Broadcast Electronics FM-5T, User Manual

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4-2
4-11.
OUTPUT COUPLING.
Energy is coupled into the transmission line by an adjustable un
tuned loop which functions in the electromagnetic field within the cavity. One end of the
output loop is connected to ground, while the other connects to the center conductor of the
output transmission line through flexible straps.
4-12.
PLATE TUNING.
Plate tuning is accomplished by adjusting a threaded rod which mechani
cally expands or contracts a beryllium copper bellows on the end of the grounded transmis
sion line center conductor inserted into the main line. Coarse frequency adjustment is ac
complished by pre-setting the length of the center conductor into the cavity.
4-13.
NEUTRALIZATION.
Neutralization is accomplished in the FM-5T/FM-5TS by distributed
inductance added in series between the screen connection and the screen bypass capaci
tors. The inductance develops a counteractive voltage swing between the screen and
ground which cancels out the voltage fed through the internal capacitances of the tube and
the stray capacitances of the tube socket. This form of self-neutralization results in very
stable operation and requires no adjustment when the power tube is replaced.
4-14.
SECOND HARMONIC SUPPRESSOR.
A patented second harmonic suppressor is in
cluded in the PA cavity. This consists of a capacitive disc and a series inductance to ground
coupled to the main transmission line at the fundamental frequency RF voltage null point.
Here the second harmonic exhibits a high voltage and the suppressor presents a low im
pedance to ground which reduces the amplitude of the second harmonic. This unique
method of harmonic suppression has minimal effect on the fundamental frequency and
does not add losses to the PA cavity at the fundamental frequency.
4-15.
OUTPUT CIRCUIT.
A separate low-pass filter is contained within the FM-5T/FM-5TS
cabinet to attenuate all residual second and higher order harmonics. This filter functions
over the entire FM broadcast band. Three RF directional couplers are mounted after the
filter in the output transmission line connection. Two of these supply filtered forward and
reflected power RF samples to the controller and the third port supplies a forward power
sample at 40 dB below carrier at 50 Ohms for external test equipment.
4-16.
TRANSMITTER CONTROLLER.
4-17. Transmitter control operations and parameter monitoring are performed by a built-in
microprocessor based controller. The controller incorporates extensive use of RFI filtering,
optical isolation, and state-of-the-art microprocessor technology to assure maximum reli
ability.
4-18. The controller is designed with 12 switch/indicators, 9 status indicators, 3 modem ports,
and a printer port. Adjustable timers are provided to determine filament warm-up time,
blower run-down time, overload-recycle time, and warm-up defeat time. In addition, the
controller is equipped with adjustable plate, screen, grid, and VSWR overload limits. The
timers and the overload limits are adjusted by controls on the main circuit board. The
range of all controls is limited, however so that the safe operating limits of the transmitter
cannot be exceeded by incorrect adjustment.
4-19. All transmitter control and monitoring operations are directed by a state-of-the-art
Z-SOFT microcontroller. The microcontroller is housed on a single plug-in daughter cir
cuit board. The circuit board plugs directly into a header on the controller main circuit
board.
4-20. The controller operates from a modular switching power supply assembly. Three LEDs on
the main circuit board monitor the status of the +5V, +15V, and -15V power supplies. A
Lithum battery backup system has been incorporated into the circuit design. The battery
is designed to maintain the controller memory during power failures and has a useful life
of approximately 2 years.