Motorola GM640 Service Information - Page 13

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UHF (403-470MHz) Transmitter Power Amplifier (PA) 25 W

produce the low IF signal at 455 kHz. The 2nd LO frequency is determined by crystal Y3101. The

low IF signal is ampliﬁed and ﬁltered by an external pair of 455 kHz ceramic ﬁlters FL3112, FL3114

for 20/25 kHz channel spacing or FL3111,FL3113/F3115 for 12.5 kHz channel spacing. These pairs

are selectable via BWSELECT. The ﬁltered output from the ceramic ﬁlters is applied to the limiter

input pin of the IF IC (pin 14).

The IF IC contains a quadrature detector using a ceramic phase-shift element (Y3102) to provide

audio detection. Internal ampliﬁcation provides an audio output level of 120 mV rms (at 60%

deviation) from U3103 pin 8 (DISCAUDIO) which is fed to the ASFIC_CMP (U0221) pin 2 (part of

the Controller circuitry).

A received signal strength indicator (RSSI) signal is available at U3101 pin 5, having a dynamic

range of 70 dB. The RSSI signal is interpreted by the microprocessor (U0101 pin 63) and in addition

is available at accessory connector J0501-15.

3.0

UHF (403-470MHz) Transmitter Power Ampliﬁer (PA) 25 W

The radio's 25W PA is a three stage ampliﬁer used to amplify the output from the VCOBIC to the

radio transmit level. All three stages utilize LDMOS technology. The gain of the ﬁrst stage (U4401) is

adjustable, controlled by pin 4 of PCIC (U4501) via U4402-1. It is followed by an LDMOS stage

(Q4421) and LDMOS ﬁnal stage (Q4441).

From VCO

Controlled

Stage

Vcontrol

To Microprocessor

Devices U4401, Q4421 and Q4441 are surface mounted. A pressure pad between board and the

radio's cover provides good thermal contact between the devices and the chassis.

3.1

First Power Controlled Stage

The ﬁrst stage (U4401) 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 U4401 is

PA-Final

Driver

Stage

Bias 2

A S F I C _ C M P

SPI BUS

Bias 1

Figure 2-2 UHF Transmitter Block Diagram

Pin Diode

Antenna

Switch

Power

Sense

PWR

SET

P C I C

Antenna

Harmonic

Filter

RF Jack

Temperature

Sense

To Microprocessor

2-3