In the block diagram of a simple modulator, a square law device generates DC and sinusoidal of frequencies \( \omega_m, 2\omega_m, \omega_c, 2\omega_c, \omega_c - \omega_m \) and \( \omega_c + \omega_m \). The device which rejects dc and unwanted frequencies and retains frequencies, \( \omega_c, \omega_c - \omega_m \) and \( \omega_c + \omega_m \) is called
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Band pass filter
In a simple amplitude modulator, the message signal and the carrier are added and passed through a square law device, which produces DC and the frequencies \( \omega_m, 2\omega_m, \omega_c, 2\omega_c, \omega_c \pm \omega_m \).
The AM wave needs only the carrier \( \omega_c \) and the two side bands \( \omega_c \pm \omega_m \), which lie in a narrow band around \( \omega_c \). A circuit that passes only a chosen band of frequencies and rejects everything below and above it is a band pass filter.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic was removed from the latest NCERT book (2026-27), and it also appears to be out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Production of Amplitude Modulated Wave – “… As shown in Fig. 15.10 this signal is passed through a band pass filter which rejects dc and the sinusoids of frequencies \( \omega_m \), \( 2\omega_m \) and \( 2\omega_c \) and retains the frequencies \( \omega_c \), \( \omega_c - \omega_m \) and \( \omega_c + \omega_m \).”
Production of Amplitude Modulated Wave – “… A band pass filter rejects low and high frequencies and allows a band of frequencies to pass through.”