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 8.  Control Panel (Fig. 1)
_____________
 A.  The control panel remotely controls receiver band switching, tuning, gain and operational modes by passive, electrical means. It is manually operated and provides the operator with visual indications of selected mode and frequency.
 B.  Frequency selection is controlled by a concentric frequency selection knob set below a frequency indicator. (AT airplanes have a dual unit consisting of two identical frequency selection knobs below the two frequency indicators). The panel also has a mode select switch (ANT/ADF Switch), a momentary pushbutton TEST switch, a BFO/OFF (TONE/OFF) switch, and an audio level VOL or Gain control knob (except on AZ231-AZ233 airplanes).
 C.  On AT airplanes, a two-position TFR (transfer) switch is provided that operates with the left and right frequency select tuning knobs and frequency readout indicators on the panel. Positioning the TFR switch left or right tunes the ADF receiver to the frequency that appears on the left or right frequency digital readout indicator as selected. The passive standby frequency is partially obscured by a horizontal bar across the face of the remaining indicator.
 EFFECTIVITY
 AF 121, 122, 152-199;

 IB 406-499;  LH 011-099; SN 303-399;  CONFIG 2 AT, AZ, GN, MD AND VM ALL  09 Page 5
   Sep 25/94

34-57-00
 D.  On AZ airplanes 231-233, the ADF control panels do not have a volume control. The ADF system audio level is adjusted using the volume (gain) control on the audio selector panel.
 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 (Sheet 1, Fig. 3). 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. The balanced modulator output is a 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 .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-MHz 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 23-51-00).
 EFFECTIVITY
 AF 121, 122, 152-199;

 IB 406-499;  LH 011-099; SN 303-399;  CONFIG 2 AT, AZ, GN, MD AND VM ALL  09 Page 6
   Sep 25/94

34-57-00
 (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 BFO/OFF or 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 outputs can be heard in the flight interphones. If the BFO position is not selected when receiving a CW station, the output of the ADF is unusable.

 (g)  
The mode select switch on the ADF control panel also 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.
　
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