CC2420RTCR Datasheet, PDF (54/94 Pages) Texas Instruments – 2.4 GHz IEEE 802.15.4 / ZigBee-ready RF Transceiver

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
C₃₈₁ C₃₉₁
The parasitic capacitance is constituted by
pin input capacitance and PCB stray
capacitance. The total parasitic
capacitance is typically 2 pF - 5 pF.
CC2420
The crystal oscillator circuit is shown in
Figure 30. Typical component values for
different values of C_L are given in Table
10.
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 must be within the
specification in order to ensure a reliable
start-up (see the Electrical Specifications
section).
XOSC16_Q1
C391
XTAL
XOSC16_Q2
C381
Figure 30. Crystal oscillator circuit
Item
CL= 16 pF
C381
27 pF
C391
27 pF
Table 10. Crystal oscillator component values
32 Input / Output Matching
The RF input / output is differential (RF_N
and RF_P). In addition there is supply
switch output pin (TXRX_SWITCH) that
must have an external DC path to RF_N
and RF_P.
In RX mode the TXRX_SWITCH pin is at
ground and will bias the LNA. In TX mode
the TXRX_SWITCH pin is at supply rail
voltage and will properly bias the internal
PA.
The RF output and DC bias can be done
using different topologies. Some are
shown in Figure 4 and Figure 5.
Component values are given in Table 2.
Using a differential antenna, no balun is
required.
If a single ended output is required (for a
single ended connector or a single ended
antenna), a balun should be used for
optimum performance.
The balun adds the signals from the RF_N
and RF_P. This is achieved having two
paths with equal amplitude response, but
180 degrees phase difference.
SWRS041c
Page 54 of 85