistance R, the RMS values of the voltage components are:
Ac
- Carrier: √
2

kA
- Each sideband: √c
2 2

2
Vrms
Using the power formula P = R
, the total transmitted power becomes:

! !
A2 (kAc )2 (kAc )2 A2 k2 k2 k2
PT = c + + = c 1+ + = Pc 1+ (2.10)
2R 8R 8R 2R 4 4 2

A2c
where Pc = 2R
is the carrier power. Thus, the sideband power is:

5
 k2
PSB = PT − Pc = Pc ·
2

Power in each sideband is:

Pc k 2
PUSB = PLSB = (2.11)
4

Maximum efficiency occurs when k = 1, giving sideband power equal to 50% of the carrier
power.

Example: A zero-mean sinusoidal message is applied to a transmitter radiating a total

AM power of 10 kW. The modulation index is k = 0.6. Compute the carrier power, the
percentage of total power in the carrier, and the power in each sideband.

Solution:

!
k2
PT = Pc 1+
2

Solving for Pc :

PT 10,000 10,000 10,000

Pc = k2
= 0.62
= = ≈ 8474.58 W
1+ 2 1+ 2 1 + 0.18 1.18

Each sideband power:

Pc k 2 8474.58 × 0.36
PUSB = PLSB = = ≈ 762.71 W
4 4

Percentage of total power in the carrier:

Pc 8474.58
× 100 ≈ × 100 ≈ 84.75%
PT 10,000

Therefore, 84.75% of the power is in the carrier, and 15.25% is shared equally between
the two sidebands.

2.2.3 Single Sideband AM

In conventional AM, both sidebands carry the same information. This redundancy allows
us to remove one sideband without loss of information. Single Sideband (SSB) modulation
transmits only the upper or lower sideband, thereby reducing the bandwidth by half.

6
Expanding the conventional AM signal in equation (2.7) gives

sAM (t) = Ac cos(2πfc t) + Ac km(t) cos(2πfc t). (2.12)

• The first term is the carrier.

• The second term generates two sidebands: one at fc + f and another at fc − f ,

where f is a frequency component of m(t).

Thus, conventional AM uses double the bandwidth of the baseband message. To

save bandwidth, SSB is constructed by suppressing one sideband while retaining all
information. This is achieved using two orthogonal carriers:

cos(2πfc t) (in-phase carrier), sin(2πfc t) (quadrature carrier).

The SSB signal is therefore expressed as

sSSB (t) = km(t) cos(2πfc t) ∓ k m̂(t) sin(2πfc t), (2.13)

where m̂(t) is the Hilbert transform of m(t). The minus sign corresponds to the upper
sideband (USB) while the plus sign corresponds to the lower sideband (LSB).

The Hilbert transform is defined as

m̂(t) = m(t) ∗ hHT (t), (2.14)

where hHT (t) is the impulse response of a Hilbert transformer. In the frequency domain,
the Hilbert transform introduces a 90◦ phase shift:

−j, f > 0,
H(f ) = 
j, f < 0.

From equation (2.13), it can be seen that multiplying m(t) by cos(2πfc t) produces both
sidebands, while multiplying m̂(t) by sin(2πfc t) produces another pair of sidebands shifted
by 90◦ . Adding or subtracting these components cancels one sideband and leaves only
the desired one, achieving single sideband transmission.

Two methods for generating SSB signals:

1. Filter Method: A double sideband AM signal is passed through a bandpass filter

that removes one of the sidebands. This requires a highly selective filter, often
implemented using crystal filters at an intermediate frequency (IF). (See Figure 2.4)

2. Balanced Modulator Method: The modulating signal is split into two paths.
One path modulates the in-phase carrier, while the other is Hilbert transformed
(phase shifted by −90◦ ) and then modulates the quadrature carrier. The sign of
the quadrature component determines whether the USB or LSB is transmitted.
(See Figure 2.5)

7
 Figure 2.4: Block diagram of SSB generation using a sideband filter.

Figure 2.5: Block diagram of SSB generation using a balanced modulator.

