BK Radio
Page 3-5
theory of operation
transmitter
The transmitter section has circuits that perform many functions including frequency generation,
modulation, buffering, and amplification. The main functional blocks in the transmitter are:
• PA Driver
• PA
• Antenna Switch
• Harmonic Filter
PA Driver
Tx / Rx
Power Amplifier
Tx Power
Control
Antenna
Switch
Harmonic
Filter
LO
Rx
DAC
From uP
transmitter Block Diagram
The PA driver is first stage of the transmitter’s output. It outputs the RF signal with the power necessary
to drive the final Power Amplifier.
The final PA module is designed to produce sufficient RF output to meet the radio’s rated transmit power.
This FET-based module operates as a class C output stage.
The Tx Power Control signal comes from the DAC and is stepped to allow three power levels (High,
Medium, and Low) as chosen by the user. The control voltage is adjusted by the microprocessor to
ensure constant output power over the bandwidth of the radio.
The Antenna Switch routes the RF signal from the transmitter to the Harmonic Filter and antenna while
the radio is in transmit mode. When in receive mode, the antenna port is routed to the receiver. In
Transmit Mode, both pin diodes in the Antenna Switch are turned on, completing a signal path to 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.
The Harmonic Filter attenuates the harmonics created by the Power Amplifier to meet or exceed the
transmit spurious and harmonic specification. The passband input and output impedances of the filter
are 50 Ohms.
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