RFHCS362F Datasheet, PDF (31/60 Pages) Microchip Technology – Code Hopping Encoder with UHF ASK/FSK Transmitter

rfHCS362G/362F
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:
f_c =
f_max + f_min
2
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 6-3: 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 6-2: EXAMPLE FSK EXTERNAL
CRYSTAL CIRCUIT
FIGURE 6-3:
LOAD CAPACITANCE
VERSUS CHANGE IN
TRANSMITTED FREQUENCY
X1
C2
C1
XTAL
FSKOUT
rfHCS362F
Fmax
Frequency
(MHz)
Fmin
C1 C1||C2
DATAFSK = 1 DATAFSK = 0
Load Capacitance (pF)
© 2002 Microchip Technology Inc.
Preliminary
DS41189A-page 31