Texas Instruments 3138 155 232931 manual Circuit Description, CC2420 simplified block diagram

CC2420

8 Circuit Description

LNA

TX/RX CONTROL

AUTOMATIC GAIN CONTROL

ADC

ADC

DIGITAL

DEMODULATOR

-Digital RSSI

-Gain Control

-Image Suppression

-Channel Filtering

-Demodulation

-Frame synchronization

Serial

voltage

regulator

SmartRF →

CC2420

Power

Control

PA Σ

XOSC

On-chip

BIAS

R

16 MHz

0

90

FREQ

SYNTH

TX POWER CONTROL

DAC

DAC

LOGIC	INTERFACE
	DIGITAL
CONTROL	WITH FIFO
	BUFFERS,
	CRC AND
	ENCRYPTION

DIGITAL

MODULATOR

- Data spreading - Modulation

Serial microcontroller interface

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 by 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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