Bandpass Modulation MCQ || Bandpass Modulation Questions and Answers

11. Average information content per symbol in a group of symbols is known as ______

  1. Entropy
  2. Isotropy
  3. Modulation
  4. None of the above

Answer.1. Entropy

Explanation:-

Average information content per symbol in a group of symbols is known as entropy and represented by H.

Entropy is the measurement of the uncertainty of a random variable where what is specifically measured is what information is missing.

Therefore the relationship between them is inverse. More information creates a lower measure of entropy. Less information creates a higher measure of entropy.

 

12. PSK and FSK are which type of modulation?

  1. Analog Modulation
  2. Digital Modulation
  3. Both Analog Modulation & Digital Modulation
  4. None of these

Answer.2. Digital Modulation

Explanation:-

AM, FM and PM are analog modulation techniques whereas ASK, PSK and FSK are digital modulation techniques.

 

13. The real part of a sinusoid carrier wave is called as

  1. In phase
  2. Quadrature
  3. In phase & Quadrature
  4. None of the mentioned

Answer.1. In phase

Explanation:-

The real part of a sinusoid carrier wave is also called the I, or in-phase, component, and the imaginary part is called the Q, or quadrature, component, where in-phase stands for 0° phase angle to a reference carrier and quadrature stands for 90° phase angle.

 

14. Which of the following digital modulation can be decoded non-coherently?

  1. QAM
  2. APSK
  3. BPSK
  4. BFSK

Answer.4. BFSK

Explanation:-

Coherent detection: In it, the receiver exploits the exact knowledge of the phase of the carrier to detect the signal better.

Non-coherent detection: It involves making some approximations to the phase information that results in a loss in performance. However, it simplifies the circuitry.

BFSK digital modulation can be decoded non-coherently. Binary Frequency Shift Keying (BFSK) is a type of digital modulation technique in which we are sending one bit per symbol i.e., ‘0’ or a ‘1’. Hence, the bit rate and symbol rate are the same. In BFSK, the information is encoded in the variation of the frequency of the carrier.

There are different methods for demodulating an FSK wave. The main methods of demodulating BFSK signals are:

1) Asynchronous detector

2) Synchronous (Coherent) detector.

 

15. Which one of the following digital modulation schemes has the bit error rate as $\dfrac{1}{2}\rm{erfc} \left( \sqrt{\dfrac{E_b}{N_0}} \right)$ ?

  1. coherent binary PSK
  2. Coherent binary FSK
  3. DPSK
  4. Noncoherent binary FSK

Answer.1. coherent binary PSK

Explanation:-

BPSK

S1(t)= Ac cos (2πfc t)

S2(t)= -Ac cos (2πfc t)

Bit error rate (BER) for BPSK given as

$(BER)_{BPSK}= Q\sqrt \frac{A_C^2\times T_b}{N_o }$

noise power =No/2

bit energy(E=Ac2Tb/2

$(BER)_{BPSK}= Q\sqrt \frac{2\times E_b}{N_o }$

Relation between Q(x) and erfc are

Q(x)=1/2 erfc (x/√ 2)

$(BER)_{BPSK}=Q\sqrt { (2E_b/N_o)}=1/2× erfc(\sqrt {(E_b/N_o)}$

 

16. Antipodal signal sets are those vectors that can be illustrated as

  1. Two 180 opposing vector
  2. Two 90 opposing vector
  3. Two 360 opposing vector
  4. None of the mentioned

Answer.1. Two 180 opposing vector

Explanation:-

Antipodal signals are ‘mirror images, or that one signal is the negative of the other, or that the signals are 180° apart. Antipodal signal sets are those vectors that can be illustrated as Two 180 opposing vectors.

 

17. If the desired transmission bit rate for a coherent binary FSK system is 4 Kbits / sec, the best possible interval between the carriers is

  1. 0.25 mS
  2. 0.5 mS
  3. 1 mS
  4. 5 mS

Answer.2. 0.5 mS

Explanation:-

Given:

Rb = 4 kbps

In FSK, the best possible interval between the carriers is given as:

ΔT = 2/Rb

ΔT = 2/4kbps

ΔT = 0.5 msec

 

18. A 70 MHz carrier is QPSK modulated by a 1.544 Mbps T1 data stream. The transmitter employs a raised-cosine filter with α = 0.2. What is the transmitted bandwidth of the signal?

  1. 3705.6 kHz
  2. 1852.8 kHz
  3. 308.8 kHz
  4. 926.4 kHz

Answer.4. 926.4 kHz

Explanation:-

The bandwidth for a raised cosine pulse with a roll-ff factor of α, is given by:

Bandwidth = $\frac{{{R_b}}}{2}\left( {1\; + \;α } \right)$ —-(1)

Rb = Bita/Data rate

Calculation:

Given:

α = 0.2, fc = 70 MHz, Rb = 1.544 Mbps

From equation (1);

$BW=\frac{{{1.544 \ Mbps}}}{2}\left( {1\; + 0.2 } \right)$

BW = 0.9264 Mbps

BW = 926.4 kHz

 

19. The FSK signal which has a gentle shift from one frequency level to another is called as

  1. Differential PSK
  2. Continuous PSK
  3. Differential & Continuous PSK
  4. None of the mentioned

Answer.2. Continuous PSK

Explanation:-

Continuous phase modulation (CPM) is a method for the modulation of data commonly used in wireless modems. Furthermore, CPM is typically implemented as a constant-envelope waveform, i.e., the transmitted carrier power is constant.

In general FSK the signal change from one frequency to another will be abrupt but in continuous FSK the signal change from one frequency to another will be gentle and gradual.

 

20. In which modulation technique does the phase of the carrier signal is changed by varying the sine and cosine inputs at a particular time.

  1. Frequency modulation
  2. Phase shift key modulation
  3. Analog modulation
  4. Pulse code modulation

Answer.2. Phase shift key modulation

Explanation:-

In PSK (phase shift keying), binary 1 is represented with a carrier signal, and binary 0 is represented with 180° phase shift of a carrier, i.e. the phase of the carrier signal is changed by varying the sine and cosine inputs at a particular time.

For binary ‘1’ → S1 ( = Acos 2π fCt

For binary ‘0’ → S2 (t) = A cos (2πfCt + 180°) = – A cos 2π fCt

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