Chapter Overview

Many different circuits have been devised to produce FM and PM signals. There are two different types of frequency modulator circuits, direct circuits and circuits that produce FM indirectly by phase modulation techniques. Direct FM circuits make use of techniques for varying the frequency of the carrier oscillator in accordance with the modulating signal. Indirect modulators produce FM via a phase shifter after the carrier oscillator stage. Frequency demodulator or detector circuits convert the FM signal back to the original modulating signal.

Chapter Outline

6-1 Frequency Modulators
6-2 Phase Modulators
6-3 Frequency Demodulators

Chapter Objectives

• Compare and contrast FM using crystal oscillator circuits with FM using varactors.
• Explain the general principles of phase modulator circuits and list the basic techniques for achieving phase shift.
• Calculate the total frequency deviation of an FM transmitter given the original oscillator frequency and the frequency multiplication factor.
• Describe the operation of slope detectors, pulse-averaging discriminators, and quadrature detectors.
• Draw a block diagram of a phase-locked loop (PLL), state what each component does, explain the operation of the circuit, and define the capture range and the lock range of a PLL.
• Explain the operation of a PLL as a frequency demodulator.