5C8
Jun 20/85 23-11-01 Page 9
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
(5)
The 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 200-mW 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. The IF AGC voltage is also used to develop the RF AGC voltage to control the input RF attenuators.
(6)
The final audio amplifier has two other inputs. While the external antenna coupler is tuning, the 1000-Hz signa1 from the tune-tone oscillator is amplified. The other input is the sidetone signal that is produced during transmit mode.
(7)
The 200-watt PEP RF output from the low-pass filter is routed through a directional wattmeter and a transmit-receive relay to the external antenna coupler. Forward and reflected power detectors, within the directional wattmeter, provide voltages that are used to generate the ALC signal and to drive the power amplifier protection control circuit. The relay disconnects the receiver from the antenna coupler and connects the transmitter to the antenna coupler. Whenever the transceiver is in the transmit mode, an internal blower provides forced-air cooling to the transmitter circuits to prevent overheating.
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.
5C8 23-11-01 Jun 20/85 Page 10
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
(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
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 will 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.
中国航空网 www.aero.cn
航空翻译 www.aviation.cn
本文链接地址：737 AMM 飞机维护手册 通讯 COMMUNICATIONS(8)
