Abstract: The structure and the performance of L-band VCO and the resonating cavity are described in this paper.I ts osillating frequency can be tunned in the range of 0.9-1.07GHz, and its output power is more than 10mW.It can operate in scan-frequency mode by using a saw tooth generator.The sample cavity is a square-gap resonator.When a cylindrical sample is inserted into the cavity, the resonating frequency changes only a little.The microwave energy is fed to the cavity by magnetic coupling.While changing the distance between the coils and the cavity the coupling degree can be changed.

Key words: ESR, Voltage controlled microwave osillator, Square-gap resonator