4–13
Principles of Operation
4.11 RF Driver (FM100/FM250)
The RF Driver assembly is mounted on a 100 mm x 100 mm plate in the under side
of the chassis. The driver amplifies the approximate 20 milliwatts from the
frequency synthesizer to about 8 watts to drive the RF power amplifier. An
MHW6342T hybrid, high-gain, wideband amplifier, operating at about 20 volts,
provides about one watt of drive to a single BLF245 MOSFET amplifier. The
BLF245 stage operates from a supply voltage of approximately 20 volts.
The circuit board provides for input/output coupling and for power supply filtering.
4.12 RF Amplifier (FM100/FM250)
The RF power amplifier assembly is mounted on back of the chassis with four
screws, located behind an outer cover plate. Access the connections to the module
by removing the bottom cover of the unit. The RF connections to the amplifier are
BNC for the input and output. Power comes into the module through a 5–pin
header connection next to the RF input jack.
The amplifier is built around a Phillips BLF278, a dual power MOSFET rated for 50
volts DC and a maximum power of about 300 watts. When biased for class B, the
transistor has a power gain of about 20 dB. (It is biased below class B in the
transmitter.)
Input transformer, T1111, is made up of two printed circuit boards. The four-turn
primary board is separated from the one-turn secondary by a thin dielectric film.
R1112–R1117 are for damping. Trim pot R1111 sets the bias.
Output transformer, T1121, has a one-turn primary on top of the circuit board and a
two-turn secondary underneath. Inductors L1121 and L1122 provide power line
filtering.
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