Broadcast Electronics FM-5T, User Manual, Page: 47 / 191

4-1
SECTION IV
THEORY OF OPERATION
4-1.
INTRODUCTION.
4-2. This section presents general theory of operation for the FM-5T/FM-5TS FM Transmit
ters.
4-3. The FM-5T/FM-5TS transmitters are divided into functional blocks which are discussed
by the following text. The functional blocks consist of the FM exciter, the power amplifier,
the transmitter controller, metering circuitry, and the associated power supply circuitry.
The power supply and RF circuitry are discussed in further detail at the end of this section.
The transmitter controller is described in detail by the CONTROLLER section of this
manual. Refer to Figure 4-1 and the overall schematic diagram in SECTION VII as re
quired for the following explanation.
4-4.
ELECTRICAL DESCRIPTION.
4-5.
FM EXCITER.
4-6. The Broadcast Electronics FM-250C is a totally solid-state wideband FM exciter providing
a continuously variable RF output from 25 to 250 watts. The FM-250C operates into a 50
Ohm load at any frequency within the 87.5 to 108 MHz FM broadcast band. The exciter
may be programmed to any frequency within the FM band in 10 kHz increments. The
FM-250C is mounted on slides to allow easy access to the internal semi-modular exciter
circuitry.
4-7. The FM-250C will accept multiple wideband composite inputs from a stereo generator or
SCA generator as well as a 600 Ohm balanced audio input. Refer to publication 597-1004
for a detailed explanation of the FM-250C exciter features.
4-8.
POWER AMPLIFIER.
4-9. The FM-5T/FM-5TS uses a single 4CX3500A tetrode to provide 5000 Watts of RF power
on a single frequency within the FM broadcast band of 87.5 MHz to 108 MHz. The power
amplifier operates in a high-gain, grid- driven class C configuration. A patented input
circuit matches the 50 Ohm output of the IPA up to the higher grid input impedance. Use
of a large coaxial cavity results in high PA efficiency for comparatively low power consump
tion. Removal of the PA tube is a simple and quick procedure due to the cavity arrange
ment. A dual-blower cooling system forces air through the tube socket, anode fins, and out
through the main transmission line chimney. A differential air pressure sensor monitors
the effectiveness of the cooling system and removes power to the tube if air flow is inter
rupted.
4-10.
POWER AMPLIFIER CAVITY.
The patented PA cavity used in the FM-5T/FM-5TS em
ploys a folded half-wave coaxial transmission line resonator constructed with aluminum
and copper tubing. This cavity design eliminates the high voltage blocking capacitors and
high current sliding contacts of conventional cavities by connecting the main transmission
line conductor directly to the anode of the power tube. A grounded concentric transmission
line center conductor tunes this cavity by varying the length inserted into the open end of a
main transmission line inner conductor. The main inner conductor is insulated from
ground and carries the anode dc potential. DC power is fed at the RF voltage null point,
approximately one-quarter wave from the anode for effective RF decoupling. A large sur
face area without sliding contacts results in minimal RF loss at this point in the cavity.