Guardian G25AMK005, Technical Manual

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2-14 G U A R D I A N V H F 1 1 0 W M O B I L E
configured for a voltage gain of 2. The outputs at U2-1 and U6-1, labeled PABIAS1 and PABIAS2 respectively, are
then routed to the gates of the power transistors. The correct DAC values for the bias current are stored in the
EEPROM (U15). The correct DAC value is obtained and stored in the EEPROM during the power amplifier bias
calibration procedure at level 2 testing. Each gate is biased such that 100 mA of current flows through each power
transistor with PWRSET set to a DAC value of zero in the transmit mode. Level 2 software monitors the U10
output line labeled IMONITOR when calibrating PA bias.
2.5.4 Keypad Microcontroller
The keypad board is controlled by an Atmel AVR4414 microcontroller. This is a Flash programmable device. The
microcontroller implements these functions:
Keypad scanning
PTT switch input
Auxiliary keys input
Volume switch input
Channel switch input
Emergency switch input
Synchronous bidirectional serial interface to main controller at 100 kHz
Keypad and LCD backlight control
A clock derived from the main reference clock, clocks the keypad microcontroller at 1.5 MHz. The keypad
microcontroller is powered by the 3.3V supply as the LCD driver device.
2.6 Audio Amplifier Board
2.6.1 Description
The Audio amplifier board contains the internal and external audio and control circuitry. It also houses the DB25
accessory connector, the DB9 power connector, and the RF control connector that protrudes through the rear panel.
The board is located in the Control Module. The board contains the following functions:
RFI and transient protection and system on/off switch
Voltage regulator and 7.8V on/off switch
Communication with transceiver
Audio power amplifiers
Power levels detector and converter and the RF and DC combiner
PA on off control
RFI and transient protection
2.6.2 Power Conditioning
Power for the Guardian 110W control head enters this board on J5 the DB9 connector. Dual low R
DS(on)
P-channel
FET Q10 serves as reverse polarity protection and on-off switching. Transistor Q7 pulls down the gates of Q10
when the ON/OFF signal is high, allowing Q10 to conduct. With ON/OFF control low, Q10 is cut off, and will not
pass forward or reverse polarity. An over-voltage condition is detected by D2 and Q6, which cause Q7 to cut off and
turn off power.
Back-to-back transorbers D1 and D4 are used to eliminate voltage spikes before the on/off switch. The input power
is routed to the clamp circuit via an LC filter. The filter eliminates unwanted signals from being conducted to the
vehicle power line.