Single-Chip Low Cost Low Power RF-Transceiver (Rev. A)

CC25003530252015Selectivity [dB]1050-3 -2 -1 0 1 2 3-5-10-15-20Frequency offset [MHz]Figure 26: Typical selectivity at 500 kbps. IF frequency is 307.4 kHz.MDMCFG2.DEM_DCFILT_OFF = 026 Crystal OscillatorA crystal in the frequency range 26-27 MHzmust be connected between the XOSC_Q1and XOSC_Q2 pins. The oscillator is designedfor parallel mode operation of the crystal. Inaddition, loading capacitors (C81 and C101)for the crystal are required. The loadingcapacitor values depend on the total loadcapacitance, C L , specified for the crystal. Thetotal load capacitance seen between thecrystal terminals should equal C L for thecrystal to oscillate at the specified frequency.1C = +1 1+C CLC parasitic81The parasitic capacitance is constituted by pininput capacitance and PCB stray capacitance.Total parasitic capacitance is typically 2.5 pF.101The crystal oscillator circuit is shown in Figure27. Typical component values for differentvalues of C L are given in Table 32.The crystal oscillator is amplitude regulated.This means that a high current is used to startup the oscillations. When the amplitude buildsup, the current is reduced to what is necessaryto maintain approximately 0.4 Vpp signalswing. This ensures a fast start-up, and keepsthe drive level to a minimum. The ESR of thecrystal should be within the specification inorder to ensure a reliable start-up (see Section4.4 on page 10).XOSC_Q1C81XTALXOSC_Q2C101Figure 27: Crystal oscillator circuitComponent C L = 10 pF C L =13 pF C L =16 pFC81 15 pF 22 pF 27 pFC101 15 pF 22 pF 27 pFTable 32: Crystal oscillator component valuesPRELIMINARY Data Sheet (Rev.1.2) SWRS040A Page 45 of 83