Jawaharlal Nehru Technological University Hyderabad B.Tech III Year ANALOG COMMUNICATIONS Previous Question Paper (March - 2021 - R16)

A switching modulator generates an AM wave by using a semiconductor diode as a high-speed switch. The circuit consists of a diode in series with a load resistor, where the modulating signal m(t) and a high-frequency carrier signal c(t) = A cos(ωct) are applied in series.

The operation relies on the assumption that the carrier amplitude is much larger than the modulating signal, causing the diode to switch between ON and OFF states at the carrier frequency. The output voltage across the load is the product of the input signal and a periodic switching function s(t), which is a square wave with the same frequency as the carrier.

The switching function s(t) can be represented by a Fourier series:

The output signal v(t) is given by:

By passing this output through a bandpass filter centered at the carrier frequency ωc, the higher-order harmonics are suppressed, leaving only the desired AM wave:

This results in an amplitude-modulated signal where the envelope follows the message signal m(t).

The phasor diagram of an AM signal with noise represents the carrier phasor as a reference vector rotating at the carrier frequency. The sidebands are represented by two smaller phasors rotating at frequencies (ωc + ωm) and (ωc - ωm), which combine to form a resultant phasor that varies in length along the direction of the carrier.

When noise is introduced, it is represented as a random phasor added to the signal phasor. The noise phasor has a random magnitude and a random phase angle relative to the carrier. The total resultant phasor is the vector sum of the AM signal phasor and the noise phasor.

In the diagram, the carrier phasor is drawn along the horizontal axis. The sideband resultant is a vertical vector oscillating at the modulation frequency. The noise component is shown as a vector originating from the tip of the signal phasor, effectively causing both amplitude and phase fluctuations in the total received signal vector.