GATE 2026 IN – Question 18
A square wave signal of frequency 20 kHz is passed through an ideal low-pass filter with cut-off frequency of 21 kHz. The output signal is a _______.
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Correct answer: (A) 20 kHz sine wave
Explanation
A symmetric square wave consists of a fundamental at 20 kHz and higher odd harmonics at 60, 100, … kHz. The ideal low-pass filter passes frequencies up to 21 kHz and removes the higher harmonics.
Only the fundamental remains, producing a **20 kHz sine wave (A)**. This uses the usual zero-mean square-wave convention.