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This paper describes a new arrangement for generating short pulses. The circuit consists essentially of a loop containing an amplifier, expandor, bandpass filter, a time delay, and a means of automatic gain control. The circuit is controlled to give a round trip gain of unity for the pulse. Under this condition a pulse travels around the loop, alternately being sharpened in the expandor and broadened in the filter. An output tap delivers identical pulses recurring at a rate determined by the loop delay, shape determined principally by the expandor and filter characteristics, and length controlled principally by the filter bandwidth. Pulses modulated in frequency may be obtained by adding a nonlinear phase-frequency characteristic to the loop delay. Microwave radio frequency pulses as short as 0.002 microsecond, and direct current pulses as short as 0.005 microsecond (at half-amplitude) have been produced directly. The principles involved may have application in pulse communication and radar, and have already found use in pulse measurements of broadband circuits.