Si4020
CRYSTAL SELECTION GUIDELINES
The crystal oscillator of the Si4020 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 (C0) value is expected for the
crystal, the oscillator is able to start up with any crystal having less than 300 ohms ESR (equivalent series loss resistance). However,
lower C0 and ESR values guarantee faster oscillator startup.
The crystal frequency is used as the reference of the PLL, which generates the RF carrier frequency (fc). Therefore, fc 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 carrier frequency error.
Maximum XTAL Tolerances Including Temperature and Aging [ppm]
Bit Rate: 2.4kbps
Transmitter Deviation [+/- kHz]
30
60
90
120
150
180
210
315 MHz
30
20
10
10
75
50
25
25
100
75
100
100
60
100
100
75
100
100
100
75
100
100
100
100
433 MHz
868 MHz
915 MHz
40
40
50
75
Bit Rate: 9.6kbps
Transmitter Deviation [+/- kHz]
30
60
90
120
150
180
210
315 MHz
433 MHz
868 MHz
915 MHz
25
15
8
70
50
25
25
100
75
100
100
60
100
100
75
100
100
75
100
100
100
100
40
8
40
50
70
75
Bit Rate: 38.3kbps
Transmitter Deviation [+/- kHz]
30
60
90
120
150
180
210
315 MHz
433 MHz
868 MHz
915 MHz
don’t use
don't use
don't use
don't use
30
20
10
10
75
50
30
25
100
75
100
100
60
100
100
75
100
100
100
75
40
40
60
75
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 C0.
Note: There may be other requirements for the TX carrier accuracy with regards to the requirements as defined by standards and/or channel
separations.
23
SILICON [ SILICON ]
