User's Manual

M889-00

General Information 1.3

1.1 Introduction

The T889 is an FM base station power amplifier designed for single or multichannel

operation within the frequency range 800 to 870MHz. The output power capability is 20

to 100W.

The PA comprises a broad band, two stage drive amplifier whose output is split to drive

four separate output stages. The outputs from these final stages are then recombined

and filtered before being fed to the output socket. This type of balanced output stage

offers two advantages over single ended types:

- improved intermodulation performance in the presence of high signal levels

from adjacent transmitters;

- enhanced reliability: if one of the four output stages fails, the transmitter can

still produce half its rated power.

VSWR and thermal protection is incorporated into the basic design, while monitoring

and alarm signals are available for both forward and reverse power. The output power

is adjustable from the front panel.

The main PCB is mounted directly on a die-cast chassis/heatsink. For long-term relia-

bility, four high quality, low loss Teflon PCBs are sweated to the main PCB in areas of

high RF current. Extensive use is also made of the latest surface mount technology.

Forced air cooling for the heatsink is provided on the T889 by a fan, which is activated

whenever the transmitter is keyed. Thermal sensors will also activate the fan automati-

cally if the internal temperature reaches an unacceptable level.

The T889 has a width of 120mm, occupying a double module in a Tait rack shelf

(T99-770) which will accommodate up to seven standard modules to give an attractive

and convenient installation.