BK Radio GPH-CMD, Service Manual

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GPH-CMD VHF Radio Theory of Operation
BK RADIO
Page 4-5
In Transmit Mode, both pin diodes in the Antenna Switch are turned on, completing a signal path
from the Transmitter and shunting the Receiver path to ground. In Receive Mode, both pin diodes
are turned off, allowing RF signals to flow to the Receiver with the Transmitter output port removed
from the circuit.
D. Power Control Circuit
The Power Control Circuit monitors the DC supply current to the Integrated Power Module. This
current increases with RF output power and is kept constant by the Power Control circuit.
Current sensing is accomplished using a small resistance in the DC supply to the Integrated Power
Module. The voltage developed across this resistance is fed to a bridge circuit and amplifier where
it is compared to the "PWR SET" voltage. Any difference in the levels is amplified and used to
control the bias current to the Integrated Power Module.
4.3.3 SYNTHESIZER
The Synthesizer generates an RF signal either to down-convert a desired receive frequency to a fixed IF
or to drive the Transmitter. The synthesizer is essentially a phase-locked loop that locks the RF output of
a Voltage Controlled Oscillator (VCO) to a very stable lower frequency reference. The DSP determines
the frequency that the synthesizer produces by programming the dividers contained within the
Synthesizer IC.
Synthesizer
IC
TCXO
Reference
Loop
Filter
Phase Error
Synthesizer
Buffer
Coarse Tune
Program
Lines
Adapt Pulse
Frequency Steering
VCO
Buffer
VCO
TX Modulation
RX/TX Band
Switch
Steering Line
Figure 4-5 Synthesizer Block Diagram