Figure 3-5. Transmitter Block Diagram - Motorola ASTRO XTSTM 4000 Detailed Service Manual

Table of Contents

Theory of Operation: Transceiver Section

3.1.3

Transmitter

The transmitter takes modulated RF from the FGU and amplifies it to the radio's rated output power

to produce the modulated transmitter carrier at the antenna.

NOTE: Refer to

transmitter-related schematics that will aid in the following discussion.

The transmitter

voltage-controlled oscillator (VCO) and a high-power output transistor. Transmitter power is

controlled by a power-control IC (PCIC) that senses the total current drawn by the transmit gain

stages and adjusts PA control voltages to maintain a constant power level. The signal passes

through an antenna switch and harmonic filter to the antenna.

RAWB+

Modulated

RF from FGU

3.1.3.1 Power Distribution

To minimize voltage drop to the power amplifiers, net RAWB+ connects to power module Q107 and

the second stage of driver amplifier U102 through components having minimal series resistance –

ferrite beads and chokes only. During receive, no RF or DC bias is applied, and leakage current

through U102 and Q107 is less than 100 microamps.

At a transmitter power of 2 Watts, the radio consumes approximately 1100 mA.

3.1.3.2 Driver Amplifier

The driver amplifier IC (U102) contains two LDMOS FET amplifier stages and two internal resistor

bias networks. Pin 16 is the RF input. Modulated RF from the FGU, at a level of +3 dBm ±2 dB, is

coupled through a blocking capacitor to the gate of FET-1. An LC interstage matching network

connects the first stage output VD1 to the second stage input G2. The RF output from the drain of

FET-2 is pin 6 (RFOUT1). Gain control is provided by a voltage applied to pin 1 (VCNTRL). Typical

output power is about +26 dBm (400 mW) with VCNTRL at 4.5 V.

L101 and C102 are the interstage matching network; capacitor C111 is a DC block.

VHF: Components C112–C115 and L103–L104 match the output impedance to maximize power

transfer to Q107.

UHF: Components C114 and L113 match the output impedance to maximize power transfer to Q107.

6871620L01-C

Table 8.1, "List of Schematics and Boards Overlays," on page 8-1

(see Figure

3-5) consists of an RF driver IC that receives its input signal from the

Sense

Resistor

Driver

Amplifier

INT

Secondary

Cutback

RFIN