Honeywell SPZ-8000 Maintenance Manual page 130

MAINTENANCE
Honeywell !!!!&!!Sh.
The radio altimeter receiver transmitter provides a dc output voltage
which is proportional to the aircraft absolute altitude above terrain.
In addition, it provides radio altitude trip points, an indicator warning
flag output, and an auxiliary radio altitude output.
The precision output is
used to drive the PFD RAD ALT display and
supplies absolute altitude information to the flight guidance system.
The RT-300 transmitter section is an all solid state, two-transistor unit
operating
in a master oscillator, power amplifier configuration. The
transmitter is collector pulse modulated by a solid state modulator. A
sample of the transmitted signal is detected and applied to the processor
assembly as a To (system sync) pulse to initiate the timing sequence.
The r-f ground return signal from the receive antenna is input to the
receiver subsystem where it is amplified and converted to produce an i-f
signal. The i-f signal is amplified and detected to produce a video
pulse which is further amplified and applied to the processor.
Within the processor, the detected video pulse is compared to the system
sync pulse, and the time difference between these two pulses is
determined. This time difference is converted to a dc analog voltage
(internal range voltage) proportional to the shortest range to the
ground. The internal range voltage is fed to the output assembly for
further processing. An automatic gain control (AGC) voltage is derived
in the range tracker and fed back to the receiver assembly to maintain
the amplitude of the video signal at a constant level.
The processor also contains the search and acquisition circuits which
enable the system to initially acquire the ground return signal or
reacquire the signal should tracking be interrupted. The search mode is
initiated upon loss of signal. A sweep signal is generated which causes
the system to search for the earliest return signal. This process
continues until track is reestablished. Once track has been properly
established, the search mode is inhibited. A search valid is output to
the output networks to cause the indicator needle to disappear from view
during the search phase. A sensitivity time control (STC) voltage is fed
back to the receiver assembly to gain-program the receiver as a function
of altitude. Gain programming prevents acquisition of the direct antenna
leakage during the interval when the transmitter is on.
The internal range voltage is processed within the output networks to
produce dc outputs for driving the indicator needle and other aircraft
systems requiring radio altitude information. Four altitude trip outputs
which supply a ground at or below the preset altitude are provided. The
flag warn signal controls the indicator flag to provide warning of system
malfunction. In addition, a track invalid signal is available which
indicates if the RT-300 is not in the track mode. Two test inhibit
inputs are provided to permit defeat of the pilot-activated self-test
when the flight control system is engaged.
The self-contained power supply operates from the aircraft 27.5 V dc bus.
It provides all RT-300 operating voltages.
22-14-00
Page 142
Aug 15/91
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