RFPIC12F675 Datasheet, PDF (50/136 Pages) Microchip Technology – FLASH-Based Microcontroller with ASK/FSK Transmitter

rfPIC12F675
9.3 Crystal Oscillator
The transmitter crystal oscillator is a Colpitts oscillator
that provides the reference frequency to the PLL. It is
independent of the microcontroller oscillator. An
external crystal or AC coupled reference signal is
connected to the XTAL pin. The transmit frequency is
fixed and determined by the crystal frequency
according to the formula:
f _transmit  f
 32
RFXTAL
Due to the flexible selection of transmit frequency, the
resulting crystal frequency may not be a standard off-
the-shelf value. Therefore, for some carrier frequencies
the designer will have to consult a crystal manufacturer
and have a custom crystal manufactured. For
background information on crystal selection see
Application Note AN588, PIC^® Microcontroller Oscilla-
tor Design Guide, and AN826 Crystal Oscillator Basics
and Crystal Selection for rfPIC™ and PIC^® Devices.
For ASK modulation the crystal can be connected
directly from RFXTAL to ground, or in series with an
additional capacitor to trim the frequency. Figure 9-2
shows how the crystal is connected and Table 9-2
shows how the frequency of a typical crystal changes
with capacitance.
The oscillator is enabled when the RFEN input is high.
It takes the crystal approximately 1 ms to start oscillat-
ing. Higher frequency crystals start-up faster than
lower frequencies. The crystal oscillator start time
(TON) is listed in Table 13-11, Transmitter AC
Characteristics. This start-up time is mainly due to the
crystal building up an oscillation, but also includes the
time for the PLL to lock on the crystal frequency.
9.4 ASK Modulation
In ASK modulation the data is transmitted by varying
the output power. The DATAASK pin enables the PA,
toggling the pin turns the RF output signal on and off. A
simple receiver using a tuned filter and peak detector
diode can capture the data. A more advanced super-
heterodyne receiver such as the rfRXD0420 can
greatly increase the range and reduce susceptibility to
interference.
In ASK mode the DATAFSK and FSKOUT pins are not
used and should both be tied to ground. An example of
a typical ASK circuit is shown in Figure 9-5. The C1
capacitor can be replaced by a short to simplify the
transmitter if the receiver has a wide enough
bandwidth. For a very narrowband receiver the C1
capacitor may need to be replaced by a trimmer cap to
tune the transmitter to the exact frequency.
FIGURE 9-2: ASK CRYSTAL CIRCUIT
XTAL
X1
rfPIC12F675K/F/H
C1
TABLE 9-2: XTAL OSC APPROXIMATE FREQ. VS. CAPACITANCE (ASK MODE) ⁽¹⁾
C1
Predicted Frequency
(MHz)
PPM from 13.55 MHz
Transmit Frequency (MHz)
(32 * fXTAL)
22 pF
13.551438
+106
433.646
39 pF
13.550563
+42
433.618
100 pF
13.549844
-12
433.595
150 pF
13.549672
-24
433.5895
470 pF
13.549548
-33
433.5856
1000 pF
13.549344
-48
433.579
Note 1: Standard Operating Conditions (unless otherwise stated) TA = 25°C, RFEN = 1, VDDRF = 3V,
fXTAL = 13.55 MHz
DS70091B-page 50
Preliminary
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