Digital Modulation MCQs

Practice Digital Modulation MCQs with four options, answers and explanations for focused Electronics & Telecommunication Engineering revision.

Question 1

Communication SystemsDigital Modulation

In amplitude-shift keying (ASK), digital information is represented by changes in the carrier’s?

AAmplitude
BFrequency
CPhase only
DPropagation speed

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Correct Answer: A. Amplitude

Explanation: ASK represents digital symbols by changing the amplitude of a sinusoidal carrier.

Reference: Standard communication systems: digital modulation

Question 2

Communication SystemsDigital Modulation

In frequency-shift keying (FSK), binary symbols are commonly represented by?

ADifferent carrier frequencies
BDifferent antenna resistances
COnly different DC supply voltages
DDifferent sampling periods with no carrier

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Correct Answer: A. Different carrier frequencies

Explanation: FSK maps different digital symbols to different carrier frequencies.

Reference: Standard communication systems: digital modulation

Question 3

Communication SystemsDigital Modulation

In binary phase-shift keying (BPSK), the two binary symbols are represented by carrier phases separated by?

A45 degrees
B90 degrees
C180 degrees
D360 degrees

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Correct Answer: C. 180 degrees

Explanation: BPSK uses two antipodal carrier phases, conventionally separated by 180 degrees.

Reference: Standard communication systems: digital modulation

Question 4

Communication SystemsDigital Modulation

How many distinct carrier phase states are used in QPSK?

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Correct Answer: B. 4

Explanation: Quadrature phase-shift keying uses four phase states, allowing each symbol to represent two bits.

Reference: Standard communication systems: digital modulation

Question 5

Communication SystemsDigital Modulation

Each QPSK symbol carries how many bits?

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Correct Answer: B. 2

Explanation: Four possible symbols correspond to log2(4)=2 bits per symbol.

Reference: Standard communication systems: digital modulation

Question 6

Communication SystemsDigital Modulation

A QPSK system transmits at 20 ksymbols/s. Ignoring coding overhead, its bit rate is?

A10 kbps
B20 kbps
C40 kbps
D80 kbps

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Correct Answer: C. 40 kbps

Explanation: QPSK carries 2 bits per symbol, so Rb=20 ksymbols/s × 2 = 40 kbps.

Reference: Standard communication systems: digital modulation

Question 7

Communication SystemsDigital Modulation

A 16-QAM system operates at 100 kbaud. Its uncoded bit rate is?

A25 kbps
B100 kbps
C200 kbps
D400 kbps

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Correct Answer: D. 400 kbps

Explanation: 16-QAM carries 4 bits/symbol, so bit rate = 100 ksymbols/s × 4 = 400 kbps.

Reference: Standard communication systems: digital modulation

Question 8

Communication SystemsDigital Modulation

A 16-QAM constellation carries how many bits per symbol?

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Correct Answer: B. 4

Explanation: Bits per symbol = log2(16)=4.

Reference: Standard communication systems: digital modulation