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It is shown theoretically and experimentally that a tunable oscillator without spurious oscillations can be obtained by using a magnetostatic forward‐volume wave resonator with wide strip transducers. First, the equivalent circuit of a rectangular resonator is presented and the unloaded Q and the external Q are expressed in terms of the dimensions and the material constants of the resonator. By means of this expression, the condition is derived for selective excitation of the dominant mode and for suppression of the higher‐order modes. A 5‐GHz tunable oscillator using this resonator was fabricated. It was demonstrated that stable oscillation characteristics without spurious oscillations can be obtained.

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A novel stepped-impedance resonator with a thin metal saucer is proposed in order to reduce spurious-response levels in a coaxial-line bandpass filter (BPF). In designing the BPF, the mode-matching technique is employed for fast and accurate calculation of coaxial-line discontinuities. A prototype BPF obtains a spurious-response level of less than ؊55 dB in the frequency band from 2f 0 to 9f 0 , where f 0 is the pass-band center frequency.