Broadcast Electronics AM-2.5E, User Manual

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4-103. Modulator Circuit Board. The modulator circuit board consists of a MOSFET forward
converter circuit and a filter network. The forward converter circuit consists of MOSFET
transistors Q1 and Q2. The filter network consists of inductors L1 through L3 and
capacitors C13 through C16. A dc operating voltage for transistors Q1 and Q2 is provided
by the power supply circuit board. The B- leg of the supply is routed through relay K1 to
the transistors. K1 is provided to immediately terminate the supply during a modulator
fault condition.
4-104. The PWM signal at a 15 volt level from the exciter circuit board is applied to the gates of
MOSFET transistors Q1 and Q2. Q1/Q2 function to switch the B- leg at a 122 kHz to 135
kHz rate. The output from Q1/Q2 is applied to the LC low-pass filter network to convert
the square-wave PWM signal to a dc voltage. The output from the filter will produce a 50
volt dc signal with: 1) a nominal PWM duty cycle of 40% and 2) no audio modulation.
The dc voltage will vary from 0 to 125 volts with -100% to +150% modulation. The output
of the filter network is applied to the drains of MOSFET amplifier transistors on the
power amplifier circuit boards.
4-105. Power Amplifier Circuit Boards. The RF circuitry on the power amplifier circuit boards
consists of a Class E MOSFET power amplifier circuit. Each power amplifier circuit
board is designed to output approximately 350 watts. The power amplifier circuit boards
are identical. Therefore, only power amplifier 1 will be discussed.
4-106. The MOSFET amplifier circuit is designed in a push-pull design Class E configuration.
Class E power amplifier characteristics consist of: 1) the transistor drain-to-source
voltage must be nominally zero immediately prior to the turn-on of the transistor and
2) the time slope of the drain-to-source voltage waveform must be nominally zero prior to
the turn-on of the transistor. The Class E circuit results in: 1) lower device dissipation
resulting in reduced transistor operating temperature which greatly increases component
life, 2) an operating efficiency of 95% or greater, and 3) increased reliability when
operated into VSWR conditions.
4-107. Additional characteristics of a Class E amplifier design is the application of dc power to
the amplifier transistors. The B+ leg of the B supply is applied to RF choke L1. The
choke is connected to the primary center tap winding of combiner transformer T1. The
transistors are connected to the primary winding of the transformers.
4-108. Two signals are applied to the power amplifier 1 circuit board: 1) an RF square-wave
signal from the driver circuit board and 2) a dc voltage from the modulator circuit board
which varies at an audio rate. The RF square-wave signal at the carrier frequency is
applied to the gates of MOSFETs Q1 and Q2. The varying dc voltage from the modulator
circuit board is applied to the source of MOSFETs Q1 and Q2. Q1/Q2 operate in a
push-pull configuration to develop approximately 350 watts of RF power at combiner
transformer T1. The power at transformer T1 is combined with the 687.5 watts of RF
power from power amplifier circuit board 2 to generate 1375 watts of RF power from the
RF power module.
4-109.
RF COMBINER.
The RF combiner components are located on the rear-panel of each
power block assembly. The combiner consists of a star combiner design. The star
combiner contains an individual transformer, an RF choke, and an impedance matching
circuit for each power amplifier circuit board. The impedance matching circuit consists of
star inductors. The circuit presents the correct impedance when a module is removed
from the chassis. This allows the transmitter to operate at an output power which is
proportional to the modules removed from the power block chassis.