G.  In VOR operation, two signals (30 HZ REF and 30 Hz VAR) radiated by a VOR ground station are fed to both the manual and automatic sections of a navigation unit. The 30-Hz REF signal is fed to both automatic and manual resolvers. The signal from each resolver is then phase compared against the 30-Hz VAR signal to produce the station bearing, VOR deviation and TO-FROM signals. In LOC operation, the deviation signal is produced by a comparison of the amplitude difference between two tone modulated signals (90 and 150 Hz) radiated by a runway localizer transmitter. Portions of these signals are summed to produce the warning flag signals. The GS deviation and warning flag signals are produced in much the same manner as the LOC deviation and warning flag signals.
H.  A VOR/ILS (LOC and GS) test circuit is provided in each VOR navigation unit to enable a check to be made of the VOR/ILS circuit function. Test switches are provided for this purpose on the control panels. Function of the test circuits is described under Operation.
2.  Navigation Unit
A.  Each navigation unit contains planar construction-type circuit boards and an interconnect unit. The circuit boards consist of VOR/LOC and glide slope receiver assemblies with associated instrumentation, digital instrumentation and self-test circuit board assemblies, an RMI driver and a power supply. These assemblies process VOR, localizer and glide slope signals and provide outputs to drive deviation, flag and to-from flight instrument displays.
B.  The chassis provides the mounting surfaces for connectors, support for two swing-out mounting frames, and a center mounting board.
C.  The center mounting board consists of a removable printed circuit board that contains the power supply, audio amplifier, and RMI driver circuits.
D.  The left swing-out mounting frame supports the VOR/LOC receiver board and the glide slope receiver board. Each board contains tuner, synthesizer, self-test, and injection oscillator circuitry. The VOR/LOC receiver is on the outer board and the glide slope receiver is on the inner board.
E.  The right swing-out mounting frame supports board for interconnections and the remaining VOR/ILS circuits. The VOR/LOC instrumentation and 2 out of 5 circuits, and optional expanded VOR monitor, programmable bearing filter, and digital bearing output circuits are on the inner board. The glide slope instrumentation, digital instrumentation, and optional VOR/ILS self-test and expanded ILS monitor circuits are on the outer board.
3.  VOR/LOC Antenna
A.  The VOR/LOC antenna assembly is integral with the fin cap installed on top of the vertical stabilizer. The fin cap has metallic leading and trailing edge tips mounted on a fiberglass shell to provide an aerodynamic surface. The antenna consists of two balanced half loops with a hybrid coupler added for increased reliability under fault conditions. The hybrid coupler is composed of two quarter wavelength lines with the dual outputs to the receivers shunted by a 100-ohm resistor. In addition, a 18.5-inch tuning stub is connected to each half loop. A lighting diverter strip is embedded along the top centerline of the antenna assembly.
4.  GS Antenna
A.  The GS antenna with dual output ports is a horizontally polarized unit installed above the weather radar antenna in the nose radome.
5C8 
34-31-01 Page 4  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Jun 20/85 

T53993
504  ILS Frequency Chart 
Feb 15/81  Figure 2  34-31-01 
Page 5 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

5.  Glide Slope Antenna Director Bar
A.  The glide slope (G/S) antenna director bar is a passive element of the glide slope antenna used to alter the G/S radiation patterns such that the navigation units have maximum glide slope sensitivity.
B.  The bar consists of a 13-inch strip of aluminum foil, pressure sensitive tape, installed horizontally inside the nose radome approximately 22 inches forward of the aft edge of the nose radome top center line.
6.  Localizer Antenna
A.  The localizer antenna with dual rf output ports is installed in the nose radome below the weather radar antenna. The rf ports are connected to separate ILS relays to provide localizer rf to the two navigation units.
7.  ILS Relays
A.  Two coaxial relays are installed on shelf No. 4 of electronic equipment rack E3 in the main equipment center. The relays are energized by approach mode logic in the AFCS when the associated navigation unit is tuned to an ILS frequency. The relays, when energized, switch the navigation unit VOR/LOC rf input from the fin tip VOR/LOC antenna to the nose radome localizer antenna. The glide slope superflag signals to flight computers A and B of the AFCS are also routed through energized contacts of the relays.
　
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