Communication Interview Questions and Answers (Topic Wise)

Amplitude Modulation Questions and Answers

Q1. Write the expression for AM wave.

Ans: The standard equation for amplitude modulated (AM) wave may be expressed as:

s(t)=x(t)\cos w_ct \;+\;A\cos w_ct or,

s(t)=[A+x(t)]\cos w_ct

Q2. What is the envelope of AM wave?

Ans: The expression for AM wave is:

s(t)=[A+x(t)]\cos w_ct or,

s(t)=E(t) \cos w_ct

• Where E(t)=[A+x(t)]. E(t) is called the envelope of AM wave. This envelope consists of the baseband signal x(t). Hence, the modulating or baseband signal may be recovered from an Am wave by detecting the envelope.

Q3. What is the bandwidth for AM waves?

Ans: The difference between these two extreme frequencies is equal to the bandwidth of AM wave. Therefore,

Bandwidth B=(w_c+w_m)-(w_c-w_m)\; or\; B=2w_m

Q4. Define modulation index for AM wave in AM system.

Ans: The modulation index is defined as the measure of the extent of amplitude variation about an unmodulated maximum carrier. It is represented by [atex]m_a[/latex].

• Mathematically,

Modulation indexm_a= \frac{\left | x(t)_{max} \right |}{Maximum\;carrie\;amplitude}

• where \left | x(t)_{max} \right | represents the maximum amplitude of modulating signal and A represents the maximum amplitude of the carrier signal.

• The modulation index is also known as the depth of modulation, degree of modulation, or modulation factor.

Q5. Define transmission efficiency in AM waves.

Ans: in AM wave, the amount of useful message power Ps may be expressed by a term known as transmission efficiency \eta.

Q6. What is demodulation?

Ans: The process of extracting a modulating or baseband signal from the modulated signal is called demodulation or detection. In other words, demodulation or detection is the process by which the message is recovered from the modulated signal at the receiver. The device used for demodulation or detection is called a demodulator or detector. For amplitude modulation, demodulators are categorized as:

1. Square law detector
2. Envelope Detector

Q7. What are the frequency components in an AM wave?

Ans. The AM signal has three frequency components as follows:

• With the help of these frequency components, we can plot the frequency spectrum of single-tone amplitude-modulated (AM) waves. Figure 1 shows the one-sided frequency spectrum of single-tone AM waves.

Q8. How can you obtain DSB-SC signal?

A DSB-SC signal is obtained by simply multiplying modulating signal x(t) with carrier signal cos ω_ct. This is achieved by a product modulator. The block diagram of a product modulator is shown in figure 2.

Q9. What are the demodulation methods for the DSB-SC signal?

Ans: The DSB-SC signal may be demodulated by following two methods:

1. synchronous detection method
2. Using envelope detector after carrier re-insertion.

Q10. Write a few properties of the Hilbert transform.

Ans: Following is the properties of the Hilbert transform:

• A signal x(t) and its Hilbert transform x_h(t) have the same energy density spectrum.
• A signal x(t) and its Hilbert transform x_h(t) have the same autocorrelation function.
• A signal x(t) and its Hilbert transform x_h(t) are mutually orthogonal.

Mathematically,

\int_{-\infty}^{\infty}x(t)x_h(t)dt=0

• If x_h(t) is a Hilbert transform of x(t), then the Hilbert transform of x_h(t) is -x(t).

Q11. What are the generating methods for the SSB-SC signal?

Ans: SSB-SC signals may be generated by two methods:

1. Frequency discrimination method or filter method
2. Phase discrimination method or phase shift method.

Q12. What do you mean by multiplexing?

Ans: Multiplexing is a technique in which several message signals are combined into a composite signal for transmission over a common channel. In order to transmit a number of these signals over the same channel, the signal must be kept apart so that they do not interfere with each other, and hence they can be separated easily at the receiver end.

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