Question

What is the equation for full-carrier AM?

(a) V(t) = (Ec + Em) × (sin⁡ ωc t)

(b) V(t) = (Ec + Em) × (sin⁡ ωm  t) + (sin⁡ ωc t)

(c) V(t) = (Ec × Em) × (sin ⁡ωm  t) × (sin⁡ ωc t)

(d) V(t) = (Ec + Em sin⁡ ωm  t) × (sin ⁡ωc t)

This question was posed to me during a job interview.

My question is taken from Demodulation of VSB Modulated Wave in chapter VSB Modulation of Analog Communications

Answer 1

Correct choice is (d) V(t) = (Ec + Em sin⁡ ωm  t) × (sin ⁡ωc t)

Easiest explanation: Amplitude modulation is the change in the amplitude of carrier wave in proportion to the instantaneous amplitude of the message signal. A carrier can be seen as a waveform with frequency higher than the message signal frequency,  that is modulated with respect to input signal for the purpose of transmitting information.  The equation for full-carrier AM is V(t) = (Ec + Em  sin⁡ωm  t) × (sin ⁡ωc  t).