(6)  
The final audio amplifier has two other inputs. While the external antenna coupler is tuning, the 1000-Hz signal from the tune-tone oscillator is amplified. The other input is the sidetone signal that is produced during transmit mode.

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E.  Antenna Tuning
(1)  
Three functions are required to match the antenna to the transmitter: sensing, control, and tuning. All three occur at the same time in the coupler. After RF power is applied to the coupler, the discriminator samples the antenna coupler antenna input and develops dc voltages. These voltages are proportional to the following: the reactive component (phasing error); the 50 ohm impedance component (loading error); the forward power level; the reflected power level.

(2)  
The two error signals indicate the degree of antenna mismatch to the servo-control circuits in order that commands can be made for corrective tuning action. The forward and reflected power levels are used by the control circuits to determine the tuning sequence start and completion. To initiate and govern the corrective tuning action, the control circuits continually decode the sensed signals and all other control inputs. This includes such inputs as band information, key and tune commands, and equipment enable.

(3)  
A complete tuning sequence consists of five basic steps. These steps are: Home, Standby, Tune, Operate and Fault.

(4)  
The first four steps are controlled by a sequence counter which can only advance to a new position if the conditions of the existing step are satisfied. When a fault sensor (fifth step) is activated, the circuit overrides all other steps. A brief description of each sequence follows:

(a)  
Home

1)  When primary power is first applied, or when a new frequency is selected on the control panel, a ground is switched to the coupler. The coupler, in turn, sets a sequence counter to run the elements to the home position. In this position (home), the series capacitor, shunt capacitor, and shunt coil are effectively set to provide minimum attenuation. When all tuning elements reach their home position, a pulse is sent to the sequence counter to advance it to the standby position.

(b) 
 Standby

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Feb 20/87  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  23-11-02 Page 9 

1)  In the standby position, the coupler is set up to receive and the coupler is ready to start a tune cycle upon receipt of a key line ground. The receive path is from the antenna through a relay switch, the isolation amplifier, and from the discriminator to the receiver. If the other HF coupler is keyed, the relay switch de-energizes and shorts the receive signal to ground. When the coupler receives the key line ground return, the sequence counter advances the system to the tune position.
(c) Tune 1) Tuning is performed in three segments: segments A, B, and C. 2) In segment A, the series capacitor is servo-tuned to a zero-phasing error position. At some frequencies, the series capacitor or the shunt coil will be inserted and positioned due to band information commands so the series capacitor can produce a zero phasing error. The sequence counter then advances to segment B. 3) Segment B determines whether or not the shunt coil is needed for tuning. If, after segment A, the system impedance is less than 50 ohms (negative loading) the shunt coil will not be able to resolve the loading to 50 ohms. In this case, the coupler wil1 advance to segment C. This is typical of frequencies with the 2- to 10-MHz range. If the loading error was positive after segment A, the shunt coil will be switched on the line and servo-tuned to obtain negative loading. The phasing error will be kept at zero during segment B by the series capacitor. When the phasing error is zero and the loading error is negative, the sequence counter advances to segment C.
4)  During segment C, either the shunt capacitor of the shunt coil (as selected in segments A and B) is servo-tuned to produce zero loading error. The series capacitor is fine-tuned to retain a zero phasing error. At this point, the sequence counter advances to the operate position.
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(d) 
 Operate

1)  Upon reaching the operate sequence, the tune power and key lines have unlatched and the coupler is ready to transmit and receive. If a new frequency is selected, the sequence counter advances to the home step, and the complete turning sequence is repeated.

(e)  
Fault

1)  The coupler has circuits that detect faults and provide such indications on the HF fault detection panel. COUPLER FAULT circuit - indicates a failure of the coupler to home or tune within 15 seconds. RF FAULT circuit - indicates a lack of RF power due to a defective transceiver, an open coaxial, or insufficient power to allow the coupler to tune. PRESSURE FAULT circuit - indicates low pressure in the coupler case. TUNE IN PROCESS circuit - indicates when the coupler is in a tune mode. OPERATE circuit - indicates when the coupler has tuned.
　
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