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 (C0 ) 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 C0 and ESR values guarantee faster oscillator startup.
The crystal frequency is used as the reference of the PLL, which generates the local oscillator frequency (fLO). Therefore, fLO 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
Maximum XTAL Tolerances Including Temperature and Aging [ppm]
Bit Rate: 2.4 kbps
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| Deviation [± kHz] |
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| 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 |
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Bit Rate: 9.6 kbps
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| Deviation [± kHz] |
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| 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 |
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Bit Rate: 38.4 kbps
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| Deviation [± kHz] |
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| 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 |
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Bit Rate: 115.2 kbps
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| Deviation [± kHz] |
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| 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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