Pag. 48
SIRIO 2000T
SIRIO 2000T
SIRIO 2000T
SIRIO 2000T
SIRIO 2000T
Electronic Broadcast Equipment
The transistor gate is biased by a zener network and a resistive trimmer which adjusts the idle current of
the transistor. RT1 is usually set to provide some 200mA drain current, with +12Vdc control voltage applied
and no RF input. Varying the control voltage towards a negative supply progressively disables the amplifier
transistor, so it effectively acts as an AGC input.
The power management circuit, in a closed loop driven by the output sense circuitry, varies this voltage.
This will accordingly vary the output power to obtain the pre-set value and cope with alarm and start-up
conditions.
The low-pass section and the directional coupler circuit shown in the electrical diagram and allowed in
the board for different applications (25W stand alone exciter) may be not present since those ciruits are not
needed for an enclosed driver.
THE RF POWER AMPLIFIER ARRAY
This stage is designed with four 500W amplifier modules, coupled with an input splitter and with an output
combiner. A low-pass in incorporated in the latter, togheter with a directional coupler.
The whole assembly is contained in a fully screened box, which may be accessed from the bottom of the
transmitter, only removing the bottom cover. A quick description of each stage follows.
The input RF power splitter
The purpose of this simple board is mainly to divide the input RF drive power in four identical signals,
one for each power amplifier module, providing a good insulation between each output port (>20 dB,
typ. 23 dB minimum on the whole FM band).
This is done by a Wilkinson type printed coupler, followed by the balancing resistor array R21
÷
R24.
A directional coupler senses the input RF level and RT1 is regulated to provide the overdrive protection
circuit trip at the right maximum input level. The board supports also the direct (or forward) and reflected
detector sensitivity trimmers, whose input comes from the output board.
Two additional lines support the RF module bias (or AGC) line and the temperature sensing PTC
thermistor on the heatsink.
