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A. A reflectometer used for maintenance testing of rf power and VSWR measurements may be permanently installed in the waveguide run at the R/T units (Fig. 1). The incident and reflected wave test ports on the reflectometer have protective covers that are removed during maintenance tests.
10. Dummy Load
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A. A dummy load mounted on the rear of the R/T mount accepts and dissipates the output RF from the R/T unit when the TEST mode is selected on the pilots' control panel. The use of the dummy load permits a check of most major functions while in the TEST mode, without RF pulse radiation from the antenna.
11. Operation
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A. Functional Description, Video Generation and Display (See figure 3.)
(1)
The timing circuits within the R/T produce a 200 PPS master trigger which is applied to drive the solid-state modulator. A high voltage pulse from the modulator causes the magnetron transmitter to produce a 9375 MHz pulse of rf energy. The high power rf is applied via a duplexer, directional coupler and waveguide switch to the antenna feed. During the TEST (test pattern generation) mode, the waveguide switch shunts the high power rf from the R/T to a dummy load. A small portion of the transmitted rf is taken from the directional coupler and applied to the AFC circuits.
(2)
The reflected radar return signal is applied from the antenna via the waveguide switch to the directional coupler in the R/T. This rf return is then passed by the duplexer through an rf limiter to the tunnel diode preamplifier. The rf limiter protects the tunnel diode from high power rf energy either from this radar or from other X-band radars in the vicinity. The tunnel diode is a low noise, solid-state amplifer that keeps the overall noise figure of the receiver low. The 9375 MHz output of the tunnel diode preamplifier is applied to a balanced mixer together with a 9315 MHz signal from the first local oscillator. The resulting 60 MHz difference frequency signal is applied to the 60 MHz preamplifier.
(3)
The output frequency of the first local oscillator is precisely controlled about the nominal 9315 MHz by the AFC circuits. The AFC mixer receives a nominal 9375 MHz signal from the directional coupler and a nominal 9315 MHz signal from the first oscillator. The resultant 60 MHz difference frequency is mixed with a stable signal from the 46.5 MHz oscillator to produce a 13.5 MHz AFC IF signal. This signal is amplified and applied to a discriminator whose output is at a null when the input is 13.5 MHz. If the input to the discriminator drifts from 13.5 MHz a control signal is produced via a pulse amplifier and sweep generator and applied to the first local oscillator. The output of the first local oscillato is thereby shifted sufficiently to cause the output of the discriminator to return to a null condition. The AFC circuits, therefore, cause the output of the first local oscillator to follow the frequency changes of the transmitted signal to maintain a constant 60 MHz first IF.
EFFECTIVITY
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(4)
The stable 60 MHz IF is amplified, applied to a second mixer, and reduced to 13.5 MHz. The 13.5 MHz is amplified and detected. The resultant radar video signal is further amplified and applied to the video amplifiers of both radar indicators.
(5)
Within the receiver, AGC circuits vary the gain of the IF amplifiers to compensate for varying signal strengths. Sensitivity Time Control (STC) circuits provide gain compensation to prevent close-in targets from being distorted in size and brightness due to the inherent properties of the radar system.
(6)
Contour circuits in the R/T provide an iso-echo display of weather targets at the operators discretion. With contour operation, the pilot can see storm areas in his flight path and can also distinguish corridors of relative calm through the storms. Contour discriminates against strong returns from high density rainfall and converts them into dark areas on the radar display. These dark areas are surrounded by lighted areas or rings which represent areas of lower rainfall rates.
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