**What is Amplitude modulation and its types with advantages and disadvantages?**

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- 1 What is Amplitude modulation and its types with advantages and disadvantages?

The type of modulation in which the

amplitudeof the carrier signalvaries linearlywith respect to theinstantaneous amplitudeof the message signal is called.Amplitude modulation

**Types of Amplitude modulation:**

There are several types of Amplitude modulations.

**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 _{m}cos ω_{m}t**

The carrier signal is a high frequency sinusoidal signal.

**c(t) = A _{c }cos ω_{c}t**

The Amplitude modulated **DSB SC** signal will be

**ϕ _{DSB-SC}(t) = A_{DSB} cos ω_{c}t**

**A _{DSB } = A_{c} + m(t)**

Substituting **A _{DSB }**

**ϕ _{DSB-SC}(t) = (A_{c} + m(t)) cos ω_{c}t ……….eq(1)**

**ϕ _{DSB-SC}(t) = A_{c }cos ω_{c}t + m(t) cos ω_{c}t**

Substituting **m(t)**

**ϕ _{DSB-SC}(t) = A_{c }cos ω_{c}t + A_{m}cos ω_{m}t cos ω_{c}t**

**ϕ _{DSB-SC}(t) = A_{c }cos ω_{c}t + 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**as shown in the figure below.

_{c}Suppose the message signal’s spectrum is

The spectrum of 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**of the message spectrum is called

_{c }+ ω_{m })**upper**

**sideband**& lower half

**(ω**of the message spectrum is called

_{c }– ω_{m })**lower**

**sideband**. Because of these two sidebands, it is called

**double**

**sideband**

**AM transmission.**

**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_{c}t **

Substituting **m(t)**

**ϕ _{DSB-SC}(t) = (A_{c} + A_{m}cos ω_{m}t) cos ω_{c}t**

**ϕ _{DSB-SC}(t) = A_{c} (1 + A_{m}/A_{c }cos ω_{m}t) cos ω_{c}t**

**ϕ _{DSB-SC}(t) = A_{c} (1 + μ cos ω_{m}t) cos ω_{c}t**

where **μ= A _{m}/A_{c }**is the modulation index.

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

**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**

Demodulationis the process ofacquiringtheoriginal signal(message signal) from themodulated 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 _{c}t**

Then the modulated signal will be

**e(t) = m(t) cos ω _{c}t cos ω_{c}t**

** e(t) = ½ m(t) (1 + cos 2ω _{c}t)**

**e(t) = ½ m(t) +½ m(t) cos 2ω _{c}t**

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.

**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 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 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 receiver.

**Demodulation:**

**DSB-FC** or **traditional AM** can be demodulated by using coherent source or envelope detector. Envelop detector is 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 **same frequency carrier** with **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 modulation of two message signals. The carrier source produces carrier signal. The carrier signal with **0° phase shift** is used with **first message signal m _{1}(t)** and the carrier signal with

**90° phase shift**is used with

**second message signal m**. Both modulated signals are then added using

_{2}(t)**summer**to make a single signal having same frequency and bandwidth.

**Mathematical expression**

Suppose two message signals are **m _{1}(t)**,

**m**

_{2}(t)The carrier signal is

**c(t)= cos ω _{c}t.**

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

**c(t)= cos (ω _{c}+90°)t = sin ω_{c}t.**

So the modulated single will be

**Φ _{QAM}(t) = m_{1}(t) cos ω_{c}t + m_{2}(t) sin ω_{c}t**

**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 **m _{1}(t)**, the received signal is multiplied with

**cos ω**.

_{c}t**e(t)= (m _{1}(t) cos ω_{c}t + m_{2}(t) sin ω_{c}t) cos ω_{c}t**

**e(t)= m _{1}(t) cos^{2} ω_{c}t + m_{2}(t) sin ω_{c}t cos ω_{c}t**

**e(t)= ½ m _{1}(t) (1+cos 2ω_{c}t) + ½ m_{2}(t) sin 2ω_{c}t**

**e(t)= ½ m _{1}(t) +½ m_{1}(t) cos 2ω_{c}t) + ½ m_{2}(t) sin 2ω_{c}t**

By passing through **Low Pass Filter**

**e(t)= ½ m _{1}(t)**

To get **m _{2}(t)**, the received signal is multiplied with

**sin ω**

_{c}t.**e(t)= (m _{1}(t) cos ω_{c}t + m_{2}(t) sin ω_{c}t) sin ω_{c}t**

**e(t)= m _{1}(t) cos ω_{c}t sin ω_{c}t + m_{2}(t) sin^{2} ω_{c}t**

**e(t)= ½ m _{1}(t) sin 2ω_{c}t + ½ m_{2}(t) (1-cos 2ω_{c}t)**

**e(t)= ½ m _{1}(t) sin 2ω_{c}t + ½ m_{2}(t) – ½ m_{2}(t) cos 2ω_{c}t)**

By passing through **Low Pass Filter**

**e(t)= ½ m _{2}(t)**

**Advantages**

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

**Disadvantages**

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

**Single sideband (SSB)**

The type of **Amplitude modulation**, in which only single side band is transmitted thorough 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 very sharp cutoff. An ideal filter.

**Vestigial sideband (VSB)**

As we know that a real **bandpass filte**r does not have a **sharp cutoff** and it does not filter all the frequencies outside of 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 **DSB** modulated signal. The bandwidth of **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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