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

OSCILLATOR BASICS

Reduced to its simplest components, the oscillator con-

sists of an amplifier and a filter operating in a positive

feedback loop (see Figure 1). The circuit must satisfy

the Barkhausen criteria in order to begin oscillation:

• the loop gain exceeds unity at the resonant fre-

quency, and

• phase shift around the loop is n2π radians (where

n is an integer)

The amplitude of the signal will grow once oscillation

has started. The amplitude of the signal must be limited

at some point and the loop gain equal unity. It is at this

point the oscillator enters steady-state operation.

FIGURE 1: SIMPLIFIED OSCILLATOR

BLOCK DIAGRAM

Looking at Figure 1, intuitively we see that the amplifier

provides the gain for the first criteria. For the second

criteria, phase shift, the amplifier is an inverting ampli-

fier which causes a π radian (180 degree) phase shift.

The filter block provides an additional π radian (180

degree) phase shift for a total of 2π radians (360

degrees) around the entire loop.

By design, the filter block inherently provides the phase

shift in addition to providing a coupling network to and

from the amplifier (see Figure 2). The filter block also

sets the frequency that the oscillator will operate. This

is done using a tuned circuit (inductor and capacitor) or

crystal. The coupling network provides light loading so

as to not overdrive the tuned circuit [2].

FIGURE 2:

SIMPLIFIED OSCILLATOR

BLOCK DIAGRAM WITH

COUPLING NETWORK

Oscillator Operation

Operation of an oscillator is generally broken up into

two phases: start-up and steady-state operation. An

oscillator must start by itself with no external stimulus.

When the power is first applied, voltage changes in the

bias network result in voltage changes in the filter net-

work. These voltage changes excite the natural fre-

quency of the filter network and signal buildup begins.

The signal developed in the filter network is small. Pos-

itive feedback and excess gain in the amplifier continu-

ously increases the signal until the non-linearity of the

amplifier limits the loop gain to unity. At this point the

oscillator enters steady-state operation. The time from

power on to steady-state operation is the oscillator

start-up time.

Steady-state operation of the oscillator is governed by

the amplifier and the tuned circuit of the filter block.

Loop gain steadies at unity due to the non-linearity of

the amplifier. The tuned circuit reactance will adjust

itself to match the Barkhausen phase requirement of 2π

radians. During steady-state operation, we are con-

cerned with the power output and loading of the tuned

circuit.

Amplifier

The amplifier circuit is typically implemented with a

bipolar junction transistor or field effect transistor

(JFET, MOSFET, etc.). Linear characteristics of the

transistor determine the starting conditions of the oscil-

lator. Non-linear characteristics determine an oscillator

operating point.

Tuned Circuits

The filter block sets the frequency that the oscillator will

operate. This is done using an LC tuned circuit (induc-

tor and capacitor) or crystal. Initially, we will look at a

few basic oscillator circuits that use a LC tuned circuit.

Later we will look at crystal basics and how crystal

oscillators operate.

Figure 3 shows a basic LC series resonator using an

inductor and capacitor. This is a simple band-pass filter

that at resonance the capacitive reactance and induc-

tive reactance are equal and cancel each other. There

is a zero phase shift and only the real resistance

remains.

FIGURE 3: BASIC LC SERIES RESONATOR

DS00826A-page 2

Since we are using an inverting amplifier, the filter block

needs to provide a π radian (180 degree) phase shift in

order to satisfy the second Barkhausen criteria. Figure

4 shows a four element shunt-C coupled LC series res-

onator that provides phase shift and a coupling network

[3].

© 2002 Microchip Technology Inc.

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