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3.22 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, CL, specified for the crystal. The total load capacitance seen between the crystal terminals
should equal CL for the crystal to oscillate at the specified frequency.
1
C
C
=
+
L
parasitic
1
1
+
C
C
81
101
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
are given in
Table
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-V
The ESR of the crystal should be within the specification to ensure a reliable start-up (see
The initial tolerance, temperature drift, aging and load pulling should be carefully specified to meet the
required frequency accuracy in a certain application.
3.22.1 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 V
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. When using a full-swing digital signal this capacitor can
be omitted. The XOSC_Q2 line must be left unconnected. C81 and C101 can be omitted when using a
reference signal.
3.23 External RF Match
The balanced RF input and output of CC11x1-Q1 share two common pins and are designed for a simple,
low-cost matching and balun network on the printed circuit board. The receive- and transmit switching at
the CC11x1-Q1 front-end is controlled by a dedicated on-chip function, eliminating the need for an
external RX/TX-switch.
A few passive external components combined with the internal RX/TX switch/termination circuitry ensures
match in both RX and TX mode.
Copyright © 2009–2010, Texas Instruments Incorporated
Figure
3-16.
XOSC_Q1
C81
Figure 3-23. Crystal Oscillator Circuit
Table 3-16. Crystal Oscillator Component Values
Component
C
= 10 pF
L
C81
15 pF
C101
15 pF
signal swing. This ensures a fast start-up, and keeps the drive level to a minimum.
pp
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SWRS076B – 11-07-22-013 - APRIL 2009 – REVISED APRIL 2010
3-23. Typical component values for different values of CL
XTAL
XOSC_Q2
C101
C
= 13 pF
C
= 16 pF
L
L
22 pF
27 pF
22 pF
27 pF
) or a sine wave of maximum 1 V peak-peak
DD
CC11x1-Q1
(10)
Section
2.9).
Detailed Description
55
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