A
737-300/400/500MAINTENANCE MANUAL
(b)
The antenna radiates the RF energy. The reflected RF energy from targets is received by the antenna. The antenna then directs this energy to the receiver circuits in the R/T thru the circulator. The TR limiter at the output of the circulator protects the receiver from high power transmitter pulses leaking through the circulator.
(c)
The received 9345 MHz is applied to the mixer along with a 9285 MHz local oscillator frequency to produce a 60 MHz IF frequency.
(d)
The local oscillator is maintained at exactly 60 MHz below the magnetron frequency by the automatic frequency control (AFC) circuits. The AFC mixer circuits receive samples of the actual magnetron frequency plus the local oscillator output. If the magnetron frequency drifts, a difference frequency will be applied to the AFC circuit, which will retune the local oscillator to 60 MHz below the magnetron frequency.
(e)
The 60 MHz mixer output is applied to an IF amp and video detector circuit along with the STC gate and video blanking pulse.
(f)
The STC gate reduces the sensitivity of the receiver for close-in targets; the video blanking circuit blanks the indicator display during the transmission interval.
(g)
The detected video is passed on to the indicator and Automatic Gain Control (AGC) circuit. When the Gain Control Knob on the indicator is in PRESET, the AGC circuit controls the gain of the receiver.
(h)
Power supply failure, overtemperature and modulator fault (when sensed by the modulator fault detector) de-energize relay K1 and disconnect power to the radar.
(i)
For maintenance purposes, a MOD ON PUSH/MOD OFF PULL switch is included in the R/T to activate/deactivate the modulator in the R/T and prevent radar pulse transmission from the antenna. Access to this switch requires removal of the R/T dust cover. When the dust cover is in place, the modulator on-off switch is interlocked in ON position.
Indicator
(a)
The weather radar system indicator unit is a high resolution color indicator which displays weather, operating mode, and fault information on a 5-inch CRT. The CRT uses an X-Y raster scan.
(b)
The indicator contains the video digitizer, level conversion, and second video amplifier circuits, a 5-inch CRT with its associated high voltage power supply, and self-test, range mark, electromagnetic deflection, and sweep circuits. It is self-contained except for dc control signals from the receiver-transmitter.
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(c)
(d)
(e)
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737-300/400/500MAINTENANCE MANUAL
The indicator uses digital circuits, integration/averaging, and
memory storage to provide a low noise non-fading display. The
digital display is refreshed at a 60.7 Hz frame rate by video
information stored in memory. The solid state memory holds the
image between sweeps, with the data sweep displayed on the CRT
moving in synchronism with the antenna's scan.
The indicator control panel contains pushbutton mode selector
switches, rotary TILT, GAIN, and INT control knobs, rotary
RANGE and STAB selector knobs, and pushbutton AZIM, SEC SCAN,
FRZ, and TGT ALRT switches.
The radar operating modes selectable on the radar indicator in
the control cabin are as follows:
1) OFF - Shuts down all operation of the weather radar system and removes operating power from weather radar components. If all mode select switches are out of their detent position, PUSH will be displayed in the mode field (Fig. 4).
2) STBY - Is selected from a cold start. The radar begins a 70-second warmup period, and STBY will be displayed in the mode field. After the warmup period the radar is ready to operate in any mode. If the WX, CYC or MAP mode are selected directly from a cold start, the system will still go through the 70-second warmup period before it will operate and WAIT (Fig. 4) will be displayed in the mode field.
3) WX (weather) - Places radar system in the normal weather detection mode of operation. The indicator shows areas of significant rainfall as colored areas on the display (Fig. 4). Areas of heaviest rainfall will appear in red, the next lower level of rainfall in yellow, and the least rainfall in green. A color-bar legend, in the upper right corner of the display, defines the colors. WX will be displayed in the mode field. Either the variable gain or the PRESET operation of the GAIN control may be selected in this mode.
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