User Manual

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© Jennic 2008 JN-DS-JN5139 v1.5 25
TX Active
ADO (DIO[12])
1
st
TX-RX Cycle 2
nd
TX-RX Cycle (1
st
Retry)
RX Active
Figure 17 Antenna Diversity ADO Signal for TX with Acknowledgement
8.2 Modem
The Modem performs all the necessary modulation and spreading functions required for digital transmission and
reception of data at 250kbps in the 2450MHz radio frequency band in compliance with the IEEE802.15.4 standard.
AGC
O-QPSK
Demodulation
Symbol
Detection
(Despreading)
Pulse Shaping
O-QPSK
Modulation
Spreading
TX
RX
TX Data
Interface
RX Data
Interface
I
Q
I
I
Q
Q
IF Signal
Gain
Figure 18: Modem Architecture
The transmitter receives symbols from the baseband processor and uses the spreading function to map each unique
4-bit symbol to a 32-chip Pseudo-random Noise (PN) sequence. Offset-QPSK modulation and half-sine pulse
shaping is applied to the resultant spreading sequence to produce two independent quadrature phase signals (I and
Q), which are subsequently converted to analogue voltages in the radio transmit path.
The Automatic Gain Control (AGC) monitors the received signal level and adjusts the gain of the amplifiers in the
radio receiver to ensure that the optimum signal amplitude is maintained during reception.
The demodulator performs digital IF down-conversion and matched filtering and is extremely tolerant to carrier
frequency offsets in excess of ±80ppm without suffering any significant degradation in performance.