(2)
When the key or PTT switch on the microphone is activated, a relay in the power supply switches +20-volt dc power to the transmitter circuits in the receiver/exciter. The key-line or (PTT) signal is also applied to a transistor switch that enables the audio amplifier. The audio amplifier, whose gain is controlled by a compression amplifier, supplies a constant audio peak voltage output to a balanced modulator. The constant optimum audio level reduces the intermodulation products generated in the modulator. A part of the audio output from the audio amplifier is the sidetone signal. Thus, the audio being used to modulate the transmitter can be heard on the audio output system.
(3)
Within the balanced modulator, the audio signal modulates the 500-kHz injection signal from the frequency synthesizer. The 500-kHz output of the modulator is amplified by an IF amplifier whose output is applied to a lower sideband (LSB) mechanical filter. The filter removes the upper sideband (USB) and 500-kHz carrier and passes only the lower sideband. Another IF amplifier then amplifies the LSB signal to a level adequate to drive the first mixer of the up-converting frequency translator. When transmitting in AME mode, this IF amplifier also receives a 500-kHz carrier signal, from the frequency synthesizer through a carrier injection switch.
(4)
Within the first mixer, the LSB signal (and 500-kHz carrier, if in AME mode) is mixed with the fixed 69.3-MHz signal from the frequency synthesizer. Only the 69.8-MHz output of the mixer passes through a 69.8-MHz crystal filter and then into a second mixer. Within the second mixer, the 69.8-MHz signal is mixed with the variable 72.6- to 96.799-MHz signal from the frequency synthesizer. The resulting output of the second mixer is a 2.800- to 26.999-MHz signal. Due to the high side injection into the mixer, the LSB input becomes the USB output. Thus, within the dual mixer circuit, the 500-kHz LSB signal is first translated down to the 2.800- to 26.999-MHz USB signal. It should be noted that the LSB mechanical filter, 69.8-MHz crystal filter, and two mixers are also used in the receiver of the transceiver and will be discussed later.
5C8 23-11-01 Jun 20/85 Page 8
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
(5)
The output from the second mixer is filtered by a low-pass (LC) filter and applied to an RF amplifier through an automatic load control (ALC) attenuator. The nominally 20-milliwatt pep output of the RF amplifier is then applied to a 4-stage power amplifier in the power amplifier. The power amplifiers raise the 20-milliwatt pep input to a nominally 200-watt pep output that is applied to a low-pass filter. The power amplifier has protective circuits that provide almost instantaneous shutdown of the power amplifiers in the event of excessive power amplifier internal power dissipation.
(6)
Within the low-pass filter, the RF signal is routed through one of the six filters that has been selected, by the motor-driven rotary band switches, for the operating frequency band. These six fixed-tuned, low-pass filters cover the frequency band from 2.8 to 26.999 MHz and remove the harmonics of the output frequency.
D. Receive Mode
(1)
In the receive mode, the transceiver receives 2.800- to 26.999-MHz USB and AM signals from the external antenna coupler. The receiver uses dual conversion to provide 200-mW received voice audio output during transmit mode.
(2)
The 2.800- to 26.999-MHz signal from the antenna coupler is routed, through the transmit-receive relay in a low-pass filter, to an RF bandpass LC filter in the receiver/exciter. The filter output is applied to a push-pull RF amplifier through an AGC-controlled pin diode RF attenuator. The output from the RF amplifier is then routed through the mixers and filters that are used in the receiver, as previously mentioned.
(3)
In the first mixer, the 2.800- to 26.999-MHz output from the RF amplifier is mixed with the variable 72.6- to 96.799-MHz signal from frequency synthesizer. If the received RF signal was a USB signal, the output of the mixer will be the LSB, due to the high side injection into the mixer. Only the 69.8-MHz output of the mixer passes through a 69.8-MHz crystal filter and then into a second mixer. Within the second mixer, the 69.8-MHz signal is mixed with the fixed 69.8-MHz signal from frequency synthesizer. The 500-kHz mixer output then branches to either the SSB or AM IF sections of the receiver.
(4)
In SSB operation, the output of the mixer is applied to the LSB 500-kHz mechanical filter. The LSB output of the filter is amplified by the IF amplifiers and then applied to the product detector. Within the product detector, the LSB signal is combined with a 500-kHz injection signal from the frequency synthesizer. The audio signal is amplified by an audio amplifier whose output is then switched to the final audio amplifier. The 200-mW output of the final audio amplifier is applied to the external audio system. The output of the first audio amplifier is also used to develop the AGC voltage for the IF amplifiers. The IF AGC voltage is also used to develop the RF AGC voltage to control the input RF attenuators.
中國航空網 www.k6050.com
航空翻譯 www.aviation.cn
本文鏈接地址：737 AMM 飛機維護手冊 通訊 COMMUNICATIONS(7)
