Types of Amplitude Modulation (AM) – Advantages & Disadvantages

Types of Amplitude Modulation with Advantages and Disadvantages

What is Amplitude Modulation (AM) ?

The type of modulation in which the amplitude of the carrier signal varies linearly with respect to the instantaneous amplitude of the message signal is called Amplitude modulation.

Types of Amplitude modulation:

There are several types of Amplitude modulations as follow with their advantages and disadvantages as applications.

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Double Sideband Suppressed Carrier(DSB SC)

Introduction

Double sideband is a type of Amplitude modulation in which the frequency spectrum of the message signal is symmetrically situated above & below the carrier signal’s frequency.

The upper & lower frequencies are known as sidebands of the modulated signal. Upper sideband (USB) has frequency components higher than the carrier frequency and the lower sideband (LSB) has lower frequency components than the carrier frequency.

Mathematical Expression:

Suppose the message signal is sinusoidal signal.

m(t) = A_mcos ω_mt

The carrier signal is a high frequency sinusoidal signal.

c(t) = A_c cos ω_ct

The Amplitude modulated DSB SC signal will be

ϕ_DSB-SC(t) = A_DSB cos ω_ct

A_DSB  = A_c + m(t)

Substituting A_DSB

ϕ_DSB-SC(t) = (A_c + m(t)) cos ω_ct     ……….eq(1)

ϕ_DSB-SC(t) = A_c cos ω_ct + m(t) cos ω_ct

Substituting m(t)

ϕ_DSB-SC(t) = A_c cos ω_ct + A_mcos ω_mt cos ω_ct

ϕ_DSB-SC(t) = A_c cos ω_ct + A_m/2 cos (ω_m + ω_c) t + A_m/2 cos (ω_m – ω_c) t

The modulated signal has three terms.

The first term represents the carrier signal. The second term represents the message signal’s frequency shifted to the left by ω_c. The third term represents the message signal’s frequency spectrum shifted to the right by ω_c as shown in the figure below.

Suppose the message signal’s spectrum is

The spectrum of the carrier signal is

The DSB SC modulated signal’s spectrum is

The message spectrum is centered at ω_c having two halves. The upper half (ω_c + ω_m ) of the message spectrum is called upper sideband & lower half (ω_c – ω_m ) of the message spectrum is called lower sideband. Because of these two sidebands, it is called double sideband AM transmission.

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Modulation Index

In Amplitude modulation, it describes the level of carrier signal amplitude over the level of the message signal.

According to the eq(1)

ϕ_DSB-SC(t) = (A_c + m(t)) cos ω_ct 

Substituting m(t)

ϕ_DSB-SC(t) = (A_c + A_mcos ω_mt) cos ω_ct

ϕ_DSB-SC(t) = A_c  (1 + A_m/A_c cos ω_mt) cos ω_ct

ϕ_DSB-SC(t) = A_c  (1 + μ cos ω_mt) cos ω_ct

where μ= A_m/A_c is the modulation index.

Modulation index plays important role in traditional AM discussed in this article below.

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Bandwidth

The bandwidth B.W of DSB-SC is the difference between the maximum and minimum frequency of the modulated signal.

B.W = (f_c+ f_m) – (f_c– f_m )

B.W = f_c+ f_m – f_c+ f_m

B.W = 2 f_m

The bandwidth of the DSB-SC modulated signal is twice the bandwidth of the message signal.

Demodulation

Demodulation is the process of acquiring the original signal (message signal) from the modulated signal (received signal).

To demodulate a DSB-SC signal, it is multiplied with the carrier signal (coherent frequency).

Assume the modulated signal is

ϕ(t) = m(t) cos ω_ct

Then the modulated signal will be

e(t)  = m(t) cos ω_ct cos ω_ct

 e(t) = ½ m(t) (1 + cos 2ω_ct)

e(t) = ½ m(t) +½ m(t) cos 2ω_ct

The demodulated signal contains two terms, a message signal and a high frequency term. The high frequency term is filtered out by passing through Low Pass Filter.

Advantages:

• The modulation process is very simple
• No need for filtering during modulation for sidebands.

Disadvantages

• Demodulation need coherent carrier source.
• It carrier less information about the carrier.
• Carrier power is wasted
• Envelop detection is not possible.
• High bandwidth compares to SSB, VSB.

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Double side-band full carrier (Traditional Amplitude Modulation)

Introduction

In DSB-FC, the carrier signal is utilized during demodulation. The message signal is stored in the envelope of the modulated signal. In order to acquire this envelope, the amplitude of the message signal in the modulated signal should not go below Zero. i.e.

A_c + m(t) ≥ 0

carrier amplitude A_c should be adjusted to satisfy the equation.

A_c – A_m ≥ 0

A_m = lowest peak amplitude of the message signal.

A_c  ≥ A_m

1  ≥ A_m/ A_c

1  ≥ μ

where μ = A_m/ A_c

0  ≥ μ  ≥ 1

Thus the modulation index μ should be between 0 & 1 for envelope detection at the receiver.

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Demodulation:

DSB-FC or traditional AM can be demodulated by using a coherent source or envelope detector. Envelop detector is a very simple and inexpensive process.

