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AN826

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

Microchip Technology 

AN826

required. Care should be taken in selecting low toler-

ance values as the price of the crystal will increase. The

product design engineer should select the crystal fre-

quency tolerance that meets the radio frequency regu-

lations at the price point desired for the product.

What temperature stability is needed?

This is the allowable frequency plus and minus devia-

tion over a specified temperature range. It is specified

in parts per million (PPM) referenced to the measured

frequency at +25 degrees C.

Temperature stability depends on the application of the

product. If a wide temperature stability is required, it

should be communicated to the crystal manufacturer.

What temperature range will be required?

Temperature range refers to the operating temperature

range. Do not confuse this with temperature stability.

Which enclosure (holder) do you desire?

There are many crystal enclosures to choose from. You

can select a surface mount or leaded enclosure. Con-

sult with the crystal manufacturer about your product

needs. Bear in mind that the smaller the enclosure, the

higher the cost. Also, the smaller the enclosure the

higher the series resistance. Series resistance

becomes an issue because it lowers the loop gain of

the oscillator. This can result in oscillator not starting or

stopping over a wide temperature range.

What load capacitance (CL) do you require?

This is the capacitance the crystal will see in the circuit

and operate at the specified frequency. Load capaci-

tance is required for parallel resonant crystals. It is not

specified for series resonant crystals.

What shunt capacitance (C0) do you require?

Shunt capacitance contributes to the oscillator circuit

capacitance. Therefore, it has to be taken into account

for circuit operation (starting and steady-state) and pul-

lability.

Is pullability required?

Pullability refers to the change in frequency in the area

of usual parallel resonance. It is important if the crys-

tal is going to be tuned (pulled) over a specific but nar-

row frequency range. The amount of pullability

exhibited by a crystal at the specified load capacitance

(CL) is a function of shunt capacitance (C0) and

motional capacitance (C1).

This specification is important for the rfPIC12C509AF

device in FSK mode. The crystal is pulled between two

operating frequencies by switching capacitance in and

out of the oscillator circuit. If pullability is not specified,

there will be a hazard of tuning the crystal out of its

operating range of frequencies.

What motional capacitance (C1) do you require?

Motional capacitance is required if the crystal is going

to be tuned (pulled) in the circuit.

It is interesting to note that motional inductance (L1) is

normally not specified. Instead it is inferred from the

crystal’s series resonant frequency (fa) and motional

capacitance (C1). Simply plug in the values into the

crystal series resonant frequency and solve for L1.

What Equivalent Series Resistance (ESR) is required?

Typically specified as a maximum resistance in ohms.

Recall this is the resistance the crystal exhibits at its

operating frequency. Do not confuse ESR with motional

resistance (R0). A lower ESR requires a lower drive

level and vice versa. A danger exists in specifying too

high an ESR where the oscillator will not operate.

What drive level is required?

The quartz crystal is driven by the oscillator amplifier

and will dissipate heat. Drive level is the amount of

power the quartz crystal will have to dissipate in the

oscillator circuit. It is specified in milli- or microwatts.

The quartz crystal can only stand a finite amount of cur-

rent drive. The product design engineer must ensure

the quartz crystal is not overdriven or failure of the crys-

tal will result.

Drive level should be maintained at the minimum levels

necessary for oscillator start-up and maintain steady-

state operation. The design engineer should specify

the drive level required by the device and ensure that

crystal is not overdriven by measuring the current flow

in the oscillator loop. Make certain that the current drive

does not exceed the drive level specified by the crystal

manufacturer.

PRODUCT TESTING

Once the crystal has been specified and samples

obtain, product testing can begin. The final product

should be tested at applicable temperature and voltage

ranges. Ensure the oscillator starts and maintains oscil-

lation. Include in the evaluation component and manu-

facturing variations.

SUMMARY

There is much to learn about crystals and crystal oscil-

lators, however, this Application Note can only cover

the basics of crystals and crystal oscillators in an effort

to assist the product design engineer in selecting a

crystal for their rfPICTM or PICmicro® based device.

The reader is encouraged to study more in-depth about

the design and operation of crystal oscillators because

they are such an important component in electronic

designs today. Additional reading material is listed in

the further reading and references sections of this

Application Note. The product design engineer should

also consult with the crystal manufacturer about their

product design needs.

DS00826A-page 10

© 2002 Microchip Technology Inc.

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