RFPIC12F675K Datasheet, PDF (53/136 Pages) Microchip Technology – 20-Pin FLASH-Based 8-Bit CMOS Microcontroller with UHF ASK/FSK Transmitter

rfPIC12F675
9.5 FSK Modulation
In FSK modulation the transmit data is sent by varying
the output frequency. This is done by loading the
reference crystal with extra capacitance to pull it to a
slightly lower frequency which the PLL then tracks.
Switching the capacitance in and out with the data
signal toggles the transmitter between two frequencies.
These two crystal based frequencies are then
multiplied by 32 for the RF transmit frequency.
Unlike the ASK transmit frequency the FSK center
frequency is not actually transmitted. It is the artificial
point half way between the two transmitted
frequencies, calculated with this formula.
f_c =
f_max +
2
f _min
The other important parameter in FSK is the frequency
deviation of the transmit frequency. This measures how
far the frequency will swing from the center frequency.
Single ended deviation is calculated with this formula.
∆f = ^{fmax − fmin}
2
In FSK mode the DATAASK pin should be tied high to
enable the PA. The FSK circuit is shown in Figure 9-6.
Use accurate crystals for narrow bandwidth systems
and large values for C1 to reduce frequency drift.
FIGURE 9-3: FSK CRYSTAL CIRCUIT
X1
C2
C1
XTAL
FSKOUT
rfPIC12F675K/F/H
FIGURE 9-4: FREQUENCY PULLING
An FSK receiver will specify its optimal value of
deviation. The single ended deviation must be greater
than data rate/4. The minimum deviation is usually
limited by the frequency accuracy of the transmitter and
receiver components. The maximum deviation is usually
limited by the pulling characteristics of the transmitter
crystal.
An extra capacitor and the internal switch are added to
the ASK design to build an FSK transmitter as shown
in Figure 9-3. The C1 capacitor in series with the crystal
determines the maximum frequency.
With the DATAFSK pin high the FSKOUT pin is open and
the C2 capacitor does not affect the frequency. When
the DATAFSK pin goes low, FSKOUT shorts to ground,
and the C2 is thrown in parallel with C1. The sum of the
two caps pulls the oscillation frequency lower as shown
in Figure 9-4.
Fmax
Frequency
(MHz)
Fmin
C1 C1||C2
DATAFSK = 1 DATAFSK = 0
Load Capacitance (pF)
TABLE 9-3: TYPICAL TRANSMIT CENTER FREQUENCY AND 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, TA = 25°C, RFEN = 1, VDDRF = 3V, fXTAL = 13.55 MHz
 2003 Microchip Technology Inc.
Preliminary
DS70091A-page 51