The parameters that control a blocking oscillator's pulse shape are reviewed. Limitations in previous techniques of pulse duration control are described. It is shown that the pulse duration may be determined either by tube (or transistor) saturation or by saturation of the transformer core material. The latter mode of operation is preferred because it confers stability against aging of tubes or transistors, variations of ambient temperature, and changes in loading. Design criteria for establishing either mode of operation are given. The means by which an externally applied magnetic bias can be used to control the pulse width over more than a 10:1 range are described. A blocking oscillator circuit designed according to the principles discussed is presented, along with specific performance data.