B110 INS NO. 2 (E2-6) B140 RECEIVER/TRANSMITTER NO. 2 (E5)
B143 ANTENNA ASSEMBLY (NOSE RADOME)
Weather Radar System Schematic
Figure 3 (Sheet 4)

EFFECTIVITY CONFIG 3 AF 121,122

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Sep 25/94
E39300

 (c)  
Pitch and roll information from the inertial navigation system, tilt information from the manually set tilt control, azimuth angle information from the azimuth (scan) synchro transmitter, and elevation angle information from the elevation synchro transmitter, are processed by the stabilization microprocessor circuitry in the R-T to produce a control phase voltage for the elevation drive motor. The elevation drive motor repositions the antenna to maintain line-of-sight.

 (d)  
The elevation drive motor, is a two-phase servomotor which operates with a reference phase voltage of 115 volts, 400 Hz supplied to one winding and a 400-Hz control phase voltage of varying amplitude applied to the other winding. The reference phase voltage is applied at all times while the control phase is applied only when needed to reposition the antenna planar array in elevation as ordered by the stabilization microprocessor circuitry in the R-T unit. The control voltage is an error voltage supplied by the microprocessor circuitry. The elevation motor repositions the antenna in elevation until the phase of the error voltage induced in the control winding reaches a null. The speed at which this correction is performed is dependent on the error voltage amplitude.

 (e)  
The R-T unit also supplied the azimuth motor drive signal which drives the antenna through the oscillating 180 degrees of azimuth (scan). Controlled by the microprocessor circuitry in the R-T unit, the 115-volt drive signal is applied to one of the dual windings in the azimuth motor. The motor rotation drives the gear train, which in turn drives the antenna to one side of the straight ahead zero-degree heading. The azimuth synchro continuously transmits the antenna angle position to the microprocessor circuitry. When the antenna reaches the 85-degree position, the microprocessor through triac switching removes the 115-volt drive signal from the azimuth motor. The antenna continues to coast in the direction of its movement until the microprocessor circuitry determines the antenna has reached 90 degrees, at which time the 115-volt drive signal is applied to the other of the dual windings in the azimuth motor to drive the azimuth motor in the reverse direction. The antenna therefore is driven in the reverse direction, from the 90-degree angle position through the straight-ahead, zero-degree position and toward the 90-degree angle position of the opposite side. The microprocessor circuitry again removes the 115 volts from the azimuth motor when the antenna position has reached 85 degrees and the antenna coasts to the 90-degree angle position. The microprocessor circuitry applies the 115-volt drive signal to the motor winding which initiates reverse rotation each time the antenna position angle is 90 degrees, causing the antenna to constantly scan the 180-degree sector straight ahead.

 (5)  Receiver/Transmitter Functions (Fig. 3)
 EFFECTIVITY

 AF 121, 122; SN 351-399  CONFIG 3  02 Page 21  Apr 25/9834-43-00
 (a)  
The R-T unit is a lightweight airborne unit consisting of a transmitter, receiver, video processing circuitry, digital interfaces, stabilization servo-loop circuitry, system monitoring circuitry, and power supply.

 (b)  
The transmitter consists of a driver stage, which receives its input from crystal-controlled reference oscillator, and a power amplifier output stage. Output of the transmitter is at 9345 MHz and is a nominal 125 watts peak; pulse width alternates between 6 and 18 microseconds. The 6-microsecond pulse echoes are processed to produce the zero to 40 nautical mile targets on the indicator; and the 18-microsecond pulses echoes are processed to produce the targets greater then 100 nautical miles. Echoes from both the 6- and 18-microsecond pulses are processed to produce the targets between 40 and 100 natical miles.

 (c)  
Because the receiver and transmitter both receive their input from a crystal-controlled reference oscillator, and afc circuit is not necessary. The receiver is a triple-conversion superheterodyne that uses two stages of Gallium-Arsenide FET amplifiers (Ga-As FET) low-noise amplifiers in the front end. The third i-f amplifier is a logarithmic amplifier that has a 60-dB dynamic range; it provides the capability to detect and process weather targets having intensity levels from 0 dBZ to 60 dBZ. The receiver design provides an overall MDS (minimum discernible signal) of -121.9 dB (typical) for the system.

 (d)  
The video circuits first convert the analog video signals into a digital format. The video processor adjusts the signal level with sensitivity timing control (STC). This allows nearby echo signals to be displayed with the same intensity as distant ones. STC is effective to about 70 Nm. In the MAP mode, STC is modified to optimize analysis of terrain features.
　
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