RFHCS362G datasheet(28/54 Pages) MICROCHIP

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rfHCS362G/362F

DS41189B-page 28

6.3.2

CRYSTAL OSCILLATOR FSK

OPERATION

The rfHCS362F crystal oscillator can be configured for

FSK operation. Figure 6-2 shows an example FSK cir-

cuit. Capacitors C1 and C2 achieve FSK modulation by

pulling the crystal. When DATAFSK = 1, FSKOUT 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 6-3 illustrates this relation-

ship. The transmit center frequency f

C is defined as:

The frequency deviation of the transmit frequency is

defined as:

TABLE 6-3:

TYPICAL TRANSMIT CENTER FREQUENCY AND FREQUENCY DEVIATION

(FSK MODE) (1)

FIGURE 6-2:

EXAMPLE FSK EXTERNAL

CRYSTAL CIRCUIT

FIGURE 6-3:

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

rfHCS362F

FSKOUT

Frequency

(MHz)

Fmax

Fmin

C1||C2

DATAFSK = 1 DATAFSK = 0

Load Capacitance (pF)

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