Motorola GM328 Service Manual page 419

Transmitter Power Amplifier (PA) 40 W
2.4 2nd Intermediate Frequency (IF) and Receiver Back End
The 44.85 MHz 1st IF signal from the second IF amplifier feeds the IF IC (U3101) at pin1. Within the
IF IC the 44.85 MHz high IF signal mixes with the 44.395 MHz second local oscillator (2nd LO) to
produce the low IF signal at 455 kHz. The 2nd LO frequency is determined by crystal Y3101. The
2nd 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 and 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 circuits).
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 µP (U0101 pin 63) and is available at
accessory connector J0501-15.
3.0 Transmitter Power Amplifier (PA) 40 W
The radio's 40W power amplifier (PA) is a four stage amplifier used to amplify the output from the
VCOBIC to the radio transmit level. It consists of the following four stages in the line-up. The first
stage is a LDMOS predriver (U4401) that is controlled by pin 4 of PCIC (U4501) via Q4473
(CNTLVLTG). It is followed by another LDMOS stage (Q4421), an LDMOS stage (Q4431), and a
bipolar final stage (Q4441).
From VCO
Controlled
Stage
Vcontrol
To Microprocessor
Device Q4401 is surface mounted. Q4421, Q4431 and Q4441 are directly attached to the heat sink.
PA
PA-Final
Driver
Stage
Bias 2
AS FI C _ C M P
PA
SPI BUS PWR
SET
Bias 1
P C I C
Figure 2-2 UHF Transmitter Block Diagram
Pin Diode
Antenna
Switch
Power
Sense
Temperature
Sense
Antenna
Harmonic
Filter
RF Jack
To Microprocessor
2-3