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  Microchip Technology Semiconductor Electronic Components Datasheet  

AN826 Datasheet

Crystal Oscillator Basics and Crystal Selection

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AN826 pdf
AN826
Crystal Oscillator Basics and Crystal Selection
for rfPICTM and PICmicro® Devices
Author: Steven Bible
Microchip Technology Inc.
INTRODUCTION
Oscillators are an im portant c omponent of radio f re-
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 ex ternal capacitors hav e bee n
left to the product design engineer. If the incorrect crys-
tal and external capacitors are selected, it can lead to a
product tha t does not op erate properly, fails prema-
turely, or will not operate over the intended temperature
range. F or product s uccess it is i mportant that th e
designer u nderstand h ow a n o scillator o perates i n
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 s everal standard v alues that c an be
selected from a catalog, distributor, or crystal manufac-
turer. The s econd step is to sa mple o r purchase th e
crystal and evaluate it in the product design.
However, in radio frequency (RF) circuitry, the selection
of the c rystal is no t as s imple. Fo r ex ample, if a
designer requires a transmit frequency (ftransmit) of 318
MHz fo r the rfPIC12C509AG, the crystal fre quency
(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 cus tom
crystal from a crystal manufacturer. When the designer
contacts the crystal manufacturer, he or she is asked a
series of c rystal specification q uestions t hat m ay b e
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 on ly exp lain th e ope ration 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 re quired fo r th e rfPI CTM or PI Cmicro®
device. In order to do this th e designer needs a cl ear
understanding of the interrelationship of the various cir-
cuits th at ma ke up an oscillator ci rcuit. Th e p roduct
design engineer should also consult with thecrystal man-
ufacturer about the needs of their product design.
OSCILLATOR MODELS
There are s everal m ethods to m odeling osc illator
behavior. One form is known as the one port view or
negative resistance model. It predicts the behavior of
the os cillator as a n ac tive n etwork g enerating a n
impedance equal to a negative real resistance so that
the equivalent parallel resistance seen by the intrinsic,
lossless tune d c ircuit i s in finite [1 ]. A se cond fo rm i s
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 m ore detailed explanation of
oscillator m odeling and operation are available in the
cited references.
© 2002 Microchip Technology Inc.
DS00826A-page 1
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  Microchip Technology Semiconductor Electronic Components Datasheet  

AN826 Datasheet

Crystal Oscillator Basics and Crystal Selection

No Preview Available !

AN826 pdf
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). Th e 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 th e 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 fil ter blo ck provides an add itional π rad ian (180
degree) pha se shift for a tot al of 2 π rad ians (36 0
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). Th e 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 o f an os cillator is generally bro ken up in to
two phases: st art-up and steady-state op eration. 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 v oltage c hanges excite t he natural f re-
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 l oop gain to unity. At th is point the
oscillator enters steady-state operation. The time from
power on to s teady-state op eration is th e oscillator
start-up time.
Steady-state operation of the oscillator is governed by
the am plifier an d th e tun ed c ircuit of t he fi lter blo ck.
Loop gain steadies at unity due to the non-linearity of
the am plifier. Th e tu ned cir cuit rea ctance w ill ad just
itself to match the Barkhausen phase requirement of 2π
radians. During ste ady-state ope ration, w e a re co n-
cerned with the power output and loading of the tuned
circuit.
Amplifier
The am plifier circuit is ty pically implemented with a
bipolar junction tra nsistor or f ield e ffect tra nsistor
(JFET, M OSFET, e tc.). Li near c haracteristics of th e
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 w e w ill loo k at cry stal b asics an d ho w c rystal
oscillators operate.
Figure 3 s hows a b asic 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 z ero ph ase sh ift a nd on ly t he r eal r esistance
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.


Part Number AN826
Description Crystal Oscillator Basics and Crystal Selection
Maker Microchip
Total Page 14 Pages
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