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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 100 Ω 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 (f LO ). Therefore, f LO 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)
Bit Rate: 2.4 kbps
Deviation [± kHz]
30
45
60 75
90
105
120
433 MHz
868 MHz
915 MHz
20
10
10
30
20
15
50 70
25 30
25 30
90
40
40
100
50
50
100
60
50
Bit Rate: 9.6 kbps
Deviation [± kHz]
30
45
60 75
90
105
120
433 MHz
868 MHz
915 MHz
15
8
8
30
15
15
50 70
25 30
25 30
80
40
40
100
50
50
100
60
50
Bit Rate: 38.4 kbps
Deviation [± kHz]
30
45
60 75
90
105
120
433 MHz
868 MHz
915 MHz
Do Not Use
Do Not Use
Do Not Use
5
3
3
20 30
10 20
10 15
50
25
25
75
30
30
75
40
40
Bit Rate: 115.2 kbps
Deviation [± kHz]
105
120
135 150
165
180
195
433 MHz
868 MHz
915 MHz
Do Not Use
Do Not Use
Do Not Use
3
Do Not Use
Do Not Use
20 30
10 20
10 15
50
25
25
70
35
30
80
45
40
? 2009-2011 Microchip Technology Inc.
Preliminary
DS70590C-page 49
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