RFPIC12C509AG Datasheet, PDF (38/104 Pages) Microchip Technology – 18/20-Pin 8-Bit CMOS Microcontroller with UHF ASK/FSK Transmitter

rfPIC12C509AG/509AF
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:
f_c
=
f_max +
2
f _min
The frequency deviation of the transmit frequency is
defined as:
∆f = ^{fmax − fmin}
2
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.
TABLE 7-3: TYPICAL TRANSMIT CENTER FREQUENCY AND FREQUENCY DEVIATION
(FSK MODE) ⁽¹⁾
C2 = 1000 pF
C2 = 100 pF
C2 = 47 pF
C1 (pF)
Freq (MHz) / Dev (kHz) Freq (MHz) / Dev (kHz)
Freq (MHz) / Dev (kHz)
22
433.612 / 34
433.619 / 27
433.625 / 21
33
433.604 / 25
433.610 / 19
433.614 / 14
39
433.598 / 20
433.604 / 14
433.608 / 10
47
433.596 / 17
433.601 / 11.5
433.604 / 8
68
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
FIGURE 7-3: EXAMPLE FSK EXTERNAL
CRYSTAL CIRCUIT
FIGURE 7-4:
LOAD CAPACITANCE
VERSUS CHANGE IN
TRANSMITTED FREQUENCY
X1
C2
C1
XTAL
FSKOUT
rfPIC12C509AF
Fmax
Frequency
(MHz)
Fmin
C1 C1||C2
DATAFSK = 1 DATAFSK = 0
Load Capacitance (pF)
DS70031A-page 36
Preliminary
© 2001 Microchip Technology Inc.