Introduction; Circuit Description - LG LW1100AP Manual

1. Introduction

This paper describes the LG AP circuits, specifications,layouts and partlist etc.

2. Circuit Description

2.1 The Architecture
The radio design is centered on the expected signal characteristics of the modulation method.
A Differential Quadrature Phase Shift Keying (DQPSK) modulation encodes the data in terms of
phase with minimal amplitude variation, improving noise immunity. This is joined with the IEEE
802.11 protocol standard Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) and with
the FCC requirement for 10dB processing gain, to allow signals to be received with approximately
0dB Signal to Noise Ratio (SNR) for a 10 -5 Bit Error Rate (BER). The CSMA/CA protocol allows
only one user, per channel, at a time (i.e., first come, first serve) making for a quieter medium. The
processing gain, achieved by spreading the signal with a PN code, allows a faint signal to be pulled
from the noise while suppressing non-correlating interferers. From the antenna, the received input is
applied to the pre-select filter FL1. This filter is a two pole dielectric design, rejecting interferers
outside the 2.4GHz ISM band and providing image rejection.
The signal enters the HFA3683 RF/IF Converter, first passing through the integrated LNA section
and then enters the down converter section of the HFA3683. Low-side local oscillator injection is
used to mix down to the single intermediate frequency, 374MHz. The IF receive filter FL3, is a
Surface Acoustic Wave (SAW) device used for channel selection within the band. The SAW output is
reactively matched to the IF input of the HFA3783 Quadrature IF Modulator/Demodulator. In receive
mode, the HFA3783 provides two limiting amplifiers, a quadrature baseband demodulator, and two
baseband low pass filters. The two limiting amplifiers or limiters provide most of the receiver gain,
giving the radio it's sensitivity.
The baseband circuit samples the waveform with 7-bit ADCs and then despreads and demodulates
the received data.
On the Transmit side, data can either be DBPSK or DQPSK modulated at 1MSPS (Mega Symbols
Per Second), resulting in a baseband quadrature signal with I and Q components These digital
.
signals are output to the HFA3783 fifth order Butterworth low pass filters, which are used to provide
shaping of the phase shift keyed (PSK) signal. The required transmit spectral mask, at the antenna,
is -30dBc at the first side-lobe relative to the main lobe.
The signals are then quadrature modulated up to IF using the same 2xLO used for the quadrature