CC2420-RTR1 datasheet(17/92 Pages) TI

17 / 92 page

CC2420

SWRS041B

Page 17 of 89

Circuit Description

LNA

DIGITAL

DEMODULATOR

- Digital RSSI

- Gain Control

- Image Suppression

- Channel Filtering

- Demodulation

- Frame

synchronization

DIGITAL

MODULATOR

- Data spreading

- Modulation

On-chip

BIAS

DIGITAL

INTERFACE

WITH FIFO

BUFFERS,

CRC AND

ENCRYPTION

AUTOMATIC GAIN CONTROL

TX POWER CONTROL

TX/RX CONTROL

XOSC

16 MHz

ADC

DAC

FREQ

SYNTH

SmartRF ®

CC2420

Power

Control

Serial

voltage

regulator

Digital and

Analog test

interface

Figure 2.

CC2420 simplified block diagram

A simplified block diagram of

CC2420 is

shown in Figure 2.

CC2420 features a low-IF receiver. The

received RF signal is amplified by the low-

noise amplifier (LNA) and down-converted

in quadrature (I and Q) to the intermediate

frequency (IF). At IF (2 MHz), the complex

I/Q signal is filtered and amplified, and

then digitized by the ADCs.

Automatic

gain control, final channel filtering, de-

spreading, symbol correlation and byte

synchronisation are performed digitally.

When the SFD pin goes active, this

indicates that a start of frame delimiter has

been

detected.

CC2420 buffers the

received data in a 128 byte receive FIFO.

The user may read the FIFO through an

SPI interface. CRC is verified in hardware.

RSSI and correlation values are appended

to the frame. CCA is available on a pin in

receive mode. Serial (unbuffered) data

modes

are

also

available

for

test

purposes.

The

CC2420 transmitter is based on direct

up-conversion. The data is buffered in a

128 byte transmit FIFO (separate from the

receive FIFO). The preamble and start of

frame

delimiter

are

generated

hardware. Each symbol (4 bits) is spread

using

the

IEEE

802.15.4

spreading

sequence to 32 chips and output to the

digital-to-analog converters (DACs).

An analog low pass filter passes the signal

to the quadrature (I and Q) upconversion

mixers. The RF signal is amplified in the

power amplifier (PA) and fed to the

antenna.

The internal T/R switch circuitry makes the

antenna interface and matching easy. The

RF connection is differential. A balun may

be used for single-ended antennas. The

biasing of the PA and LNA is done by

connecting TXRX_SWITCH to RF_P and

RF_N

through an external DC path.

The frequency synthesizer includes a

completely on-chip LC VCO and a 90

degrees phase splitter for generating the I

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