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Maxim > Design Support > Technical Documents > Tutorials > Automotive > APP 5265
Maxim > Design Support > Technical Documents > Tutorials > Communications Circuits > APP 5265
Maxim > Design Support > Technical Documents > Tutorials > General Engineering Topics > APP 5265
Keywords: crystal oscillator, load capacitance, negative resistance, ISM radio, ISM, crystal model, crystal
aging, drive level dependency, Colpitts Oscillator, Pierce Oscillator, oscillator startup time, three point
oscillator, crystal temperature stability
TUTORIAL 5265
Design a Crystal Oscillator to Match Your
Application
By: Theron Jones, Principal Member of the Technical Staff
Sep 18, 2012
Abstract: Quartz crystals are mechanical resonators with piezoelectric properties. The piezoelectric
properties (electric potential across the crystal is proportional to mechanical deformation) allow their use
as electrical circuit elements. Crystals are widely used as resonant elements in oscillators due to their
high quality factor (QF), excellent frequency stability, tight tolerance, and relatively low cost. This tutorial
explains the primary design considerations to be addressed in a design of a simple crystal oscillator
using AT-cut crystals. The basic qualities of a crystal oscillator and factors that can affect their
performance in a variety of applications are described. The topics discussed here are the compilation of
issues encountered over a decade of design and applications for ISM-band radios. These topics include
load capacitance, negative resistance, startup time, frequency stability versus temperature, drive-level
dependency, crystal aging, frequency error, and spurious modes.
A similar version of this article appears on Electronic Design,
September 07, 2012.
Basics of a Crystal Model
Quartz crystals are modeled electrically as a series RLC branch in
parallel with a shunt capacitance (Figure 1). The series RLC
branch, often called the motional arm, models the piezoelectric
coupling to the mechanical quartz resonator. The shunt capacitance
represents the physical capacitance formed by both the parallel
plate capacitance of the electrode metallization and the stray package capacitance.
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