Broadcast Electronics Marti Electronics ME-40, Manual

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©2011 Broadcast Electronics
Input Amplifier. The input amplifier consists of thick-film hybrid amplifier U2, and resistor pad R6
and R7. A 1 milliwatt RF input signal from the modulated oscillator is input to U2. This stage
provides approximately 1 watt of output power across R6 and R7 to the following stage.
Input amplifier U2 operates from a DC potential of +20 volts which is routed through input mute
transistor Q5. Inductor L1 and capacitors C11 and C12 provide power supply isolation.
Driver Amplifier. The driver amplifier consists of transistor Q3, an impedance matching network,
resistor R8, and inductor L3. The matching network converts the 50 Ohm output of U2 to the low
input impedance required by Q3. This stage provides approximately 8 watts of output power to
the following stage. L3 provides a DC return path for Q3 and R8 ensures stable amplifier
operation.
Driver amplifier Q3 operates from a DC potential of +20 volts. Inductors L4 and L5, and
capacitors C19, C22, and C23 provide power supply isolation.
Power Amplifier. The power amplifier consists of power transistor Q4, an impedance matching
network, resistor R10, and PA bias control R17. The matching network converts the output
impedance of Q3 to the low input impedance required by Q4 R10 provides isolation from the bias
network and R17 establishes the quiescent drain current for Q4. This stage provides 50 watts of
output power to the associated transmitter
The drain of Q4 connects to an impedance matching network which operates as: 1) a broadband
impedance step-up transformer to establish an output impedance of 50 Ohms, and 2) a second
harmonic notch filter. Capacitor C36 functions as a DC blocking capacitor.
Power amplifier Q4 operates from an adjustable DC potential of+3 to +24 volts. The adjustable
potential is preset by circuitry on the power supply/control circuit board and is automatically
maintained by feedback from the forward directional coupler. Inductors L7 and L8, and capacitors
C28 through C31 provide power supply isolation.
5.12.2.2
DIRECTIONAL COUPLER CIRCUITS.
The directional couplers provide two DC signals obtained by rectifying a sample of the RF output
signal. Due to the polarity of the samples, one signal will represent the forward output signal and
the other will represent the reflected.
Forward Directional Coupler. The forward voltage sample is obtained from a microstrip on the
circuit board near the output line. This signal is rectified and filtered by diode D1, capacitors C38
and C39, and resistor R15. Capacitor C37 establishes the broadband characteristics of the circuit.
Reflected Directional Coupler. The reflected voltage sample is obtained from a microstrip on the
circuit board near the output line. This signal is rectified and filtered by diode D2, capacitors C40
and C41, and resistor R13. Capacitor C41 establishes the broadband characteristics of the circuit.
The directivity of the circuit is adjusted by null control R12.
5.12.2.3
INPUT MUTE CIRCUIT
The input mute circuit consists of transistors Q5 and Q6 During normal operation, +20 volts is
routed to input amplifier U2 through Q5. When the exciter is muted, the final +V supply is
terminated. The loss of this potential will bias Q6 OFF and disable Q5 which terminates the +20
volts to U2.