Texas Instruments 3138 155 232931 - Manual (Page 54)

CC2420
SWRS041B Page 54 of 89
C381 C391
XTAL
XOSC16_Q1 XOSC16_Q2
C381 C391
XTAL XTAL
XOSC16_Q1 XOSC16_Q2
Figure 30. Crystal oscillator circuit
Item C
L
= 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.
33 Transmitter Test Modes
CC2420 can be set into different transmit
test modes for performance evaluation.
The test mode descriptions in the following
sections requires that the chip is first
reset, the crystal oscillator is enabled
using the SXOSCON command strobe and
that the crystal oscillator has stabilised.
33.1 Unmodulated carrier
An unmodulated carrier may be
transmitted by setting
MDMCTRL1.TX_MODE
to 2 or 3, writing
0x1800 to the DACTST register and issue
a STXON command strobe. The transmitter
is then enabled while the transmitter I/Q
DACs are overridden to static values. An
unmodulated carrier will then be available
on the RF output pins.
A plot of the single carrier output spectrum
from
CC2420 is shown in Figure 31 below.