CC2400 Datasheet, PDF (49/84 Pages) Texas Instruments – 2.4 GHz Low-Power RF Transceiver

CC2400
43 Crystal Oscillator
An external clock signal or the internal
crystal oscillator can be used as main
frequency reference. The reference
frequency must be 16 MHz. Because the
crystal frequency is used as reference for
the data rate as well as other internal
signal processing functions, other
frequencies cannot be used.
If an external clock signal is used this
should be connected to XOSC16_Q1,
while XOSC16_Q2 should be left open. If
rail-to-rail (1.8V) square-wave signal is
used, the MAIN.XOSC16M_BYPASS bit
must be set. It is also possible to use a
sine-wave input. A voltage swing of 200
mV peak-to-peak is recommended in this
case.
Using the internal crystal oscillator, the
crystal must be connected between the
XOSC16_Q1 and XOSC16_Q2 pins. The
oscillator is designed for parallel mode
operation of the crystal. In addition,
loading capacitors (C5 and C6) 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
C 421
+
1
C 431
+ C _parasitic
The parasitic capacitance is constituted by
pin input capacitance and PCB stray
capacitance. The total parasitic
capacitance is typically 5 pF.
The crystal oscillator circuit is shown in
Figure 23. Typical component values for
different values of C_L are given in Table
20. Note that these values will depend on
the PCB layout and the crystal used.
Determination of the values should be
done by measuring RF frequency on
several boards and adjusting the values of
the loading capacitors accordingly.
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
a stable oscillation. 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 the
Electrical Specifications section).
A small SMD crystal is used in the
reference design; note that the crystal
package strongly influences the price. In a
low-cost design, it may be preferable to
use a larger crystal package.
The required accuracy of the crystal is
determined by the receive filtering. Figure
19 shows how sensitivity varies with the
frequency offset between the transmitter
and the receiver. It is important to take the
total tolerance of the crystal into
consideration; this consists of the initial
tolerance, drift due to temperature and
aging.
XOSC16_Q1
XTA L
XOSC16_Q2
C421
C431
Figure 23. Crystal oscillator circuit
Item
C421
C431
C_L= 16 pF
22 pF
22 pF
SWRS042A
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