Crystal Oscillator Basics and Crystal Selection
for rfPICTM and PICmicro® Devices
Author: Steven Bible
Microchip Technology Inc.
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
= 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
• 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
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.
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
© 2002 Microchip Technology Inc.