A Resonator is a device or system that exhibits resonance or resonant behavior. That is, it naturally oscillates with greater amplitude at some frequencies, called resonant frequencies, than at other frequencies. The oscillations in a resonator can be either electromagnetic or mechanical (including acoustic). Resonators are used to either generate waves of specific frequencies or to select specific frequencies from a signal. Musical instruments use acoustic resonators that produce sound waves of specific tones. Another example is quartz crystals used in electronic devices such as radio transmitters and quartz watches to produce oscillations of very precise frequency.
A cavity resonator is one in which waves exist in a hollow space inside the device. In electronics and radio, microwave cavities consisting of hollow metal boxes are used in microwave transmitters, receivers and test equipment to control frequency, in place of the tuned circuits which are used at lower frequencies. Acoustic cavity resonators, in which sound is produced by air vibrating in a cavity with one opening, are known as Helmholtz resonators.
The resonator acts as a clock in the circuit, providing a pulse signal of a specific frequency for the entire circuit, so that each component in the circuit can complete the commands rhythmically under this signal.
An electrical circuit composed of discrete components can act as a resonator when both an inductor and capacitor are included. Oscillations are limited by the inclusion of resistance, either via a specific resistor component, or due to resistance of the inductor windings. Such resonant circuits are also called RLC circuits after the circuit symbols for the components.
Coupled resonators formed on both sides of a printed circuit board (PCB) are proposed. In the resonators, the lower of the resonant frequencies of two coupled modes generated by strong coupling of the two resonators on opposite sides of the substrate is much smaller than that of a single resonator. In addition, by making use of the extremely large difference between the resonant frequencies, a resonator with small size and a good spurious characteristic is realized. In the present paper, of the structures with especially strong coupling, coupled resonators of open ring, multi-open ring, and spiral shapes are studied.