An envelope detector is a simple diode, capacitor and resistor circuit. It does not need any coherent source or any low pass filter.

The message signal is rectified out using this envelope detector.

Advantages

• The receiver is very simple and low cost.
• Broadcasting is very efficient.

Disadvantages

• Waste too much power.
• Overall low efficiency (about 33%).
• AM is affected by noise.

Quadrature Amplitude Modulation (QAM)

Quadrature Amplitude Modulation is the type of Amplitude modulation in which two different message signals are transmitted on the same frequency carrier with a different phase shift.

The DSB modulated signal has double bandwidth of the modulating signal. To overcome excessive bandwidth, QAM is applied by sending two message signals on the same frequency carrier signal with 90° phase difference.

Block Diagram

The block diagram of QAM is given below:

This block diagram shows the modulation of two message signals. The carrier source produces carrier signal. The carrier signal with 0° phase shift is used with the first message signal m₁(t) and the carrier signal with 90° phase shift is used with the second message signal m₂(t). Both modulated signals are then added using summer to make a single signal having same frequency and bandwidth.

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Mathematical expression

Suppose two message signals are m₁(t), m­₂(t)

The carrier signal is

c(t)= cos ω_ct.

So the 90° degree phase shifted signal of c(t) will be

c(t)= cos (ω_c+90°)t = sin ω_ct.

So the modulated single will be

Φ_QAM(t) = m₁(t) cos ω_ct + m₂(t) sin ω_ct

Demodulation

QAM modulated signal cannot be demodulated using envelope detection technique because it contains two message signals. The messages signals are demodulated by multiplying the QAM signal with its coherent carrier signal as follows.

To get m₁(t), the received signal is multiplied with cos ω_ct.

e(t)= (m₁(t) cos ω_ct + m₂(t) sin ω_ct) cos ω_ct

e(t)= m₁(t) cos² ω_ct + m₂(t) sin ω_ct cos ω_ct

e(t)= ½ m₁(t) (1+cos 2ω_ct) + ½ m₂(t) sin 2ω_ct

e(t)= ½ m₁(t) +½ m₁(t) cos 2ω_ct) + ½ m₂(t) sin 2ω_ct

By passing through Low Pass Filter

e(t)= ½ m₁(t)

To get m₂(t), the received signal is multiplied with sin ω_ct.

e(t)= (m₁(t) cos ω_ct + m₂(t) sin ω_ct) sin ω_ct

e(t)= m₁(t) cos ω_ct sin ω_ct + m₂(t) sin² ω_ct

e(t)= ½ m₁(t) sin 2ω_ct + ½ m₂(t) (1-cos 2ω_ct)

e(t)= ½ m₁(t) sin 2ω_ct + ½ m₂(t) – ½ m₂(t) cos 2ω_ct)

By passing through Low Pass Filter

                 e(t)= ½ m₂(t)

Advantages

• Can carry more than one message signal.
• Utilize low bandwidth as compared to the information transmitted.

Disadvantages

• It has a complex design of transmitter and receiver.
• Envelope detection technique does not work.

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Single sideband (SSB)

The type of Amplitude modulation, in which only single sideband is transmitted through an antenna is called single sideband communication.

Unlike DSB, the SSB modulated signal has only single side-band either upper side-band (usually) or lower side-band.

The SSB modulated signal is made from DSB signal by passing it through a bandpass filter. The bandpass filter cutoff the DSB modulated signal at ω_c­ and filter out either upper sideband or lower sideband as shown in fig below.

Bandwidth

The bandwidth of the SSB signal is equal to the bandwidth of the message signal.

Demodulation

If the received signal is SSB suppressed carrier signal then the demodulator needs a coherent source. which generates the same frequency carrier as the received signal. After demodulation, the signal is passed through low pass filter to filter out high-frequency components.

If the received signal is SSB full carrier signal then it is best to use an Envelope detector or Rectifier. The SSB full carrier transmission is a type of SSB transmission in which the carrier amplitude is very large compared to message signal amplitude.

Advantages

• Bandwidth is equal to message signal.
• Save half power by transmitting one sideband.

Disadvantages

• Filtering one sideband is very difficult and add complexity to the transmitter circuit
• It needs a bandpass filter with a very sharp cutoff. An ideal filter.

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Vestigial sideband (VSB)

As we know that a real bandpass filter does not have a sharp cutoff and it does not filter all the frequencies outside of the cutoff region. A real filter allows some frequency content outside of the cutoff region. Because of this problem vestigial sideband is implemented.

In VSB, one sideband and a little portion (25%) of the second sideband is transmitted as shown in the figure below.

Bandwidth

The bandwidth of VSB modulated signal is greater than SSB but it is lower than the DSB modulated signal. The bandwidth of the VSB modulated signal is 25% greater than the bandwidth of the message signal.

B.W = f_m + 25% f_m

Demodulation

If the received signal is VSB suppressed carrier signal, then the demodulation only needs coherent carrier source. The rest of the demodulation process is same as SSB and DSB suppressed carrier demodulation.

If the received signal is VSB with the carrier, then an envelope detector can also demodulate the signal. The VSB full carrier needs very large carrier amplitude as compared to message signal’s amplitude.

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