777 AIRCRAFT MAINTENANCE MANUAL
HF SYSTEM -NORMAL TUNING
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HF SYSTEM -OPERATION SEQUENCE
General
The HF system operation is a series of modes:
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Home mode
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Standby/receive mode
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Tune mode
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Operate (receive/transmit) mode.
In all modes, the transceiver and antenna coupler constantly monitor for faults.
Home Mode
System tuning requires the system to enter the home mode. This happens when either of two events occurs:
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The system is off and you apply HF power
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You change the selected frequency during operation.
At power up or change of selected frequency, the coupler drives the tuning elements toward the home position of 2 MHz. This tuning disables the PTT signal. When the elements reach home, the home mode is complete. The system moves to the standby/ receive mode. The antenna coupler enables the PTT signal. The coupler sends the transceiver a fault signal if the elements fail to get to the home position in less than 15 seconds.
Standby/Receive Mode
The elements stay in the standby/receive mode until you push any mic switch. The system enters the tune mode when you push a mic switch.
Tune Mode
The PTT signal makes a key latch signal in the coupler. The key latch signal from the coupler keeps the transceiver in the tune mode. The transceiver sends low power RF to the antenna coupler. A 1 KHz sidetone from the transceiver tells the flight crew that tuning is in progress.
The coupler measures the phase difference between RF voltage and current. The coupler moves the elements for zero phase difference. The tuning elements adjust for a total impedance of approximately 50 ohms. The coupler maintains the voltage standing wave ratio (VSWR) so the RF power load is less than or equal to 1.3:1. The transceiver continues to send the 1 kHz sidetone to the flight crew. The coupler moves into the operate mode when all steps of the tuning mode are complete. The coupler sends the transceiver a fault signal for either of these conditions:
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The elements fail to tune in less than 15 seconds
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The VSWR is greater than 1.3:1.
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HF SYSTEM -OPERATION SEQUENCE
Operate Mode
In the operate mode, the key latch signal and the 1 KHz sidetone stop.
The system is ready to receive or transmit on the selected frequency. Push any mic switch to transmit.
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777 AIRCRAFT MAINTENANCE MANUAL
HF SYSTEM -OPERATION SEQUENCE
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HF SYSTEM -HOME/RECEIVE FUNCTIONAL DESCRIPTION
General
The left and right HF systems are similar. This example shows the left system. The home position is the starting point for the tuning circuits.
Home Sequence Mode
The radio tuning panel (RTP) sends tuning and modulation mode (AM or USB) data to the HF transceiver control circuits. When the RTP sends a change in frequency, the transceiver control circuits send a rechannel pulse to the antenna coupler control logic. This causes the coupler to enter the home sequence mode. The home sequence mode also starts at power up.
During the home sequence, the coupler control logic has these functions:
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Tells the tuning circuits to go to the home position
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Energizes relay K6
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De-energizes relays K4 and K5.
The tuning circuits have these components:
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Motors
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Variable capacitors
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A variable inductor.
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The control logic energizes K6 so that the HF system can receive during home sequence. When the tuning elements are in the home position, the HF system goes to the receive/standby mode. 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：777 AMM 飞机维护手册 通讯 Communications 1(6)
