RFPIC12C509AF datasheet(38/104 Pages) MICROCHIP

38 / 104 page

rfPIC12C509AG/509AF

DS70031A-page 36

Preliminary

7.3.2

CRYSTAL OSCILLATOR FSK

OPERATION

The rfPIC12C509AF crystal oscillator can be config-

ured for FSK operation. Figure 7-3 shows an example

FSK circuit. Capacitors C1 and C2 achieve FSK modu-

lation by pulling the crystal. When DATAFSK = 1, FSK-

OUT is high-impedance effectively coupling only

capacitor C1 to the crystal and the resulting transmit

frequency equals fMAX.When DATAFSK = 0, FSKOUT

is grounded to VSSRF and will parallel capacitor C2 with

C1. The resulting transmit frequency will equal fMIN.

Selecting the appropriate values for C1 and C2 sets the

center frequency and frequency deviation. Capacitor

C1 sets fMAX and capacitors C1 and C2 in parallel set

fMIN. The graph in Figure 7-4 illustrates this relation-

ship. The transmit center frequency f

C is defined as:

The frequency deviation of the transmit frequency is

defined as:

TABLE 7-3:

TYPICAL TRANSMIT CENTER FREQUENCY AND FREQUENCY DEVIATION

(FSK MODE) (1)

FIGURE 7-3:

EXAMPLE FSK EXTERNAL

CRYSTAL CIRCUIT

FIGURE 7-4:

LOAD CAPACITANCE

VERSUS CHANGE IN

TRANSMITTED FREQUENCY

Layout considerations - Avoid parallel traces in order

to reduce circuit stray capacitance. Keep traces as

short as possible. Isolate components to prevent cou-

pling. Use ground traces to isolate signals.

min

max

min

max

−

∆

C2 = 1000 pF

C2 = 100 pF

C2 = 47 pF

C1 (pF)

Freq (MHz) / Dev (kHz)

433.612 / 34

433.619 / 27

433.625 / 21

433.604 / 25

433.610 / 19

433.614 / 14

433.598 / 20

433.604 / 14

433.608 / 10

433.596 / 17

433.601 / 11.5

433.604 / 8

433.593 / 13

433.598 / 9

433.600 / 5.5

100

433.587 / 8

—

Note 1: Standard Operating Conditions (unless otherwise stated) TA = 25°C, RFEN =1, VDDRF =3V,

fXTAL =13.55 MHz

XTAL

rfPIC12C509AF

FSKOUT

Frequency

(MHz)

Fmax

Fmin

C1||C2

DATAFSK =1 DATAFSK =0

Load Capacitance (pF)

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