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AN826

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

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internal capacitance, and stray capacitance (CS). The

product design engineer selects the values of capaci-

tors C2 and C3 to match the crystal CL using the below

equation:

CL

=

----C----2---C----3----

C2 + C3

+

CS

Stray capacitance can be assumed to be in the range

of 2 to 5 pF. PCB stray capacitance can be minimized

by keeping traces as short as possible. A desirable

characteristic of the Pierce oscillator is the effects of

stray reactances and biasing resistors appear across

the capacitors C2 and C3 in the circuit rather than the

crystal.

If the circuit load capacitance does not equal the crystal

CL, the operating frequency of the Pierce oscillator will

not be at the specified crystal frequency. For example,

if the crystal CL is kept constant and the values of C2

and C3 are increased, the operating frequency

approaches the crystal series resonant frequency (i.e,

the operating frequency of the oscillator decreases).

Care should be used in selecting values of C2 and C3.

Large values increase frequency stability but decrease

the loop gain and may cause oscillator start-up prob-

lems. Typically the values of C2 and C3 are equal. A

trimmer capacitor can be substituted for C2 or C3 in

order to manually tune the Pierce oscillator to the

desired frequency. Select capacitors with a low temper-

ature coefficient such as NP0 or C0G types.

Colpitts Crystal Oscillator

The Colpitts crystal oscillator (Figure 16) is a parallel

resonant circuit for Fundamental mode crystals [3]. The

Colpitts is designed to look into a high impedance

across the crystal terminals [6]. The series combination

of C2 and C3, in parallel with the effective transistor

input capacitance, form the crystal loading capacitance

[3]. The effects of stray reactances appear across the

crystal. The biasing resistors are also across the crys-

tal, which can degrade performance as mentioned in

the LC version.

FIGURE 16: COLPITTS CRYSTAL

OSCILLATOR

In the particular Colpitts configuration shown in Figure

16, the capacitive divider off the tuned circuit provides

the feedback as in a classic LC Colpitts. However, the

crystal grounds the gate at the series resonant fre-

© 2002 Microchip Technology Inc.

AN826

quency of the crystal, permitting the loop to have suffi-

cient gain to sustain oscillations at that frequency only

[4]. This configuration is useful because only one pin is

required to connect the external crystal to the device.

The other terminal of the crystal is grounded.

A trimmer capacitor can be placed in series with the

crystal to manually tune the Colpitts oscillator to the

desired frequency.

SPECIFYING A CRYSTAL

Now that we know how a crystal behaves in an oscilla-

tor circuit, let’s review the specification questions asked

by the crystal manufacturer:

What crystal frequency do you require?

This is the frequency stamped on the crystal package.

It is the desired operational crystal frequency for the cir-

cuit. It depends on the mode of operation (fundamental

or overtone, series or parallel resonant), and load

capacitance. Recall that parallel resonant crystals

operate at the specified frequency at the specified load

capacitance (CL) that you request.

Which mode of operation?

Fundamental or overtone. This Application Note

focused primarily on Fundamental mode since the

rfPIC and PICmicro MCU oscillators generally operate

below 30 MHz, which is the upper frequency limit of AT-

cut quartz crystals.

Series or parallel resonant?

This tells the crystal manufacturer how the crystal will

be used in the oscillator circuit. Series resonant crys-

tals are used in oscillator circuits that contain no reac-

tive components in the feedback loop. Parallel

resonant crystals are used in oscillator circuits that con-

tain reactive components. As mentioned, there is no

difference in the construction of a series or parallel res-

onant crystal.

For the Pierce and Colpitts oscillators reviewed in this

Application Note, the crystal is used at its parallel reso-

nant frequency. Therefore, a load capacitance must be

specified in order for the crystal to operate at the fre-

quency stamped on the package.

What frequency tolerance do you desire?

This is the allowable frequency deviation plus and

minus the specified crystal frequency. It is specified in

parts per million (PPM) at a specific temperature, usu-

ally +25 degrees C.

The designer must determine what frequency toler-

ance is required for the product design. For example, a

PICmicro device in a frequency insensitive application

the frequency tolerance could be 50 to 100 ppm. For a

rfPIC device, the crystal frequency is multiplied up to

the transmit frequency. Therefore, the tolerance will be

multiplied. The tolerance required depends on the radio

frequency regulations of the country the product will be

used. Tolerances of 30 ppm or better are generally

DS00826A-page 9

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