2.2.4 Pilot Tone SSB

Single Sideband (SSB) modulation is highly bandwidth-efficient, but its performance

degrades significantly in fading channels. In such environments, small frequency
mismatches between the transmitted carrier and the receiver’s local oscillator lead to
distortions in the demodulated signal, especially in audio applications where pitch shifts
may become noticeable. Furthermore, Doppler spreading and Rayleigh fading can cause
both frequency and amplitude variations that affect demodulation accuracy.

To address these issues, a low-level pilot tone is transmitted along with the SSB signal.
This pilot tone serves as a frequency and phase reference at the receiver, where a
phase-locked loop (PLL) uses it to synchronize the local oscillator. As long as the
pilot and the information-bearing signal undergo correlated fading, the receiver can
compensate for amplitude and phase distortions through feedforward signal regeneration
(FFSR). This technique significantly enhances SSB robustness in mobile and fading
environments.

2.2.5 Demodulation of AM Signals

Demodulation is the process of recovering the original message signal from a modulated
carrier. AM, demodulation techniques are generally categorized as either coherent or
noncoherent. Coherent detection requires the receiver to have a reference oscillator
synchronized in both frequency and phase with the transmitted carrier, whereas

8
noncoherent detection requires no such phase reference and is simpler to implement.

A commonly used coherent demodulator is the product detector, also known as a phase
detector. As illustrated in Figure 2.6, this circuit multiplies the incoming AM signal with
a locally generated carrier. If the received AM signal is expressed as
R(t) cos(2πfc t + θr ),
where R(t) is the instantaneous amplitude of the received signal, including the modulated
message, i.e.,
R(t) = Ac [1 + km(t)],
and the local oscillator generates A0 cos(2πfc t + θo ), the output of the multiplier is:

v1 (t) = R(t)A0 cos(2πfc t + θr ) cos(2πfc t + θo ). (2.15)

Using the trigonometric identity

1
cos A cos B = [cos(A − B) + cos(A + B)] ,
2
this becomes
A0 R(t)
v1 (t) = [cos(θr − θo ) + cos(4πfc t + θr + θo )] . (2.16)
2

The high-frequency term is removed by a low-pass filter, leaving the baseband output:
vout (t) = KR(t),
where K is a constant. Since R(t) contains the modulated message, this output effectively
recovers the original message signal m(t).

Figure 2.6: Block diagram of a product detector.

For simpler implementations, noncoherent demodulation can be done using an envelope

detector. This circuit extracts the envelope of the AM signal without requiring a phase
reference. If the input is R(t) cos(2πfc t + θr ), the envelope detector outputs:
venv (t) = K|R(t)|.

Envelope detectors work reliably when the signal power is significantly higher than the
noise power, typically by at least 10 dB. In contrast, product detectors are better suited
for low signal-to-noise ratio (SNR) conditions and can operate effectively even when SNR
is below 0 dB.

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2.3 Angle Modulation

Angle modulation varies the phase angle of a carrier signal according to the message
signal, while keeping the amplitude constant. The two important types of angle
modulation are Frequency Modulation (FM) and Phase Modulation (PM).

2.3.1 Frequency Modulation (FM)

Frequency modulation (FM) is a type of angle modulation where the instantaneous

frequency of the carrier varies linearly with the baseband message signal m(t). The FM
signal can be expressed as:
 
h i Z t
SFM (t) = Ac cos 2πfc t + θ(t) = Ac cos 2πfc t + 2πkf m(τ ) dτ , (2.17)
−∞

where

• Ac is the carrier amplitude,

• fc is the carrier frequency,

• kf is the frequency deviation constant (in Hz per volt),

• m(t) is the modulating signal,

• τ is a dummy variable of integration.

Note: The integral includes the modulating signal m(τ ). The lower limit −∞ is a
theoretical convention; in practice, any constant lower limit only adds a fixed phase offset
and does not affect the FM waveform.

If the modulating signal is sinusoidal with amplitude Am and frequency fm :

m(t) = Am cos(2πfm t), (2.18)

then the FM signal becomes

 
kf Am
SFM (t) = Ac cos 2πfc t + sin(2πfm t). (2.19)
fm

R 1
Here, the sine term appears because cos(2πfm t) dt = 2πfm
sin(2πfm t).

