Motorola CDM Series Detailed Service Manual page 533

UHF Band 2 (450–512/520 MHz) 25–40 W Bipolar Transmitter Power Amplifier (PA) 40W
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 ASFIC_CMP U0221, pin 2 (part of the
controller circuits).
A Receive Signal Strength Indicator (RSSI) signal is available at U3101, pin 5, which has 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 UHF Band 2 (450–512/520 MHz) 25–40 W Bipolar Transmitter
Power Amplifier (PA) 40W
The radio's 40W power amplifier (PA), shown in Figure 10-2, is a four-stage amplifier used to amplify
the output from the VCOBIC to the radio transmit level. The first stage is an LDMOS pre-driver
(U5401) controlled by pin 4 of PCIC (U5501) via Q5473 (CNTLVLTG). This stage is followed by
another LDMOS stage (Q5421), LDMOS stage Q5431, and a bipolar final stage (Q5441). Device
Q5401 is surface mounted and Q5421, Q5431, and Q5441 are directly attached to the heat sink.
From VCO
Controlled
Stage
Vcontrol
To Microprocessor
3.1 Power Controlled Stage
The first stage (U5401) amplifies the RF signal from the VCO (TXINJ) and controls the output power
of the PA. The output power of the transistor U5401 is controlled by a voltage control line feed from
the PCIC (U5501, pin 4). The control voltage simultaneously varies the bias of two FET stages
within U5401. This biasing point determines the overall gain of U5401 and therefore its output drive
level to Q5421, which in turn controls the output power of the PA.
PA
PA-Final
Driver
Stage
Bias 2
A S F I C _ C M P
PA
SPI BUS PWR
SET
Bias 1
P C I C
Figure 10-2 UHF Transmitter Block Diagram
Pin Diode
Antenna
Harmonic
Switch
Filter
Power
Sense
Temperature
Sense
Section 10: 2-3
Antenna
RF Jack
To Microprocessor
6881091C63-F