POWER AMPLIFIERS (TUBE) William C. Brown, Raytheon Company, Waltham, Massachusetts 02154 The microwave tube devices that have been proposed to meet the SPS transmitter requirements of very high efficiency, low mass, long life, high temperature operation, and low radio frequency interference are the klystron, magnetron, amplitron, and two new devices, the gyrotron and the photoklystron. The klystron and the magnetron in its directional amplifier form are the furthest advanced and have received the most attention. The klystron approach proposes a 70 kilowatt design with depressed collectors and recycled DC power for high efficiency and a heat pipe system to radiate the heat. The magnetron directional amplifier approach proposes an efficient 3-5 kilowatt tube scaled from the microwave oven magnetron and an attached radiator made from pyrographite to passively radiate heat. The operating principles of the klystron and the crossed field device in either its magnetron or amplitron configuration are shown in Figure 1. In the klystron the energy of the power supply is converted into kinetic energy of the electron stream. The electron stream is then velocity modulated so that the electrons bunch together. These bunches are then abruptly slowed as they pass through the output cavity and most of their energy is converted into micro wave energy. The left over energy may be partially extracted in the form of DC power from a series of depressed collectors. The DC power is reprocessed and added to the power feeding from the power supply. The crossed field device works on a different principle in that the electrons are just given a small amount of kinetic energy to become synchronous with the microwave circuit. From that time on there is a direct conversion of the potential energy of the power supply into micro wave energy. The microwave generator’s ability to operate efficiently and to dispose of waste heat by operating at a high temperature dominate the design of the microwave transmitter. Figure 2 shows the amount of microwave power that can be radiated per unit area as a function of the efficiency and operating temperatures of the tube, and indicates the comparative capability of crossed field generators, klystrons, and solid state devices. The maximum efficiency that has been achieved from a klystron is 75% while the efficiency that has been achieved by both magnetrons and amplitrons is in the 83 to 85% range. Top efficiency from a klystron after a substantial development program is expected to be 85%. A similar effort could increase the crossed field device efficiency to 90%. Because this symposium places emphasis upon recent technology developments much of the remainder of this extended abstract will review an ongoing investigation of a power scaled version of the microwave oven magnetron as a potential generator for the SPS. A principle item of interest is the noise measurements that have recently been made on the common microwave oven magnetron. Making use of a special measuring technique in which a high-power, narrow-band notch filter rejects all but one part in 100,000 of the carrier signal to permit a spectrum analyzer to be exposed to the full level of the noise output, signal to noise ratios of 180 to 190 dB/Hz in selected tubes have been measured. The measurement sensitivity is still limited at frequencies outside of a 60 MHz band centered on 2450 MHz by the residual noise level of the spectrum analyzer. To place these measurements in perspective, such high ratios means that an 8 gigawatt SPS transmitter would radiate less than 2.5 milliwatts of noise for each megacycle of the frequency spectrum.
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