Some Characteristics and Design Notes for Crystal Feedback Oscillators

Overview

This technical document provides engineering guidance for designing reliable clock circuits for NXP (formerly Motorola) communications processors, including the MPC860, MC68360, and MC68302. It compares three methods for clock generation: hybrid crystal oscillators, external clock sources, and self-oscillating crystal circuits. The text focuses on the physics of self-oscillating circuits, detailing the amplifier's linear zone biasing, crystal resonance characteristics (series vs. parallel), and the Barkhausen criteria for oscillation. It also addresses practical design challenges such as PCB layout optimization to avoid noise, component selection for frequency trimming, and the impact of semiconductor manufacturing shrinks on oscillator stability and output impedance.

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