AN826 データシートの表示（PDF） - Microchip Technology

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AN826

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

Microchip Technology 

AN826

unique in that the impedance of the crystal changes so

rapidly with frequency that all other circuit components

can be considered to be of constant reactance, this

reactance being calculated at the nominal frequency of

the crystal. The frequency of oscillation will adjust itself

so that the crystal presents a reactance to the circuit

which will satisfy the Barkhausen phase requirement

[5].

Figure 12 again shows a simplified oscillator circuit

drawn with only the RF components, no biasing resis-

tors, and no ground connection [3]. The inductor has

been replaced by a crystal. We shall see for the Pierce

and Colpitts crystal oscillators, the crystal will appear

inductive in the circuit in order to oscillate.

FIGURE 12: SIMPLIFIED CRYSTAL

OSCILLATOR CIRCUIT

WITHOUT RF GROUND

frequency of the crystal, it poses more or less phase

shift such that the total is not equal to 360 degrees.

Therefore, steady-state operation is maintained at the

crystal frequency. However, this only happens in an

ideal circuit.

FIGURE 14: PIERCE CRYSTAL

OSCILLATOR, IDEAL

OPERATION [6]

Pierce Crystal Oscillator

The Pierce crystal oscillator (Figure 13) is a series res-

onant circuit for Fundamental mode crystals. It oscil-

lates just above the series resonant frequency of the

crystal [3]. The Pierce oscillator is designed to look into

the lowest possible impedance across the crystal termi-

nals [6].

FIGURE 13: PIERCE CRYSTAL OSCILLATOR

In actual circuit operation (Figure 15), the phase shift

through the transistor is typically more than 180

degrees because of increased delay and the tuned cir-

cuit typically falls short of 180 degrees. Therefore the

crystal must appear inductive to provide the phase shift

needed in the circuit to sustain oscillation.

FIGURE 15: PIERCE CRYSTAL

OSCILLATOR, ACTUAL

OPERATION [6]

In the Pierce oscillator, the ground point location has a

profound effect on the performance. Large phase shifts

in RC networks and large shunt capacitances to ground

on both sides of the crystal make the oscillation fre-

quency relatively insensitive to small changes in series

resistances or shunt capacitances. In addition, RC roll-

off networks and shunt capacitances to ground mini-

mize any transient noise spikes which give the circuit a

high immunity to noise [6].

At series resonance, the crystal appears resistive in

the circuit (Figure 14) and the phase shift around the

circuit is 2π radians (360 degrees). If the frequency of

the circuit shifts above or below the series resonant

DS00826A-page 8

Thus the output frequency of the Pierce crystal oscilla-

tor is not at the crystal series resonant frequency. Typi-

cally a parallel resonant crystal is specified by

frequency and load capacitance (CL). CL is the circuit

capacitance the crystal expects to see and operate at

the desired frequency. The circuit load capacitance is

determined by external capacitors C2 and C3, transistor

© 2002 Microchip Technology Inc.

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