PlacementPrepA signal x(t) is periodic if:
Solution
A periodic signal repeats every T seconds.
Answer: (A) x(t) = x(t + T) for some T > 0
Technical · Electronics and Communication (ECE)
Signals and Systems MCQs for placement tests
20 technical questions of moderate difficulty, as asked in campus placement and company tests. Each has a short explanation.
A signal satisfying x(t) = x(−t) is called:
Solution
Even signals are symmetric about t = 0.
Answer: (C) Even
The Fourier transform of a rectangular pulse is a:
Solution
A rect in time gives a sinc in frequency.
Answer: (D) Sinc function
The Laplace transform of the unit step function u(t) is:
Solution
∫₀^∞ e−st dt = 1/s.
Answer: (A) 1/s
The Laplace transform of e−atu(t) is:
Solution
∫₀^∞ e−(s+at) dt = 1/(s + a).
Answer: (C) 1/(s + a)
By the Nyquist theorem, a signal with maximum frequency 4 kHz must be sampled at least at:
Solution
The minimum sampling rate is twice the highest frequency.
Answer: (D) 8 kHz
Sampling below the Nyquist rate causes:
Solution
High frequencies fold back and appear as lower frequencies.
Answer: (C) Aliasing
An LTI system is completely described by its:
Solution
The output is the convolution of the input with the impulse response.
Answer: (D) Impulse response
A system is causal if its output depends on:
Solution
Causal systems do not depend on future inputs.
Answer: (C) Present and past inputs only
For an LTI system to be BIBO stable, its impulse response must be:
Solution
∫|h(t)| dt < ∞ for continuous time, or Σ|h[n]| < ∞ for discrete time.
Answer: (B) Absolutely integrable (summable)
The Z-transform of the unit impulse δ[n] is:
Solution
Only n = 0 contributes: Σ δ[n] z−n = 1.
Answer: (D) 1
Convolution in the time domain corresponds to which operation in the frequency domain?
Solution
This is the convolution property of the Fourier transform.
Answer: (B) Multiplication
The fundamental period of x(t) = sin(4πt) is:
Solution
ω = 4π, so T = 2π/ω = 0.5 s.
Answer: (B) 0.5 s
The energy of a periodic signal over all time is:
Solution
Periodic signals have finite power but infinite energy.
Answer: (B) Infinite
A filter that passes frequencies below a cut-off and blocks those above is a:
Solution
Low frequencies pass; high frequencies are attenuated.
Answer: (D) Low-pass filter
The region of convergence of a Z-transform of a causal sequence is:
Solution
For right-sided (causal) sequences, the ROC is the exterior of a circle.
Answer: (B) Outside a circle (|z| > r)
Multiplying a signal by cos(ω₀t) shifts its spectrum by:
Solution
This is the modulation property.
Answer: (C) ±ω₀
The DFT of an N-point sequence has how many points?
Solution
The N-point DFT gives N frequency samples.
Answer: (A) N
The FFT reduces the computation of an N-point DFT from O(N²) to:
Solution
The FFT exploits symmetry and periodicity of the twiddle factors.
Answer: (D) O(N log N)
Parseval's theorem relates:
Solution
Total energy is the same computed in either domain.
Answer: (A) The energy in the time domain to the energy in the frequency domain
Want guided preparation? Placement Papers Prep courses by Dr. Kiran Derle, with video lessons and full-length mocks.
See placement courses ↗