JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II ANALOG COMMUNICATIONS Previous Question Paper (July/August - 2021 - R16)

A ring modulator is a popular circuit for generating a DSB-SC signal. It consists of four diodes arranged in a ring configuration connected between two center-tapped transformers. The modulating signal m(t) is applied to the input transformer, and a high-frequency square wave carrier c(t) is applied to the other transformer.

During the positive half-cycle of the carrier, the diodes D1 and D2 are forward-biased, acting as short circuits, while D3 and D4 are reverse-biased. During the negative half-cycle, D3 and D4 are forward-biased, and D1 and D2 are reverse-biased. This effectively performs a switching operation, multiplying the message signal by a square wave carrier. The output is the product of the message signal and the square wave carrier, which contains the DSB-SC signal along with higher-order harmonics that can be removed using a bandpass filter.

The primary significance of DSB-SC modulation is power efficiency. In standard AM, a significant portion of the transmitted power is wasted in the carrier and the redundant sidebands. By suppressing the carrier, all transmitted power is concentrated in the sidebands, which contain the actual information, leading to better signal-to-noise ratio performance.

In the time domain, the DSB-SC signal is represented as:

This expression shows that the amplitude of the carrier is directly modulated by the message signal m(t). When m(t) is zero, the output is zero, meaning no power is transmitted in the absence of a modulating signal, which is a major advantage over conventional AM where the carrier remains present.