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Analog communication is the transmission of information using continuously varying signals. The message, such as speech or music, modulates a high-frequency carrier by changing its amplitude, frequency or phase. Examples include AM and FM radio. The subject covers modulation, transmitters, receivers, bandwidth, power and the effects of noise.

Modulation shifts a low-frequency message to a high carrier frequency. This allows practical antenna sizes, lets many stations share the spectrum through frequency division, improves range and, in FM, can trade bandwidth for better noise performance. Without modulation, antennas for audio signals would need to be kilometres long.

The AM modulation index is the ratio of the message amplitude to the carrier amplitude, μ = Am ÷ Ac. From an oscilloscope trace it can be found as (Vmax − Vmin) ÷ (Vmax + Vmin). It should stay at or below 1, because a value above 1 causes over-modulation and distortion of the envelope.

When the modulation index is greater than 1, the envelope crosses zero and the carrier is cut off during part of each cycle. The envelope no longer follows the message, so an envelope detector produces distorted output, and extra frequency components spread into neighbouring channels, causing interference.

The maximum transmission efficiency of standard AM is 33.3%, reached at a modulation index of 1. Efficiency is μ² ÷ (2 + μ²), so at μ = 1 it equals one third. Most transmitted power stays in the carrier, which carries no information, which is why DSB-SC and SSB are more power-efficient.

AM varies the carrier's amplitude with the message, while FM varies its frequency and keeps the amplitude constant. AM uses less bandwidth and simpler receivers but is more affected by noise. FM uses more bandwidth but gives better noise performance and sound quality, which is why it is used for music broadcasting.

Carson's rule estimates the bandwidth of an FM signal as B ≈ 2(Δf + fm), where Δf is the peak frequency deviation and fm is the highest message frequency. It covers about 98% of the signal power. For broadcast FM with 75 kHz deviation and 15 kHz audio, it gives a bandwidth of about 180 kHz.

Narrowband FM has a modulation index well below 1, so its bandwidth is close to 2fm, similar to AM, and it is used in two-way radio. Wideband FM has a large modulation index, needs much more bandwidth and gives better noise performance, which suits broadcast radio. The boundary is usually taken around β = 1.

A superheterodyne receiver mixes the incoming signal with a local oscillator to produce a fixed intermediate frequency, commonly 455 kHz for AM and 10.7 MHz for FM. Most amplification and filtering then happen at this fixed frequency, which gives good selectivity and sensitivity across the whole tuning range.

The image frequency is an unwanted frequency that also produces the intermediate frequency after mixing in a superheterodyne receiver. When the local oscillator is above the signal, it equals the signal frequency plus twice the IF. The RF stage must filter it out, and image rejection improves with a higher intermediate frequency.

Noise figure measures how much a device or receiver degrades the signal-to-noise ratio. It is the ratio of input SNR to output SNR, usually expressed in decibels. A perfect noiseless device has a noise figure of 0 dB. In a chain of stages, the Friis formula shows the first stage has the biggest effect.

Single sideband modulation transmits only one sideband, upper or lower, and suppresses both the carrier and the other sideband. It halves the bandwidth compared with AM and saves power, making it popular for long-distance HF and amateur radio. It needs a more complex coherent receiver to recover the message.

FM signals are demodulated by circuits that convert frequency changes into amplitude changes. Common methods include the slope detector, the Foster–Seeley discriminator, the ratio detector, the quadrature detector and the phase-locked loop. Modern receivers often use PLL or digital methods because they are linear and reliable.

Yes. An electronics tutor explains the theory behind each formula, works through AM, FM, receiver and noise numericals with you, and shows how to draw spectra and waveforms. You solve the problems yourself during the session, so you can handle university exams, GATE questions and lab reports independently.