9. Operation
A. Functional Description (Fig. 3)
(1)
The ADF system operation consists of two modes of operation: an operational mode and a test mode. The operational mode is the normal operation of the ADF receiver using antenna signals. The test mode uses only internal signals to assure operation of the receiver.
(2)
Operational Mode
(a)
The rf signal is coupled from the directional fixed loop antenna through the quadrantal error corrector to the goniometer in the receiver. The angular position of the goniometer rotor determines the input amplitude and phase to the loop rf amplifier. The loop rf amplifier output is applied to the balanced modulator where the signal is modulated at 90 Hz by the switching oscillator. The balanced modulator output is double-sideband, suppressed-carrier signal.
(b)
The sense signal originating in the sense antenna is transferred through the sense antenna coupler to the sense rf amplifier where the signal is amplified and applied to the signal mixer. The signal mixer output is a phase-modulated signal with a 90-Hz modulation frequency. The direction and phase is determined by the difference between the rotor position of the goniometer and the null direction of the station.
(c)
The amplified loop and sense antenna signals are coupled through a 0.19- to 1.75-MHz bandpass filter to the first rf mixer and heterodyned with the frequency synthesizer (vco) output. The first rf mixer signal is then applied to the 15-MHz crystal filter. The 15-MHz crystal filter output is routed through the agc attenuation circuit to the 15-MHz IF amplifier. The agc attenuator and 15-MH z IF amplifier are gain controlled by the agc amplifier.
(d)
The second mixer inputs are 15 MHz from the IF amplifier and 18.6 MHz from the vco. From the second mixer, the 3.6-MHz difference frequency is filtered, amplified and applied to the bearing detector for phasing information. The detector output is applied to the servo-phase detector along with 90 Hz from the switching oscillator to be amplified and applied to the loop servomotor for nulling the goniometer. Also, the detector applies outputs to the audio compressor and the agc amplifier. The audio compressor signal is amplified and applied to the flight interphone system (Ref 23-52-0).
(e)
The loop servomotor also drives the bearing synchro which applies outputs to the loop antenna position indicator on the front of the ADF receiver and also the bearing indicator on the RMI's.
(f)
The TONE/OFF switch allows for a BFO tone (1020 Hz) to be injected into the 3.6-MHz IF amplifier. When a CW station is being received, the 1020-Hz output can be heard in the flight interphones. If the TONE position is not selected when receiving a CW station, the 1020-Hz signal will not be heard.
(g)
The mode select switch on the ADF control panel allows selection of the ADF or ANT positions. In the ADF mode, both the loop and sense antennas are used to give an output to the bearing indicator (RMI) and the flight interphone system. In the ANT mode, the sense antenna is used to receive a station and is only used for monitoring the audio of the station selected.
(h)
Frequency selection is accomplished with the use of the frequency selector knobs on the ADF control panel. BCD (binary coded decimal) information is fed to the vco and the band logic circuits. The BCD signals are changed to an incremental voltage and applied to the bandpass filter to change the frequency receiving characteristics.
(3)
Test Mode
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34-57-01 Page 8 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. Aug 15/79
(a) When the self-test switch is energized, a test enable signal is applied to the test generator and the beat frequency oscillator (BFO). The test generator applies a predetermined signal to the goniometer and the sense rf amplifier. The goniometer phasing is unbalanced and causes the receiver to slew to a null position. With a signal applied to the sense rf amplifier, a predetermined sense signal is processed by the ADF receiver. By comparing the loop and sense outputs, a relative bearing of 315 degrees is displayed during self-test. In TEST mode a relay (if installed) disables input from the loop antenna to prevent strong broadcast signals from entering the test loop.
B. Controls
(1)
Prior to operating the ADF system, electrical power must be provided to the airplane (Ref 24-50-0, Electrical Load Distribution).
(2)
The INST XFMR-1, INST XFMR-2, ADF 1 and ADF 2 circuit breakers on the P18 and P6 electronic circuit breaker panels must be closed.
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