Section 7: 2-4 Low Band Transmitter Power Amplifier (PA) 60 W
6881091C63-F
If the repetition rate of noise impulses is so rapid that the noise blanker can no longer blank them
individually, as indicated by a large increase in high-frequency content at the output of Q1604, stage
Q1607 amplifies this level and turns on level detector Q1609. Its output is highly filtered into a DC
voltage level which is proportional to the repetition rate of noise impulses, and this is applied to the
AGC input pin 5 of U1601, reducing its gain and therefore the amount of noise pulses which are
detected and processed.
3.0 Low Band Transmitter Power Amplifier (PA) 60 W
The radio’s 60W power amplifier (PA), shown in Figure 7-2, is a three-stage amplifier used to amplify
the output from the VCO to the radio transmit level. The line-up consists of three stages which utilize
LDMOS technology. The first stage is a pre-driver (U1401) that is controlled by pin 4 of PCIC
(U1503) via Q1504 and Q1505 (CNTLVLTG). It is followed by driver stage Q1401, and final stage
utilizing two devices (Q1402 and Q1403) connected in parallel. Q1402 and Q1403 are in direct
contact with the heat sink.
To prevent damage to the final stage devices, a safety switch has been installed to prevent the
transmitter from being keyed with the cover removed.
Figure 7-2 Low Band Transmitter Block Diagram
3.1 Power Controlled Stage
The first stage (U1401) 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 U1401 is
controlled by a DC voltage applied to pin 1 from the power control circuit (U1503 pin 4, with
transistors Q1504-5 providing current gain and level-shifting). The control voltage simultaneously
varies the bias of two FET stages within U1401. This biasing point determines the overall gain of
U1401 and therefore its output drive level to Q1401, which in turn controls the output power of the
PA.
Pin Diode
Antenna
Switch
RF Jack
Antenna
Harmonic
Filter
PA-Final
Stage
PA
Driver
From VCO
Controlled
Stage
BIAS
To Microprocessor
Temperature
Sense
DC AMP
PASUPLVLTG
(2 Lines)
SPI Bus
TXINJ
Sense
Current
Sense
Current
ASFIC_CMP
PCIC
INT
24
4
29
5
6
BIAS
Power
Power Adjust
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