Broadcast Electronics FX-50, Instruction Manual

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©2011 Broadcast Electronics
4 THEORY OF OPERATION
This section presents overall theory of operation for the FX-50/E FM Exciters.
For the purpose of definition, the FX-50/E Exciter is divided into functional subassemblies in the following text.
A detailed description of each subassembly is presented in Part II of this manual. A block diagram of the FX-
50/E FM Exciter is presented in Figure 4-1.
4.1 FUNCTIONAL DESCRIPTION.
4.1.1 POWER SUPPLY/CONTROL CIRCUITS.
The power supply/control circuit board contains the exciter power supply and control circuitry. The proceeding
text will describe the power supply circuitry followed by the control circuitry.
POWER SUPPLY CIRCUIT. Primary ac power to the exciter is applied through a voltage selector and line filter
module. This device provides overload protection for the entire ex citer and allows selection of a wide range of
ac input potentials. On FX-50E models, the ac power is routed through an additional ac line filter to meet CE ac
line related specifications.
All dc circuitry in the exciter operates from an unregulated potential of +30V dc and three pre-regulated
potentials of +20 volts, -20 volts and +5 volts. All supplies are full-wave rectified, filtered, and electronically
regulated to assure stable equipment operation.
The +20 volt, -20 volt, and +5 volt supplies are low-current circuits which are protected from over-voltage,
over-current, reverse-voltage, and short-circuit conditions. These potentials are distributed throughout the
exciter to various subassemblies and re-regulated to lower voltages on each circuit board. Front-panel LEDs
provide status indication of the +20 volt, -20 volt, and +5 volt operating potentials.
The fi20 volt supply associated with the RF amplifier is regulated by the control circuitry in response to
preset level controls and feedback loops. This supply contains over-voltage, over-current, reverse-voltage, short-
circuit, and over-temperature circuitry to protect the exciter sub-assemblies.
CONTROL CIRCUIT. The control circuitry regulates operation of the RF amplifier within preset limits dependent
upon several parameters such as forward RF power output, reflected power, RF amplifier heat sink temperature,
dc current, dc supply voltage, an external mute control potential, and an external RF power adjust potential.
The control circuit assembly also contains amplifiers for the forward and reflected power directional couplers,
over temperature circuitry, and the VSWR circuitry.
The control circuit compares the sum of the forward and reflected powers to a reference for automatic control
of power output. If the reflected power becomes excessive, the power output will be reduced by the amount
required to maintain safe operation of the RF output transistor. If excessive VSWR exists, a front-panel VSWR
indicator will illuminate.
In addition, the control circuit monitors the total RF amplifier assembly heat sink temperature and limits RF
output accordingly. This assures operation at safe transistor temperatures under the worst case conditions of
high VSWR, high ambient temperatures, or failure of the cooling fan. If an over-temperature condition exists, a
front-panel TEMP indicator will illuminate.
Automatic protection of the RF devices from excessive voltage is provided by an MOV and crowbar circuit, and
short circuit protection is provided by foldback current limiting and a fuse. If an over-current condition exists, a
front-panel RF indicator will illuminate.