Broadcast Electronics FX-50, Instruction Manual

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©2011 Broadcast Electronics
WARNING
DISCONNECT EXCITER PRIMARY POWER BEFORE
PROCEEDING.
WARNING
A. Disconnect the primary power to the exciter.
B. Follow the REMOVAL PROCEDURE in reverse order.
8.4 THEORY OF OPERATION
This section presents the theory of operation for the exciter modulated oscillator assembly.
8.5 FUNCTIONAL DESCRIPTION.
MECHANICAL ASSEMBLY.
The modulated oscillator circuit board is enclosed in a cast aluminum housing which is se cured to a heavy steel
plate. Mechanical vibrations are reduced by a foam rubber pad between the steel plate and the chassis. The
increased mass of the assembly also lowers the mechanical resonance below the frequency of vibrations from
external sources.
In addition, a foam rubber pad attached to the inside top-cover restricts movement of circuit board
components to reduce mechanically introduced noise modulation and increase the frequency stability of the
oscillator.
ELECTRICAL DESCRIPTION.
Figure 8-1 presents a simplified schematic diagram of the modulated oscillator circuit board. Refer to Figure 8-1
as required for a description of the following circuits.
A. Modulator/Oscillator
B. Buffers and Output Amplifier
C. Power Supply
MODULATOR/OSCILLATOR. The oscillator section is a modified Colpits configuration consisting of transistor Q2,
inductors L3 and L2, capacitors C1 and C2, and varactor diodes D1 through D8. C2 provides positive feedback
to sustain oscillation. Tuning is accomplished by the 2V to 9V (dependent upon the carrier frequency) potential
applied to the varactor diodes from the AFC/PLL circuit board through L1/L6.
Varactor diodes D1 through D8 also operate as a linear FM modulator. The modulation voltage applied to the
diodes through L1/L6 varies the capacitance across the oscillator tank circuit to provide direct FM modulation.
Capacitor C3 prevents ground loops between the AFC/PLL circuit board ground and modulated oscillator
assembly ground. The oscillator output amplitude is maintained at a constant level by limit diode D9/D10/D11.
BUFFERS AND OUTPUT AMPLIFIER. Three RF stages provide isolation between the oscillator and output load,
harmonic suppression, and a low output impedance.
The modulated RF at Q2 is coupled to the base of buffer/amplifier Q3 through capacitor C8. The output of Q3
is applied to buffer/amplifier Q4 through C11. The output of Q4 is applied to the base of output amplifier Q5
through a low-pass filter consisting of C15, C16, and L5. The output of Q5 is routed through C18 to resistors
R23 and R24 which establish a 50 Ohm output impedance.
Two identical signals are output from the modulated oscillator assembly. The signal at R24 provides drive to the
RF amplifier and the signal at R23 provides a frequency sample to the AFC/PLL circuit board.