Glideslope Receiver - Honeywell Bendix/King KX 165A Maintenance Manual

BENDIX/KING KX 165A
4.2.4

GLIDESLOPE RECEIVER

4.2.4.1 Principles of glideslope system
The glideslope signal is radiated by a directional antenna array located near the approach end of
the runway. The signal consists of two intersecting lobes of RF energy. The upper lobe contains
90Hz modulation and the lower lobe contains 150 Hz modulation. The equal tone amplitude in-
tersection of these two lobes forms the glide path. A typical glide angle is 2.5 degrees. If the air-
craft is on the glide path, equal amplitudes of both tones will be received and the deviation bar will
be centered. If the aircraft is above the glide path, 90 Hz modulation predominates and the visual
display is displaced downward. If below the glide path, 150 Hz predominates and the display is
displaced upward.
4.2.4.1.1 GS Receiver
Refer to the Figure 4-7 for a block diagram of Glideslope RECEIVER BOARD 200-09447-0000.
The Glideslope Board contains a receiver that may be tuned throughout the Glideslope frequency
band of 329.15 to 335.00 MHz. The Glideslope operation frequency is paired with a selected Lo-
calizer frequency. Therefore, Glideslope operating frequency is not viewed/selected directly from
the front panel. The radio selects the proper Glideslope frequency based upon the Localizer chan-
nel.
Digital and analog I/O comes from the VHF Main Board via a 12 pin ribbon cable. The receiver
gets its synthesizer tuning, and DC power from the VHF Main board. The RF Input comes in ex-
ternally through the antenna connection on the back of the radio. There is a rear edge card con-
nector that is used to output analog converter signals necessary to drive an external indicator.
The RF signal coming from the antenna connector passes through a dual gate FET VGA (variable
gain amplifier) and a fixed gain MMIC amplifier. This amplified signal is the RF input to a mixer
with a LO 30 kHz below the RF and an IF of 30 kHz. The IF Filter and Amplifiers consist of three
fixed gain IF amplifiers, one IF VGA amplifier, and a 50 kHz 9-pole low pass filter. The 30 kHz IF
is passed through an AM detector resulting in an AC audio composite and a DC level. The AC
composite audio signal contains the 90 and 150 Hz tones needed to produce an up/down indica-
tion on an aircraft's indicator. The DC level is used by the IF and RF AGC amplifiers to maintain
a constant Detector input power.
The audio composite is digitally processed on the main board to determine the level of Glideslope
external indicator deflection and flag level. These values are sent from the Main Board to the
Glideslope RECEIVER BOARD via the serial data buss. A quad digital potentiometer receives the
serial data. One digital potentiometer controls the flag level. Two other digital potentiometers con-
trol the Glideslope indicator deflection. An IF AGC signal is also sent back to the VHF main board
for use in automatic test and for automatic detection of the presence of the 200-09447-0000
Glideslope Board. The IF AGC voltage is also available at the rear edge card connector for auto-
matic test use.
Rev 0, Jun/2000 15610M00.RCD Page 4-17