Motorola GM340 Service Information - Page 19

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VHF (136-174MHz) Transmitter Power Amplifier (PA) 45 W

signal into a digital code that is applied to the loop divider, thereby causing the carrier to deviate. The

balance attenuator is used to adjust the VCO's deviation sensitivity to high frequency modulating

signals. The output of the balance attenuator is present at the MODOUT port (U3201-41) and

connected to the VCO modulation diode D3362 via R3364.

5.0

VHF (136-174MHz) Transmitter Power Ampliﬁer (PA) 45 W

The radio's 45 W PA is a four stage ampliﬁer used to amplify the output from the VCOBIC to the

radio transmit level. The line-up consists of three stages which utilize LDMOS technology, followed

by a ﬁnal stage using a bipolar device. The gain of the ﬁrst stage (U3401) is adjustable, controlled by

pin 4 of PCIC (U3501) via Q3501 and Q3502 (VCONT). It is followed by an LDMOS pre-driver stage

(Q3421), an LDMOS driver stage (Q3431) and a bipolar ﬁnal stage (Q3441).

From VCO

Controlled

Stage

Vcontrol

To Microprocessor

Devices U3401 and Q3421 are surface mounted. The remaining devices are directly attached to the

heat sink.

5.1

Power Controlled Stage

The ﬁrst stage (U3401) is a 20dB gain integrated circuit containing two LDMOS FET ampliﬁer

stages. It ampliﬁes the RF signal from the VCO (TXINJ). The output power of stage U3401 is

controlled by a DC voltage applied to pin 1 from the power control circuit (U3501 pin 4, with

transistors Q3501 and Q3502 providing current gain and level-shifting). The control voltage

simultaneously varies the bias of two FET stages within U3401. This biasing point determines the

overall gain of U3401 and therefore its output drive level to Q3421, which in turn controls the output

power of the PA.

In receive mode the voltage control line is at ground level and turns off Q3501-2, which in turn

switches off the biasing voltage to U3401.

Pre

PA-Final

Driver

Stage

Bias 2

A S F I C _ C M P

SPI BUS

Bias 1

Figure 2-1 VHF Transmitter Block Diagram

Pin Diode

Antenna

Switch

Power

Sense

PWR

SET

P C I C

To Microprocessor

Antenna

Harmonic

Filter

RF Jack

Temperature

Sense

To Microprocessor

2-9