Silicon Laboratories SI4421 - Manual (Page 32)

Si4421
CRYSTAL SELECTION GUIDELINES
The crystal oscillator of the Si4421 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 to 16 pF in 0.5
pF steps. With appropriate PCB layout, the total load capacitance value can be 10 pF to 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
0
) value is expected for the
crystal, the oscillator is able to start up with any crystal having less than 100 ohms ESR (equivalent series loss resistance). However,
lower C
0
and ESR values guarantee faster oscillator startup.
The crystal frequency is used as the reference of the PLL, which generates the local oscillator frequency (f
LO
). Therefore, f
LO
is
directly proportional to the crystal frequency. 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 and C
0
.
Maximum XTAL Tolerances Including Temperature and Aging [ppm]
Bit Rate: 2.4 kbps
Deviation [± kHz]
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
Bit Rate: 9.6 kbps
Deviation [± kHz]
30 45 60 75 90 105 120
433 MHz 15 30 50 70 80 100 100
868 MHz 8 15 25 30 40 50 60
915 MHz 8 15 25 30 40 50 50
Bit Rate: 38.4 kbps
Deviation [± kHz]
30 45 60 75 90 105 120
433 MHz don't use 5 20 30 50 75 75
868 MHz don't use 3 10 20 25 30 40
915 MHz don't use 3 10 15 25 30 40
Bit Rate: 115.2 kbps
Deviation [± kHz]
105 120 135 150 165 180 195
433 MHz don't use 3 20 30 50 70 80
868 MHz don't use don't use 10 20 25 35 45
915 MHz don't use don't use 10 15 25 30 40
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