User's Manual

M889-00

Circuit Operation 2.3

2.1 Introduction

Figure 2.1 High Level Block Diagram

The T889 comprises a three stage RF power amplifier with extensive control circuitry.

The final stage is composed of four transistors (Q3, Q4, Q5 & Q6) whose outputs are

combined to provide the rated output power.

The configuration of each of the main circuit blocks may be seen on a functional level in

Figure 2.1.

2.2 RF Circuitry

The driver stage of the T889 consists of a two stage transistor amplifier (Q1, Q2) which

delivers a minimum of 40W. This signal is split via three 3dB couplers (L13, L14 & L15)

and used to drive the four final amplifiers (Q3, Q4, Q5 & Q6). These outputs are recom-

bined by L40, L41 & L42 and passed to the antenna socket via the directional coupler

and low pass filter.

The directional coupler senses forward and reflected power, which is rectified (D1, D2)

and passed to the control circuitry for metering, alarm and power control purposes.

Power control is via a series pass transistor (Q16), which controls the supply voltage on

the collector of the driver transistor (Q1).

RF Out

Q1 Q2

Pad

RF In

LPF

4W 40W

30W

60W

120W

100W

Power

Control

Vsupply

Q16

L40L13

L15 L42

L14 L41

Directional

Coupler