CC2500 Datasheet, PDF (44/76 Pages) Texas Instruments – Single Chip Low Cost Low Power RF Transceiver

CC2500
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
22. 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 0.4Vpp 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).
The initial tolerance, temperature drift, aging
and load pulling should be carefully specified
in order to meet the required frequency
accuracy in a certain application. By specifying
the total expected frequency accuracy in
SmartRF^® Studio together with data rate and
frequency deviation, the software calculates
the total bandwidth and compares this to the
chosen receiver channel filter bandwidth. The
software reports any contradictions, and a
more accurate crystal is recommended if
required.
XOSC_Q1
XOSC_Q2
XTAL
C81
C101
Figure 22: 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
26.1 Reference Signal
The chip can alternatively be operated with a
reference signal from 26 to 27 MHz instead of
a crystal. This input clock can either be a full-
swing digital signal (0 V to VDD) or a sine
wave of maximum 1 V peak-peak amplitude.
The reference signal must be connected to the
XOSC_Q1 input. The sine wave must be
connected to XOSC_Q1 using a serial
capacitor. The XOSC_Q2 line must be left un-
connected. C81 and C101 can be omitted
when using a reference signal.
Preliminary Data Sheet (rev.1.1.) SWRS040
Page 44 of 77