Amplitude Modulation Questions and Answers

Answer: C

Statements 1 and 3 are correct, which is option (C). A linear multiplier followed by a low pass filter is a synchronous (product) detector, which recovers the message from an AM wave. A diode followed by a low pass filter is the familiar envelope detector. A high-pass filter after the multiplier (statement 2) would remove the low-frequency message and keep only the high-frequency products, and a band-stop filter (statement 4) would not select the message either. So (A) 1 only, (B) 3 only and (D) 4 only each leave out a valid method or include an invalid one.

Answer: C

An AM demodulator can be built with (i) and (iii), the third option. A linear multiplier, or product detector, followed by a low pass filter recovers the message, and a diode followed by a low pass filter is the envelope detector.

A multiplier followed by a high pass filter would remove the message, which lies at low frequency, and a multiplier followed by a band-stop filter would not pick out the baseband signal. So (ii) and (iv) are not correct, and the other options are incomplete.

Answer: C

The carrier frequency in decreasing order is the optical carrier of WDM (hundreds of terahertz), then the microwave carrier used by CDMA cellular systems (about a gigahertz), then the VHF carrier of FM (about 100 MHz) and finally the AM carrier (about one megahertz or less). That is 4, 3, 2, 1. The other orders place AM or FM above the optical or cellular carriers.

Answer: A

Amplitude modulation is the type that causes envelope distortion of a baseband signal, option (A). In AM the message is carried in the envelope of the carrier, and if the carrier is overmodulated or the detector is not matched, the envelope no longer follows the message.

In frequency modulation, phase modulation and angle modulation in general (options B, C and D) the message is carried in the frequency or phase while the amplitude of the carrier stays constant, so there is no message-carrying envelope to be distorted.