(3)  
Power:

 (a)  Both the control panel and the transceiver are supplied with 28v dc from a single circuit breaker directly to the units with no switching. In the control panel the power supply generates operating power for the electronics. In the transceiver the power supply generates dc for electronics use, part of which is supplied to a switching device for controlling dc that is used during transmission only.

 (4)  
Frequency Selection:

 (a)  
The frequency selection data originates in the control panel as parallel bcd code, converted to serial bcd, formatted into an ARINC 429 word and transmitted to the transceiver over an ARINC 429 data bus.

 (b)  
GUN 051; In the transceiver, the data is routed to the microprocessor through port B of the port select switch. The microprocessor then issues serial frequency data to the synthesizer and a variable receiver tuning voltage to the receiver rf section.

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757 MAINTENANCE MANUAL
 (c)  
GUN 001-050, 052-999; In the transceiver, the data is routed to the microprocessor through port B of the port select switch. The microprocessor then issues serial frequency data to the synthesizer.

GUN 051; Transmit:

 (a)  
Transmission is initiated by grounding the PTT line. The PTT line causes the transmit dc to be applied to the transmission portions of the transceiver and to the antenna switch, which connects the transmitter output to the antenna while blocking the receiver input.

 (b)  
The PTT line is also routed to the SELCAL decoder via the pilot's call panel. Grounding the PTT line resets the SELCAL switch light associated with the transceiver selected on the audio selector panel.

 (c)  
The frequency is set by the microprocessor input to the synthesizer which in turn drives the transmitter rf drivers in the transmitter's rf sections. The modulator amplitude modulates the carrier in the transmitter rf section where it is further amplified and applied to the antenna switch, and on to the antenna.

 (d)  
The frequency synthesizer output is low level amplitude modulated by audio from the interphone system (or data from the ACARS) and amplified for transmission. The forward power output is sampled and fed back to the modulator where it is used to linearize the modulated drive signal, ensuring that the modulated drive signal accurately follows the modulation. Forward power is also sampled to generate sidetone audio to the voice audio output. The RF output is then routed through the antenna switch to the antenna.

 (e)  
In the transmitter rf section, the output power is sampled and used to regulate the output power to a nominal 25 watts and to generate a sidetone enable signal. The side tone enable signal, when true, gates the side tone audio into the audio output amplifier and on to the flight interphone system and the transceiver's front panel headset jack.

 (f)  
The PTT (or data key) signal is routed out of the transceiver as a key event signal. This is used by the digital flight data recorder system to record the times during which the transceiver is keyed.

GUN 001-050,052-999; Transmit:

 (a)  
Transmission is initiated by grounding the PTT line. This activates the antenna switch and switches the frequency synthesizer output to the transmitter section.

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757
MAINTENANCE MANUAL

 (b)  
The PTT line is also routed to the SELCAL decoder via the pilot's call panel. Grounding the PTT line resets the SELCAL switchlight associated with the transceiver selected on the audio selector panel.

 (c)  
The frequency synthesizer output is low level amplitude modulated by audio from the interphone system and amplified for transmission. The forward power output is sampled and fed back to the modulator where it is used to linearize the modulated drive signal, ensuring that the modulated drive signal accurately follows the modulation. Forward power is also sampled to generate sidetone audio to the voice audio output. The rf output is then routed through the antenna switch to the antenna.
　
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