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A system is described whereby an oscillator is either swept continuously across its tuning range or is tracked to an incoming signal. The novelty of the system is that it automatically switches from the sweeping regime to the tracking regime when a signal is encountered and returns to the sweeping regime when the signal vanishes. The control is applied to a superheterodyne receiver whose intermediate frequency is 30 Mc. In the searching mode, the local oscillator is mechanically swept back and forth across its entire tuning range. When a signal is received in the intermediate-frequency channel, the sensing circuit transfers the system to the tracking regime so that the local oscillator keeps the signal centered on the intermediate-frequency passband. In this application, the system is capable of varying the frequency of a mechanically tuned ultra-high-frequency triode oscillator over a 100-Mc range, or of holding it to within 15 parts per million at 1,300 Mc.