User Guide

After Sales

Technical Documentation

System Module

THX–4

Amendment 10/97 Page 4–17

The final two stages of the PA use Gallium Arsenide (GaAs) technology to

maximise efficiency. Because of the ’spread ’ inherent in GaAs devices,

provision has been made to trim the DC bias point of both the driver and

PA stages using discrete DACs based around V413 and V415 driven from

the port of D404. Microstrip components are used to form low loss

matching networks.

The PA output from V7 is fed through a directional coupler embedded

within the PCB. This detects a fraction of the transmitter power (–16dB)

for the Automatic Level Control (ALC) circuit. A negative voltage

proportional to TX power is compared to V_ALC in N10 to produce a DC

feedback voltage VALC which is applied to V11. The different power

levels are set by using calibration values in EEPROM which are factory

set during production alignment.

Duplexer and Spurious Rejection

The Transmitter output level at full power is  +28dBm to the antenna.

The Receiver sensitivity is < –110dBm for 20dB SINAD. In order to

operate together in duplex mode, more than 140dB (1x10

) of filtering

and screening is required. The Duplex filtering (Z4) is achieved with

helical filter technology in a pre–aligned filter. As well as separating the

TX and RX components, the Duplexer also removes transmitter noise

from the Cellular Receive Band. Filtering of unwanted harmonics from the

Transmitter, and radiation of the RX Local Oscillator from the Receiver is

also accomplished.