BEI Electronics LLC 7EPTX-AM5E User Manual

1-1

SECTION I

RF POWER MODULE THEORY OF OPERATION

1-1.

INTRODUCTION.

1-2.

This section presents a general description of the Broadcast Electronics AM-2.5E/AM-5E

transmitter RF power module.

1-3.

 GENERAL 

DESCRIPTION.

1-4.

An RF power module is a plug-in assembly containing two RF amplifier circuit boards 

and a modulator circuit board.  Each RF power module is designed to produce 1375 watts

of RF power.  The modular design of the RF power assemblies allow the modules to be re

moved from the transmitter for maintenance.  The remaining power modules will provide

power to maintain on-air operation.  The following text presents a description of the RF

power modules.

1-5.

 

MODULATOR CIRCUIT BOARD.

1-6.

MODULATOR CIRCUIT.

  The modulator circuit board is designed to convert the CMOS

level PWM signal from the exciter circuit board into a dc voltage which varies at the au-

dio modulation rate (refer to Figure 1-1).  The duty cycle of the 122 kHz to 135 kHz PWM

signal is 40% with no audio modulation.  The duty cycle varies to allow modulation of the

transmitter from -100% to +150%.  The PWM signal from the exciter circuit board is ap

plied to integrated circuit U1.  U1 is a high-speed optical coupler designed to provide 

isolation for the transition of the signal from the exciter circuit board ground system to 

the modulator circuit board ground system.  The output of U1 is applied to level converter

U11A.  U11A converts the 5 volt signal to a 15 volt peak-to-peak signal.  The output of

U11A is applied to two MOSFET driver stages.  A 9.7 volt dc bias signal is incorporated

into the PWM signal by resistor R10, and zener diodes D17/D18.

1-7.

Integrated circuits U2 and U13 are MOSFET driver stages.  The outputs switch to:  1) a

logic 1 at 2 volts and 2) a logic 0 at 0.8 volts.  The output of U2 is applied to the gate of 

forward converter transistor Q1.  The output of U13 is applied to the gate of forward con

verter transistor Q2.  Q1 and Q2 are switched on/off by the PWM signal.  The transistors

convert the 125 volt B- supply to approximately 50 volts with a nominal PWM duty cycle

of 40%.  Catch diodes D2 and D3 clamps inductors L1 and L2 to prevent transistor dam-

age from high switching voltages during transistor turn-off operations.  A dc operating

potential for Q1 and Q2 is provided by the B- supply from the power supply circuit board.

Control of the B- leg is provided by relay K1.  K1 immediately terminates the power sup

ply during a power supply or modulator failure.

1-8.

The output of transistors Q1 and Q2 are applied to an LC low-pass filter network consist

ing of:  1) inductor L1/L3 and capacitor C13 and 2) inductor L2/L3 and capacitor C14.  The

LC networks function with inductor L4 and capacitors C15/C16/C49 as a fifth-order 

Bessel low-pass filter designed to remove the 125 kHz frequency from the output signal.

The output from L4/C15/C16/C49 is routed:  1) to a monitor circuit and 2) for application

to the power amplifier circuit boards.

1-9.

FAULT DETECTION CIRCUITS.  

The modulator circuitry is monitored for proper operation

by four fault detection circuits.  The fault detection circuits consist of:  1) a PWM drive 

detector, 2) a modulator fault detector, 3) a B+ supply fuse fault detector, and 4) a +20 volt

power supply fault detector.