Texas Instruments 3138 155 232931 - Manual (Page 19)

CC2420
SWRS041B Page 19 of 89
9 Application Circuit
Few external components are required for
the operation of
CC2420. A typical
application circuit is shown in Figure 4.
The external components shown are
described in Table 1 and typical values
are given in Table 2. Note that most
decoupling capacitors are not shown on
the application circuits. For the complete
reference design please refer to Texas
Instrument’s web site:
http://www.ti.com .
9.1 Input / output matching
The RF input/output is high impedance
and differential. The optimum differential
load for the RF port is 95+j187
Ω.
When using an unbalanced antenna such
as a monopole, a balun should be used in
order to optimise performance. The balun
can be implemented using low-cost
discrete inductors and capacitors only or
in combination with transmission lines.
Figure 3 shows the balun implemented in
a two-layer reference design. It consists of
a half wave transmission line, C81, L61,
L71 and L81. The circuit will present the
optimum RF termination to
CC2420 with a
50 Ω load on the antenna connection. This
circuit has improved EVM performance,
sensitivity and harmonic suppression
compared to the design in Figure 4.
Please refer to the input/output matching
section on page 54 for more details.
The balun in Figure 4 consists of C61,
C62, C71, C81, L61, L62 and L81, and will
present the optimum RF termination to
CC2420 with a 50 Ω load on the antenna
connection. A low pass filter may be
added to add margin to the FCC
requirement on second harmonic level.
If a balanced antenna such as a folded
dipole is used, the balun can be omitted. If
the antenna also provides a DC path from
the TXRX_SWITCH pin to the RF pins,
inductors are not needed for DC bias.
Figure 5 shows a suggested application
circuit using a differential antenna. The
antenna type is a standard folded dipole.
The dipole has a virtual ground point;
hence bias is provided without
degradation in antenna performance.
9.2 Bias resistor
The bias resistor R451 is used to set an
accurate bias current.
9.3 Crystal
An external crystal with two loading
capacitors (C381 and C391) is used for
the crystal oscillator. See page 53 for
details.
9.4 Voltage regulator
The on chip voltage regulator supplies all
1.8 V power supply inputs. C42 is required
for stability of the regulator. A series
resistor may be used to comply with the
ESR requirement.
9.5 Power supply decoupling and
filtering
Proper power supply decoupling must be
used for optimum performance. The
placement and size of the decoupling
capacitors and the power supply filtering
are very important to achieve the best
performance in an application. Texas
Instruments provides a compact reference
design that should be followed very
closely..