Vhf (136-174 Mhz) 25-45 W Bipolar Transmitter Power Amplifier; Power Controlled Stage - Motorola CDM Series Detailed Service Manual

VHF (136–174 MHz) 25–45 W Bipolar Transmitter Power Amplifier (PA) 45 W
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 U3101 pin 8 (DISCAUDIO) which is fed to the ASFIC_CMP (U0221) pin 2 (part of
the Controller circuits).
A Receive 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 in addition is available
at accessory connector J0501-15.
3.0 VHF (136–174 MHz) 25–45 W Bipolar Transmitter Power
Amplifier (PA) 45 W
The radio's 45W 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.
3.1

Power Controlled Stage

The first stage (U3401) is a 20 dB 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
Pre
PA
PA-Final
Driver
Driver
Stage
Bias 2
A S F I C _ C M P
SPI BUS
Bias 1
Figure 8-2 VHF Transmitter Block Diagram
Pin Diode
Antenna
Switch
Power
Sense
PA
PWR
SET
Temperature
P C I C
Section 8: 2-3
Antenna
Harmonic
Filter
RF Jack
Sense
To Microprocessor
6881091C63-F