(5)  Receive Mode
 (a)
In the receive mode, the HF transceiver receives 2.000 to

 29.999 MHz USB/LSB/SSB or AM signals from the external HF antenna coupler. The receiver uses dual conversion to provide 100-mW received voice audio output.

 (b)  
The 2.000 to 29.999 MHz signal from the HF antenna coupler is routed through the transmit-receive relay to an RF attenuator. The RF attenuator output is applied to an RF amplifier. The output from the RF amplifier is then routed to the receiver/exciter.

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737-300/400/500MAINTENANCE MANUAL
 (c)  
In SSB operation, the output of the mixer is applied to the SSB mechanical filter. The output of the filter 500 kHz is amplified by the IF amplifiers and then applied to the product detector. Within the product detector, the signal is combined with a 500 kHz injection signal from the frequency synthesizer. The output of the product detector is the detected audio signal. The audio signal is amplified by an audio amplifier. The output of this amplifier is then switched to the final audio amplifier. The output of the final audio amplifier is the audio output. The output of the first audio amplifier is also used to develop the AGC voltage for the IF amplifiers.

 (d)  
In AM operation, the output of the mixer is applied to an AM mechanical filter that is capable of passing both the LSB and USB signals. The 500-kHz output of the filter is amplified by the IF amplifiers and then applied to the AM envelope detector. The audio output of the detector is amplified by an AGC-controlled AM/SELCAL audio amplifier whose output is then switched to the final audio amplifier. The output of the final audio amplifier is applied to the external audio system. If a SELCAL signal has been received, the 0.5-volt audio signal is not switched but is applied directly to the external SELCAL audio system. The output of the first audio amplifier is also used to develop the AGC voltage for the IF amplifiers.

 (6)  Fault Lights
 (a)  
The LRU fail light is activated by faults such as frequency synthesizer out of lock, low transmitter power output, low power supply voltages, or failure of the frequency control board microcomputer.

 (b)  
The KEY INTERLOCK FAIL light is activated by HF antenna coupler faults such as excessive tuning time (more than 15 seconds), HF antenna coupler power failure, or excessive tuning reactance.

 (c)  
The CONTROL INPUT FAIL light is activated by such problems with frequency selection as absence of parallel data with HF label, or invalid format or faulty content of input from the HF control panel.

 F. Antenna Tuning
 (1)  Three functions are required to match the HF antenna to the transmitter: sensing, control, and tuning. All three occur at the same time in the HF antenna coupler. After RF power is applied to the HF antenna 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.
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 (2)

(3)

(4)

(5)

(6)

(7)

A 737-300/400/500MAINTENANCE MANUAL
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.A tuning sequence consists of four basic steps. These steps are: Home, Standby, Tune, and Operate. For airplanes with a coupler status annunciator panel, an additional step is needed: Fault.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:Home
　
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