CC2500_06 Datasheet, PDF (45/84 Pages) Texas Instruments – Single Chip Low Cost Low Power RF Transceiver

CC2500
35
30
25
20
15
10
5
0
-3
-2
-1
0
1
2
3
-5
-10
-15
-20
Frequency offset [MHz]
Figure 26: Typical selectivity at 500 kbps. IF frequency is 307.4 kHz.
MDMCFG2.DEM_DCFILT_OFF = 0
26 Crystal Oscillator
A crystal in the frequency range 26-27 MHz
must be connected between the XOSC_Q1
and XOSC_Q2 pins. The oscillator is designed
for parallel mode operation of the crystal. In
addition, loading capacitors (C81 and C101)
for the crystal are required. The loading
capacitor values depend on the total load
capacitance, C_L, specified for the crystal. The
total load capacitance seen between the
crystal terminals should equal C_L for the
crystal to oscillate at the specified frequency.
C_L =
1
1₊ 1
+ C _parasitic
C₈₁ C₁₀₁
The parasitic capacitance is constituted by pin
input capacitance and PCB stray capacitance.
Total parasitic capacitance is typically 2.5 pF.
The crystal oscillator circuit is shown in Figure
27. Typical component values for different
values of C_L are given in Table 32.
The crystal oscillator is amplitude regulated.
This means that a high current is used to start
up the oscillations. When the amplitude builds
up, the current is reduced to what is necessary
to maintain approximately 0.4 Vpp signal
swing. This ensures a fast start-up, and keeps
the drive level to a minimum. The ESR of the
crystal should be within the specification in
order to ensure a reliable start-up (see Section
4.4 on page 10).
XOSC_Q1
XOSC_Q2
XTAL
C81
C101
Figure 27: Crystal oscillator circuit
Component
C81
C101
C_L= 10 pF
15 pF
15 pF
C_L=13 pF
22 pF
22 pF
C_L=16 pF
27 pF
27 pF
Table 32: Crystal oscillator component values
PRELIMINARY Data Sheet (Rev.1.2) SWRS040A
Page 45 of 83