Introduction

Every wireless message you receive — a radio broadcast, a mobile call, a television signal — begins as a low-frequency information signal that, on its own, simply cannot travel far or be radiated efficiently. This chapter of Karnataka 2nd PUC Electronics explains the process that makes long-distance electronic communication possible: modulation, the act of impressing a low-frequency message onto a high-frequency carrier wave for transmission.

We first look at why modulation is unavoidable (antenna size, operating range, wireless radiation, mixing, multiplexing and noise reduction), then study the two families you must know for the II PUC board exam in detail — amplitude modulation (AM) and frequency modulation (FM). For AM we derive the modulated-wave equation, its frequency spectrum, sidebands, bandwidth, modulation index and power relations, and see how AM is generated and transmitted. For FM we build its equation from frequency deviation and carrier swing, work out its modulation index, spectrum and bandwidth, and study varactor-diode and reactance modulators together with a complete FM transmitter. Worked numericals throughout show how these relations are applied in practice.