The output voltage and current characteristics of the solar array will vary with time, so that there is a problem of matching the DC characteristics of the solar array with the DC input characteristics of the tube. DC transformers are non-existent, except through transformation to AC power and back again, a procedure that introduces mass and heat losses intolerable in the SPS. Fortunately, all power conditioning of this kind can be eliminated with the magnetron by providing it with a buckboost coil and feedback loop which allow adjustment of the tube to the operating voltage that matches that of the solar array. The feedback arrangement holds the power output of the magnetron to a preset value within 3% even though the input voltage as provided by the solar array is changing by a large amount. In Fig. 5, for example, if the desired power output is set at 700 watts, this value is held to within 2% even though the input voltage is changing from 3400 to 4500 volts, or in the ratio of 1.32. These data were obtained experimentally, but are accurately predicted from formal control theory. Fig. 6 shows how the voltage-current characteristic of the magnetron equipped with the feedback loop matches the typical voltage-current characteristic of the solar array. The total system will operate where the voltage-current characteristics cross each other. The buckboost coil has other functions that have been investigated experimentally. It is used to hold off the anode current while the filament is being initially heated for a few seconds. It can also be used to shut off the tube should the rf drive be removed for some reason. Achieving High Gain and Phase Lock Simultaneously It is mandatory that microwave generators have a large amount of gain so that they can operate in the high temperature back side region of the transmitter while their low
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