si4420 Silicon Laboratories, si4420 Datasheet - Page 25

• Manufacturer Part Number: si4420
• Description: Si4420 Universal Ism Band Fsk Transceiver
• Manufacturer: Silicon Laboratories

CRYSTAL SELECTION GUIDELINES
The crystal oscillator of the Si4420 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
oscillator is able to start up with any crystal having less than 300 ohms ESR (equivalent series loss resistance). However, lower C
values guarantee faster oscillator startup.
The crystal frequency is used as the reference of the PLL, which generates the local oscillator frequency (f
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
Maximum XTAL Tolerances Including Temperature and Aging [ppm]
Bit Rate: 2.4kbps
Bit Rate: 9.6kbps
Bit Rate: 38.3kbps
315 MHz
433 MHz
868 MHz
915 MHz
315 MHz
433 MHz
868 MHz
915 MHz
315 MHz
433 MHz
868 MHz
915 MHz
don't use
don't use
don't use
don't use
30
25
20
10
10
30
20
15
30
8
8
45
50
30
20
15
45
50
30
15
15
45
7
5
3
3
60
75
50
25
25
60
70
50
25
25
60
30
20
10
10
Deviation [+/- kHz]
Deviation [+/- kHz]
Deviation [+/- kHz]
100
75
70
30
30
75
75
70
30
30
75
50
30
20
15
0
.
100
100
90
90
40
40
90
80
40
40
90
75
50
25
25
0
) value is expected for the crystal, the
LO
105
100
100
105
100
100
105
100
). Therefore f
50
50
50
50
75
30
30
LO
Si4420
120
100
100
120
100
100
120
100
0
is directly
60
50
60
50
75
40
40
and ESR
25