CC2420
SWRS041B Page 17 of 89
8 Circuit Description
Serial
microcontroller
interface
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
CONTROL LOGIC
AUTOMATIC GAIN CONTROL
TX POWER CONTROL
TX/RX CONTROL
XOSC
16 MHz
R
ADC
ADC
DAC
DAC
0
90
FREQ
SYNTH
SmartRF
CC2420
Power
Control
PA
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 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
CC2420有一个低
IF接收器。接收的
RF信号通过LNA
放大,并且与IF通
过Down方向求
积。在IF2M范围
内,复杂的I/Q滤波
并放大,并且由
ADC进行数字
化。自运增益调
整,最终通道 滤波,
信号解悉,字节同
步同时进行。
当SFD引脚转为
有效,这表示开始
头初接收到。
CC2420将接收到
的数据存到
128FiFo中。用户
可以通过SPI接口
把FIFO的数据读
出。CRC校验由
硬件完成。这个
信息包的RSSI相
关值被存储。在
接收方式中,CCA
(看看通道空不空)
有效。
CC2420发送器
通过向上转
换。数据通过
128FiFo缓存。
同步段和帧头
自动被硬件插
入。每一个标
识(四位)由硬件
通过
IEEE802.15.4
标准格式化输
出给DAC.
模拟信号通低
通滤波器传到I/
Q混频.RF信号
通过PA放大给
天线.
内部的T/R切换
电路使能天线
接口很容易匹
配.RF连 接是
不同的,单端天
线要接一个不
平衡变压器.PA
和LNA不同的
偏置通过
TXRX-SWitch
在RF_P和
RF_N进行偏
置.
频率合成器包括一个完全在片内的VCO和一个90
度相位分割器.