Rev. 1.0
Principles of Operation
4—9
For delay times, see Table 4-1, Program Delay Settings. The times are proportional
to the value of R721; times can be doubled by doubling the value of R721.
The power regulator board is the shorter of two boards mounted under the chassis
toward the front of the unit. The board has the isolating diode for the battery input,
the switch-mode voltage regulator for the RF power amplifier, and circuitry for PA
supply current metering. A diode in series with the battery input, together with the
AC-supply diode bridge, provides diode OR-ing of the AC and DC supplies. A
metering shunt samples the output current for use by the metering and display
board.
The RF Driver/Amplifier 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 30 watts. A hybrid, high-gain, wideband amplifier,
operating at about 18 volts, provides about one watt of drive to a single MOSFET
amplifier. The stage operates from a supply voltage of 28 volts in the FM30.
The circuit board has components for input and output coupling and for power
supply filtering.
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. A hybrid,
high-gain, wideband amplifier, operating at about 18 volts, provides about one watt
of drive to a single MOSFET amplifier operating from a supply voltage of
approximately 13 volts.
The circuit board provides for input/output coupling and for power supply filtering.
The RF power amplifier assembly is mounted on back of the chassis with four
screws, located behind an outer cover plate. 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 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.
JP701 Jumper
Delay
3 and 4
2–minute
5 and 6
4–minute
7 and 8
8–minute
Table 4-1 Program Delay Settings
