Operating instructions

DMH Series VHF Radio General Information
BK RADIO Page 4-5
C. Antenna Switch
The antenna switch connects either the power amplifier or the receiver to the antenna. It
also provides isolation between the receiver and the power amplifier when the radio is
transmitting.
D. Directional Coupler
A directional coupler and associated detectors provide DC voltages proportional to the
forward and reflected power at the antenna connector. The voltages are used for power
leveling and high VSWR protection.
E. Power Control
The core microprocessor controls the output power of the transmitter by setting the
reference voltage of a feedback control loop. The microprocessor sets the value of U510, a
digital to analog converter. An output of the U510 sets the reference voltage of the power
control loop which consists of U105 and associated circuitry.
4.3.4 OPTIONS BOARD
A. Audio Power Amplifier
U1, U2, U3, U4 and U5 provide audio power amplification to drive the internal and external
speakers.
4.3.5 CONTROL BOARD
A. Microprocessor
U101 controls the interface between the radio and the user. During normal operation, U101
monitors the front panel and push-to-talk switches, and provides data to the display. In the
radio programming mode, U101 interprets commands from the serial bus and provides a
transparent interface to the external programming source.
B. Voltage Regulator
Voltage regulators provide the necessary voltages for the display and microprocessor
circuitry.
C. Vacuum Fluorescent Display Driver
The Display Driver provides the necessary drive signals for the vacuum fluorescent display.
4.3.6 VCO BOARD
The VCO is a varactor tuned oscillator. A steering voltage controls the frequency of oscillation. A
low sensitivity input for modulation is provided.
The output cascode amplifier provides a nominal drive level of 0 dBm in receive and transmit
modes.