An ideal low-pass filter is characterized by which of the following?

• A. A A constant gain with zero phase distortion in the pass band
• B. B A variable gain dependent on frequency
• C. C A sharp transition at high frequencies but undefined phase
• D. D A phase shift proportional to frequency (linear phase)

Answer: (A)

An ideal low-pass filter is all about passing frequencies in the passband without changing their amplitudes or their relative timing. That means the magnitude response in the passband is flat (constant gain) and the phase response does not distort the waveform (zero phase distortion). With constant gain, every frequency component that makes it through is transmitted equally; with no phase distortion, these components stay aligned in time, so the output is essentially the input waveform scaled and/or delayed but not distorted. That combination—unchanged amplitude across the passband and no phase-induced timing changes—best describes the ideal low-pass filter. The other descriptions imply altering amplitudes with frequency, or introducing phase behavior that would distort the waveform, which is not characteristic of an ideal passband.