Chapter Summary

This chapter completes the study of how a transmitted message is recovered and reproduced, and how the signal is guided and radiated.

• Demodulation (detection) is the reverse of modulation — recovering the baseband signal from the modulated carrier. It is necessary because a loudspeaker cannot follow a radio-frequency wave and the carrier lies above the audible range. A detector both rectifies the modulated wave and separates the audio from the carrier. AM detectors are of the square-law and envelope (diode) types; the diode/envelope detector, using a diode in its linear region with an R-C low-pass filter, is the most widely used. FM is demodulated by a discriminator (slope, Foster-Seeley, ratio, PLL or quadrature type) whose output voltage tracks the instantaneous frequency.
• A radio receiver intercepts, selects, amplifies, detects and reproduces the signal. Its quality is judged by sensitivity, selectivity, fidelity, signal-to-noise ratio and stability. The superheterodyne receiver converts every station to a fixed intermediate frequency (455 kHz for AM, 10.7 MHz for FM) by heterodyning with a local oscillator kept above the signal frequency, giving uniform selectivity and gain. Key ideas include AGC, the compromise choice of IF, the reason for keeping the oscillator above the signal, and the image frequency $f_{S2} = 2\,IF + f_{S1}$. The FM receiver adds a limiter, discriminator, de-emphasis network and AFC.
• A transmission line guides electrical energy between equipment and antenna. Its four primary constants are R, L, C and G per unit length; its secondary constants are the attenuation constant $\alpha$, phase-shift constant $\beta$ and propagation constant $\gamma$, with $\gamma = \alpha + j\beta$. …