CC2420-RTR1 datasheet(19/92 Pages) TI

19 / 92 page

CC2420

SWRS041B

Page 19 of 89

Application Circuit

Few external components are required for

the

operation

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

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

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..

CC2420-RTR1 数据表(PDF) 19 Page - Texas Instruments

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