As shown in Figure 2.7, the FM signal amplitude remains constant, but its waveform
compresses and expands over time. This behavior reflects the instantaneous frequency
variations imposed by the message signal, illustrating the fundamental principle of
frequency modulation.

10
 Figure 2.7: Time-domain plot of the FM signal.

2.3.2 Phase Modulation (PM)

Phase modulation is a form of angle modulation where the instantaneous phase of the
carrier is varied linearly with the message signal m(t):
SP M (t) = Ac cos [2πfc t + kp m(t)] , (2.20)
where kp is the phase deviation constant in radians per volt.

Figure 2.8: Time-domain plot of the PM signal.

Figure 2.8 shows the message signal m(t), the carrier c(t), and the resulting
phase-modulated signal SP M (t). The PM signal maintains the same amplitude as

11
the carrier, but its waveform is shifted in time according to the instantaneous value
of the message. Positive values of m(t) advance the carrier phase, compressing the
cycles, while negative values delay the phase, expanding the cycles. Where the message
crosses zero, the PM signal aligns with the carrier, clearly illustrating how information
is encoded in the carrier’s phase rather than its amplitude or frequency.

2.3.3 Relation Between FM and PM

An FM signal can be viewed as a PM signal with the message signal integrated before
modulation:  Z t 
SF M (t) = Ac cos 2πfc t + kp m(τ )dτ . (2.21)
−∞

Conversely, a PM signal can be generated by differentiating the message signal and

applying FM modulation.

2.3.4 Modulation Indices

The frequency modulation index βf relates the peak frequency deviation ∆f to the
maximum frequency fm in the message signal:
∆f kf Am
βf = = .
fm fm

The phase modulation index βp is the peak phase deviation:

βp = kp Am .

Example

A sinusoidal message signal m(t) = 4 cos (2π × 4 × 103 ) t is applied to an FM modulator

with frequency deviation constant kf = 10 kHz/V. Find:

(a) Peak frequency deviation ∆f ,

(b) Frequency modulation index βf .

Solution:

(a) The peak frequency deviation occurs at the maximum value of m(t), which is 4 V:
∆f = kf × Am = 10 kHz/V × 4 V = 40 kHz.

(b) The frequency modulation index is

∆f 40 kHz
βf = = = 10.
fm 4 × 103

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2.4 Spectra and Bandwidth of FM Signals

For a sinusoidal modulating signal

m(t) = Am cos(2πfm t),

the FM signal spectrum contains a carrier and an infinite number of sidebands spaced at
integer multiples of fm .

The amplitude of each spectral component is given by Bessel functions of the first kind,
Jn (βf ), where the modulation index is

∆f
βf = ,
fm
with ∆f being the peak frequency deviation.

The bandwidth containing approximately 98% of the total transmitted power is

approximated by Carson’s rule:

BT = 2(∆f + fm ) ≈ 2∆f, (2.22)

where BT is the total bandwidth of the FM signal.

For small modulation indices (βf < 1), the bandwidth approaches 2fm , while for larger
indices, it approaches 2∆f .

As a practical example, the U.S. AMPS cellular system uses a modulation index βf = 3
and fm = 4 kHz. Applying Carson’s rule gives a bandwidth approximately

BT = 2(∆f + fm ) = 2(3 × 4 kHz + 4 kHz) = 32 kHz.

Example An 880 MHz carrier is frequency modulated by a 100 kHz sinusoidal wave with
peak deviation ∆f = 500 kHz. Find the IF bandwidth necessary to pass the signal.

Solution: Calculate the modulation index:

∆f 500 kHz
βf = = = 5.
fm 100 kHz

By Carson’s rule, the bandwidth is:

BT = 2(∆f + fm ) = 2(500 kHz + 100 kHz) = 1200 kHz.

Thus, the IF filter bandwidth should be at least 1.2 MHz.

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2.4.1 FM Modulation Methods

There are two primary methods to generate an FM signal: the direct method and the
indirect method.

