User's Manual

Model 1150A DVOR
Rev. - November, 2008
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to others for any purposes without written permission from SELEX Sistemi Integrati Inc.
2-21
From U21 the demodulated signal is sent through a non-inverting unity gain buffer amplifier, U22:D. The output of
U
22:D is split into three different paths: CSB Power, Reflected Shutdown, and Detected.
The CSB Power circuit is non-inverting amplifier (U22:C) circuit that uses R85 and R87 to set the gain of the
circuit. Potentiometer R87 is adjusted to set the voltage level at TP20 to 3.0V with a 100W output CW RF power.
The output of the CSB Power circuit is split into two signals, one is sent to the backplane via the DB37 pin
connector, the other is sent through a non-inverting unity gain buffer amplifier, U22:A, to the front panel of the
module for an easily accessible test equipment connection point.
The demodulated signal from U22:D is sent through a voltage divider network R84 and R81. After the signal is
divided down, it is sent to the positive input of the voltage comparator (U23:C). The reflected carrier demodulated
signal is sent from the backplane via the DB37 connector to a differential amplifier U22:B. The gain is set for
U22:B with the potentiometer R76 and resistor R73. From the differential amplifier, the signal is amplified and sent
to the negative input of the voltage comparator (U23:C). The comparator is set such that a 3V reflected input
voltage will trip the comparator. The reflected input voltage is routed from the RF monitor, which scales the voltage
that is used for the reflected shutdown circuit within the power amplifier module. When the comparator trips, the
output voltage travels to a low state causing the one-shot timer U24:A to take the output low for a time period of 33
mS. The time constant for U24:A is set with R92 and C124. The Reflected Shutdown output is connected to the
backplane via the DB37 connector, to the reflected shutdown control, and to the latch U8:A for monitoring the
reflected shutdown through a serial interface.
The demodulated signal from U22:D serves as a feedback signal to the negative input of the error amplifier U12:A.
The positive input of the error amplifier is the reference that is generated by the audio generator CCA within the
VOR system. There is a shutdown control circuit consisting of U11, and Q1-Q5 for the input reference signal in the
event a fault in the power amplifier should occur. The carrier modulation voltage is routed from the backplane via
the DB37 connector to a differential amplifier, U10:A. The differential amplifier can be configured for single mode
using the J5 selection. The audio signal may be a sine wave input with a DC offset up to 10 KHz. The amplitude of
the sine wave may be a level capable of producing a minimum of 50% modulation on the output of the carrier
amplifier module. The output of the U10:B buffer amplifier is then sent to the input of R18 for scaling the input
audio signal for a 100W output. The output of the U11 analog switch shutdown circuit will travel to a ground
potential in the event a fault should occur in the amplifier module. The faults monitored for a shutdown to occur
within the amplifier assembly are Over Temperature, Over Power, and Carrier Reflected. The Q2, Q3, and Q4 fault
transistors are connected in series to the gate of Q1. If any of the fault transistors stop conducting, the gate of Q1 is
pulled high, pulling the gate of Q5 low and to a non-conducting state. The gate of Q5 may also be pulled low
externally to shut down the power amplifier assembly.
The SPI interface communicates with the amplifier assembly through U1:A. U1:A is used as a buffer for the SPI
bus. The SPI uses the general purpose I/O expander U2 along with U4, U6, and U7 for address decoding for serial
communication to the EEPROM, U3 and temperature sensor, U5. U1:B is only used for the SPI serial out of the
amplifier assembly. The general purpose I/O expander U2 in conjunction with U8 and U9 reports the status of faults
that have occurred in the power amplifier assembly.
The Over Power shutdown circuit serves to protect the amplifier assembly if an over power condition should occur.
The over power circuit consists of a voltage comparator U23:A and a one-shot timer circuit U24:B. The CSB power
signal from the detector is sent to the negative input of the voltage comparator. The negative input of the voltage
comparator also contains a low pass filter circuit R98, R99, and C128 that serves to reduce the modulation on the
line to for an average output power. The positive input of the voltage comparator is set with R100 and R101 voltage
divider. Once the power level out of the amplifier assembly reaches approximately 200 watts, the comparator output
will go low. The negative edge from the comparator triggers the one-shot timer low pulse for approximately 33mS.
The pulse time for the one-shot timer is set with R109 and C51. The over power signal is sent to the back plane, Q3,
and U8:B for an error status signal. When the over power circuit pulses low, Q3 will stop conducting and disable
the amplifier from transmitting for the period set by the one-shot timer.
The over temperature shutdown circuit consists of a voltage comparator U23:B with positive feedback R104 for
hysteresis purposes. Thermistor R108 is thermally connected to the amplifier heat sink. The value of the thermistor