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AN826

Crystal Oscillator Basics and Crystal Selection for rfPICTM and PICmicro® Devices

Microchip Technology 

AN826

Crystal Oscillator Basics and Crystal Selection

for rfPICTM and PICmicro® Devices

Author: Steven Bible

Microchip Technology Inc.

INTRODUCTION

Oscillators are an important component of radio fre-

quency (RF) and digital devices. Today, product design

engineers often do not find themselves designing oscil-

lators because the oscillator circuitry is provided on the

device. However, the circuitry is not complete. Selec-

tion of the crystal and external capacitors have been

left to the product design engineer. If the incorrect crys-

tal and external capacitors are selected, it can lead to a

product that does not operate properly, fails prema-

turely, or will not operate over the intended temperature

range. For product success it is important that the

designer understand how an oscillator operates in

order to select the correct crystal.

Selection of a crystal appears deceivingly simple. Take

for example the case of a microcontroller. The first step

is to determine the frequency of operation which is typ-

ically one of several standard values that can be

selected from a catalog, distributor, or crystal manufac-

turer. The second step is to sample or purchase the

crystal and evaluate it in the product design.

However, in radio frequency (RF) circuitry, the selection

of the crystal is not as simple. For example, if a

designer requires a transmit frequency (ftransmit) of 318

MHz for the rfPIC12C509AG, the crystal frequency

(fxtal) will equal:

fxtal = f---t-r--a---n---s--m----i-t

32

= 3----1---8---,-0---0---0----,-0---0---0--

32

= 9,937,500 Hz

The frequency 9.9375 MHz is not a standard crystal fre-

quency. Therefore, the designer must order a custom

crystal from a crystal manufacturer. When the designer

contacts the crystal manufacturer, he or she is asked a

series of crystal specification questions that may be

unfamiliar, such as:

• What crystal frequency do you require?

• Which mode of operation?

• Series or parallel resonant?

• What frequency tolerance do you desire?

• What temperature stability is needed?

• What temperature range will be required?

• Which enclosure (holder) do you desire?

• What load capacitance (CL) do you require?

• What shunt capacitance (C0) do you require?

• Is pullability required?

• What motional capacitance (C1) do you require?

• What Equivalent Series Resistance (ESR) is

required?

• What drive level is required?

To the uninitiated, these are overwhelming questions.

What effect do these specifications have on the opera-

tion of the oscillator? What do they mean? It becomes

apparent to the product design engineer that the only

way to answer these questions is to understand how an

oscillator works.

This Application Note will not make you into an oscilla-

tor designer. It will only explain the operation of an

oscillator in simplified terms in an effort to convey the

concepts that make an oscillator work.

The goal of this Application Note is to assist the product

design engineer in selecting the correct crystal and exter-

nal capacitors required for the rfPICTM or PICmicro®

device. In order to do this the designer needs a clear

understanding of the interrelationship of the various cir-

cuits that make up an oscillator circuit. The product

design engineer should also consult with the crystal man-

ufacturer about the needs of their product design.

OSCILLATOR MODELS

There are several methods to modeling oscillator

behavior. One form is known as the one port view or

negative resistance model. It predicts the behavior of

the oscillator as an active network generating an

impedance equal to a negative real resistance so that

the equivalent parallel resistance seen by the intrinsic,

lossless tuned circuit is infinite [1]. A second form is

known as the two port view or feedback model consist-

ing of an amplifier with gain G and a frequency selec-

tive filter element with a linear transfer function in the

positive feedback path. This Application Note will use

simplified forms of each view to explain the basic oper-

ations of an oscillator. A more detailed explanation of

oscillator modeling and operation are available in the

cited references.

© 2002 Microchip Technology Inc.

DS00826A-page 1

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