Direct Method: In the direct method, the carrier frequency is directly varied
according to the amplitude of the modulating signal. This is typically achieved using a
voltage-controlled oscillator (VCO), where a voltage-variable reactance element, such as
a varactor diode, controls the oscillator frequency. Figure 2.9 shows a simple reactance
modulator.

Figure 2.9: A simple reactance modulator where the capacitance of a varactor diode is varied to
change the frequency of the oscillator, generating FM signals.

The varactor diode acts as a voltage-controlled capacitor. Its capacitance changes

with the applied modulating signal (audio or information signal). When the varactor
is connected in parallel with the tank circuit (L and C), the total capacitance of
the LC circuit varies, which in turn changes the oscillation frequency of the carrier.
Mathematically, the carrier frequency is determined by the resonance of the LC circuit:

1
fc = √
2π LC

Thus, instead of varying the amplitude, the frequency of the carrier is varied according to
the amplitude of the modulating signal, producing a frequency-modulated (FM) signal.

While VCOs provide a straightforward way to generate narrowband FM signals,

maintaining frequency stability for wideband FM is more challenging. Frequency
stability can be significantly improved by using a phase-locked loop (PLL), which locks
the oscillator frequency to a stable crystal reference.

Indirect Method: The indirect method, introduced by Major Edwin Armstrong,

14
Figure 2.10: Block diagram of the indirect FM generation method. A narrowband FM signal is
generated using a balanced modulator and then frequency multiplied to obtain wideband FM.

generates narrowband FM by approximating the FM signal as the sum of a carrier and

a single sideband (SSB) signal phase-shifted by 90°. For small phase deviations, the FM
signal sF M (t) can be approximated by:

sF M (t) ≈ Ac cos(2πfc t) − Ac βf sin(2πfc t) sin(2πfm t) (2.23)

where the first term represents the carrier and the second term the sideband.

A balanced modulator produces this narrowband FM signal by modulating a

crystal-controlled oscillator. Frequency multipliers then increase both the carrier
frequency and frequency deviation to achieve wideband FM. However, the phase noise
increases proportionally with the multiplication factor N . A simple block diagram of the
indirect FM transmitter is shown in Figure 2.10.

2.5 FM Detection Techniques

Frequency Modulation (FM) detection is the process of recovering the original message
signal m(t) from the modulated FM signal. The objective of all FM demodulators is
to produce an output voltage that is proportional to the instantaneous frequency of the
input FM signal. In essence, FM detection circuits perform a frequency-to-amplitude
conversion.

Several techniques exist for FM demodulation, including:

• Slope Detection

• Zero-Crossing Detection

• Phase-Locked Loop (PLL) Detection

• Quadrature Detection

15
2.5.1 Slope Detection

This method approximates the FM demodulation process by differentiating the FM signal,

followed by envelope detection. The FM signal is first passed through an amplitude limiter
to suppress any amplitude variations. The output of the limiter is given by:

 Z t 
v1 (t) = V1 cos (2πfc t + θ(t)) = V1 cos 2πfc t + 2πkf m(τ ) dτ (2.24)
−∞

The slope filter (a differentiator with gain increasing linearly with frequency) then yields:

v2 (t) ∝ sin (2πfc t + θ(t)) (2.25)

Envelope detection of this signal provides an output that contains a DC component

(related to fc ) and the original message signal m(t).

The key idea of the envelope detector is that the amplitude of v2 (t) varies slightly
according to the instantaneous frequency deviation of the FM signal. By passing v2 (t)
through a simple envelope detector—typically a diode followed by a capacitor—the
variations in amplitude are converted into a voltage proportional to the original message
signal m(t).

The output of the envelope detector contains two components:

1. A DC component related to the carrier frequency, and

2. The desired message signal m(t).

The DC component can be removed using a capacitor filter. The block diagram for this
method is shown in Figure 2.11.

