Datasheet

© 2009-2011 Microchip Technology Inc. Preliminary DS70590C-page 49
MRF49XA
3.6 Crystal Selection Guidelines
The crystal oscillator of MRF49XA requires a 10 MHz
Parallel mode crystal. The circuit contains an
integrated load capacitor in order to minimize the
external component count. The internal load
capacitance value is programmable from 8.5 pF –
16 pF in 0.5 pF steps. With appropriate PCB layout, the
total load capacitance value can be 10 pF – 20 pF, so a
variety of crystal types can be used.
When the total load capacitance is not more than 20 pF,
and a worst case 7 pF Shunt Capacitance (C
S) value is
expected for the crystal, the oscillator is able to start-up
with any crystal having less than 100Ω Equivalent
Series Loss Resistance (ESR). However, the low C
S
and ESR values ensure the faster oscillator start-up.
The Crystal Frequency (f
ref) is used as the reference of
the PLL, which generates the Local Oscillator
Frequency (fLO). Therefore, fLO is directly proportional to
f
ref. The accuracy requirements for production tolerance,
temperature drift and aging can thus be determined from
the maximum allowable local oscillator frequency error.
Whenever a low-frequency error is essential for the
application, it is possible to “pull” the crystal to the
accurate frequency by changing the load capacitor
value. The widest pulling range can be achieved if the
nominal required load capacitance of the crystal is in
the “midrange”; for example, 16 pF. The “pull-ability” of
the crystal is defined by its Motional Capacitance (C
m)
and shunt capacitance.
FIGURE 3-5: MAXIMUM CRYSTAL TOLERANCES INCLUDING TEMPERATURE AND AGING (ppm)
30 45 60 75 90 105 120
433 MHz 15 30 50 70 80 100 100
868 MHz 8 152530405060
915 MHz 8 152530405050
Deviation [± kHz]
Bit Rate: 9.6 kbps
30 45 60 75 90 105 120
433 MHz 20 30 50 70 90 100 100
868 MHz 10 20 25 30 40 50 60
915 MHz 10 15 25 30 40 50 50
Deviation [± kHz]
Bit Rate: 2.4 kbps
30 45 60 75 90 105 120
433 MHz Do Not Use 5 20 30 50 75 75
868 MHz Do Not Use 3 10 20 25 30 40
915 MHz Do Not Use 3 10 15 25 30 40
Deviation [± kHz]
Bit Rate: 38.4 kbps
105 120 135 150 165 180 195
433 MHz Do Not Use 3 20 30 50 70 80
868 MHz Do Not Use Do Not Use 10 20 25 35 45
915 MHz Do Not Use Do Not Use 10 15 25 30 40
Deviation [± kHz]
Bit Rate: 115.2 kbps