PlacementPrepIn amplitude modulation, what varies with the message signal?
Solution
AM changes the carrier amplitude in step with the message.
Answer: (C) The amplitude of the carrier
Technical · Electronics and Communication (ECE)
Communication Systems MCQs for placement tests
20 technical questions of moderate difficulty, as asked in campus placement and company tests. Each has a short explanation.
For a tone-modulated AM wave, the maximum power efficiency (at 100% modulation) is:
Solution
Efficiency = μ²/(2 + μ²); with μ = 1, it is 1/3.
Answer: (D) 33.3%
The bandwidth of a DSB-SC AM signal with message bandwidth W is:
Solution
Both sidebands are transmitted, each W wide.
Answer: (A) 2W
Which modulation transmits only one sideband?
Solution
Single-sideband saves bandwidth and power.
Answer: (B) SSB
By Carson's rule, an FM signal with frequency deviation 75 kHz and message frequency 15 kHz needs a bandwidth of:
Solution
B = 2(Δf + fm) = 2(75 + 15) = 180 kHz.
Answer: (B) 180 kHz
FM is preferred over AM for music broadcasting mainly because it:
Solution
Noise mostly affects amplitude, which FM receivers ignore.
Answer: (A) Is less affected by noise
The intermediate frequency of a standard AM superheterodyne receiver is:
Solution
AM receivers use 455 kHz; FM receivers use 10.7 MHz.
Answer: (A) 455 kHz
Shannon's capacity formula is:
Solution
Capacity depends on bandwidth and signal-to-noise ratio.
Answer: (A) C = B log₂(1 + S/N)
In PCM, increasing the number of bits per sample:
Solution
Each extra bit improves the signal-to-quantisation-noise ratio by about 6 dB.
Answer: (B) Reduces quantisation noise
Which digital modulation varies the phase of the carrier?
Solution
Phase-shift keying represents data by phase changes.
Answer: (B) PSK
QPSK transmits how many bits per symbol?
Solution
Four phases represent two bits each.
Answer: (D) 2
An 8-level PCM system needs how many bits per sample?
Solution
2³ = 8 levels.
Answer: (B) 3
Delta modulation encodes:
Solution
It sends 1 bit per sample: up or down.
Answer: (A) The change from the previous sample using 1 bit
Slope overload in delta modulation occurs when:
Solution
Too small a step or too low a sampling rate for a steep signal causes slope overload.
Answer: (C) The signal changes faster than the step size can follow
Which multiplexing technique gives each user a different frequency band?
Solution
Frequency-division multiplexing separates users by frequency.
Answer: (B) FDM
In TDM, users share the channel by:
Solution
Time-division multiplexing gives each user a time slot.
Answer: (C) Taking turns in time slots
An antenna's gain is usually expressed in:
Solution
dBi is gain relative to an isotropic antenna.
Answer: (D) dBi
The wavelength of a 300 MHz signal in free space is:
Solution
λ = c/f = 3 × 10⁸ / 3 × 10⁸ = 1 m.
Answer: (B) 1 m
A signal-to-noise ratio of 1000 expressed in decibels is:
Solution
10 log₁₀(1000) = 30 dB.
Answer: (C) 30 dB
Which of these is a digital-to-digital line code with no DC component?
Solution
Manchester coding has a transition in every bit, so its average is zero.
Answer: (A) Manchester coding
Want guided preparation? Placement Papers Prep courses by Dr. Kiran Derle, with video lessons and full-length mocks.
See placement courses ↗