Figure 2.11: Block diagram of a slope detector FM demodulator

2.5.2 Zero-Crossing Detector

In this method, the number of zero crossings of the FM signal is counted over time.
The rate of zero crossings is proportional to the instantaneous frequency. A limiter
converts the FM signal into a constant amplitude square wave. The resulting signal is
passed through a differentiator to detect transitions, then a monostable multivibrator
("one-shot") generates pulses. A low-pass filter averages these pulses to recover m(t).

16
 Figure 2.12: Block diagram of a zero-crossing (pulse-averaging) discriminator

2.5.3 Phase-Locked Loop (PLL) Detection

A PLL is a feedback control system that locks a Voltage-Controlled Oscillator (VCO) to

the frequency of the incoming FM signal. The PLL contains:

• Phase comparator

• Low-pass filter

• Voltage-controlled oscillator (VCO)

The input signal and the VCO output are compared. The phase difference is used to
adjust the VCO frequency. Once locked, the control voltage to the VCO represents the
demodulated signal m(t).

Figure 2.13: Block diagram of a Phase-Locked Loop (PLL) FM detector

2.5.4 Quadrature Detection

Quadrature detection uses a phase-shift network that shifts the phase of the FM signal
by an amount proportional to its instantaneous frequency. The original FM signal and
the phase-shifted signal are fed into a multiplier (product detector), whose output is
proportional to their phase difference.

Because the phase shift is proportional to frequency, this technique performs

frequency-to-amplitude conversion. This method is cost-effective and widely used
in integrated circuits. However, for optimal performance, the phase shift across the FM
bandwidth should be small (typically ±5◦ ).

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2.6 Frequency Modulation vs. Amplitude Modulation

Frequency Modulation (FM) and Amplitude Modulation (AM) are two fundamental
analog modulation techniques. Understanding their differences is important when
selecting a modulation scheme for communication systems.

FM Characteristics: In frequency modulation, the carrier frequency varies in

accordance with the message signal, while the amplitude remains constant. This means
that all information is encoded in the frequency of the carrier, not its amplitude. FM
signals are considered constant-envelope signals, which allows them to be amplified using
power-efficient nonlinear amplifiers such as Class C.

FM systems offer:

• Excellent noise immunity. FM is less affected by amplitude noise (such as

atmospheric or impulse noise), since the information is not carried in the
amplitude.

• The capture effect, where the receiver locks onto the strongest signal and ignores
weaker signals on the same frequency. This improves resistance to co-channel
interference.

• Better performance in fading environments, as FM signals are less sensitive to

variations in amplitude caused by multipath fading.

• A tradeoff between bandwidth and performance: FM systems can improve the

signal-to-noise ratio (SNR) by increasing the modulation index and hence the
bandwidth.

AM Characteristics: In amplitude modulation, the amplitude of the carrier is varied

in proportion to the message signal, while frequency and phase remain constant. The
information is carried in the amplitude of the signal.

AM systems:

• Have a linear relationship between received signal power and signal quality. Weaker
signals lead to proportionally worse reception.

• Are more susceptible to amplitude noise and fading, but modern AM systems often
use pilot tones and automatic gain control (AGC) to mitigate these issues.

• Require linear amplifiers (Class A or AB), which are less power-efficient. This is a
limitation in battery-powered systems.

• Occupy less bandwidth than FM systems — which is advantageous when spectral

efficiency is a concern.

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Table 2.1: Comparison between Frequency Modulation (FM) and Amplitude Modulation (AM)

Feature FM AM
Modulation Frequency Amplitude
Parameter
Noise Immunity High Low
Bandwidth High Low
Requirement
Amplifier Efficiency High (Class C) Low (Class A/AB)
Resilience to Fading Better (constant envelope) Poorer
Receiver Complexity Higher (uses discriminator) Lower (envelope detector)
Capture Effect Present Absent

FM provides superior noise immunity and performs better in fading environments due
to its constant envelope nature. It also allows for more efficient amplification using
high-efficiency classes of amplifiers. However, these advantages come at the cost of
increased bandwidth requirements and greater complexity in both the transmitter and
receiver design. In contrast, AM is simpler to implement and more bandwidth-efficient,
but it is significantly more susceptible to noise and distortion. A detailed comparison of
the two techniques is presented in Table 2.1.

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