(e)
The reflected signal is received by the antenna during the interval between transmission pulses, and applied through the duplexer to the receiver circuits of the R/T. The receiver consists of the preamplifier, mixer and second intermediate frequency (IF) amplifier. The limiter in the preamplifier protects the receiver from power pulses, allowing low power received signals to go to the mixer in the receiver.
(f)
The mixer uses a local oscillator (L0) frequency from the frequency source to develop the first IF frequency. The first IF amplifier provides a 25 dB gain. The second IF is mixed with the second and third lo frequencies from the frequency source. The second IF amplifier gain is controlled by the automatic gain control (AGC) and sensitivity time control (STC) circuits from the digital sampler circuits.
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(h)
(i)
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737-300/400/500MAINTENANCE MANUAL
The amplified second if signal is power divided into two signals that are applied to a synchronous detector containing two mixers. A third lo signal developed by the frequency source is phase split and also applied to the two mixers. One output of the second if is in-phase I return signals. The other output is out of phase with the transmitted frequency which is the Q return signal.The I and Q return signals are applied to the digital weather processing circuits. The weather processor consists of a sampler, digital signal processor (DSP) and circuits within the central processing unit. The DSP has three DSP's that process the signals. Each DSP performs a specific function in the processing of the returning signals. The DSP get the digitized returns from the sampler and process these return signals. The I return and Q return signals are converted to digital signals within the sampler. The AGC and the STC circuits are used to normalize the voltage applied to the analog-to digital converters. This feedback to the second if amplifiers improves the performance of the signal processor by minimizing any noise from the second if amplifier on lower level signals. The digital information is applied to a DSP in the sampler circuit which also provides driver and transmitter triggers to the transmitter 777.77 MHz driver.Weather Processing 1) The detected I and Q returns are 90 degrees out of phase
that represent the magnitude of the I and Q component of the returned signal. These signals are sent from the second IF to an analog-to digital converter that converts the signals to a digital format. The conversion produces digital range samples. The digital information is arranged into 256 range bins. Each range bin represents the magnitude of the return signal at a particular distance from the airplane along the azimuth line the antenna was pointed. The range bins then are applied to the azimuth filter that act in a similar manner to the range filter. The azimuth bin data is then applied to a dual port RAM that can be accessed by the DSP. Much of the signal processing is handled by the software loaded into the DSP from the CPU.
2) Digital signal processor DSP 1 controls the system timing, receiving control data from the CPU, and continues the signal processing of data received from DSP 0 in the sampler circuit. The results of the DSP 1 signal processor are sent to DSP 2. DSP 1 gets control data from the CPU. Control data consists of system status, mode, tilt, gain, range and ground suppression. DSP 1 is the main communication between the CPU and the other DSP processors within the R/T.
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737-300/400/500MAINTENANCE MANUAL
3) When the R/T is operating in a weather radar mode, DSP 1 calculates power and varience values for each range bin. The weather processor is split into two processes. The weather process is run during both left-to right and right-to left scans. a) Selected range less than 60 nmi b) Selected range greater than 60 nmi. c) Digital signal processor DSP 2 continues the signal
processing of data received from DSP 1. The results of the DSP 2 signal processing are sent to DSP 3. DSP 2 determines when the normal scan or self-test process will be engaged. In the weather mode DSP 2 will send varience and magnitude data for each range bin to the DSP 3. An encoder in the weather processor takes the output of the azimuth filter and separates the returns into a 3-bit bin representing the rainfall intensity. This 3-bit bin is for a certain range and azimuth. DSP 2 also passes along system status information (slave, ground clutter, stabilization, faults, mode, tilt, gain, and airplane data such as IRU information to DSP 3. The ground clutter circuit starts operation when the IDENT (IDNT) switch on the WXR control panel is selected.
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