What are Tunable Resonators, and What do They Have to do With Filters and Oscillators? Part 1

A tunable resonator is a device used either to generate RF frequencies or select specific frequencies from a signal. It exhibits resonance in that it naturally oscillates at frequencies known as resonant frequencies. These oscillations can be either electromagnetic or mechanical. Tunable resonators are used in a wide range of radio devices, specifically tunable radios, test equipment, and frequency agile radar.

Types of Resonators

Resonators are key to the performance of oscillators and filters. The types of common resonators include transmission line and coaxial resonators, dielectric, crystal, ceramic, surface acoustic wave (SAW), bulk acoustic wave (BAW), and yttrium iron garnet (YIG).

Inductor/Capacitor (LC) Tank Circuit Resonator Example

Using a simple crystal radio as an example, the inductor/capacitor (LC) oscillator acts as the tuner for the radio. When radio signals are received that match the resonant frequency of the LC resonator circuit, they pass through with relatively low resistance. However, frequencies outside of the resonant frequency of the LC circuit are heavily attenuated. In this example, either the capacitor or the inductor in the resonator is adjustable so that when the tuner knob on the radio is turned, a variable element is adjusted which changes the resonant frequency of the resonator. Most RF and microwave filters are made up of one or more coupled resonators; the quality factor, or Q factor, of the resonator will determine the selectivity the filter can achieve.

Transmission Line Resonators and Coaxial Resonators

Transmission lines are structures that allow broadband transmission of electromagnetic waves at radio or microwave frequencies. Due to the nature of transmission lines, a section of transmission line can be selected to resonate at a particular frequency, and in essence, function as a tuned LC circuit with a very high Q factor. Planar transmission line resonators are used in coplanar, stripline, and microstrip transmission lines, can be compact in size, and are used in microwave circuitry. Coaxial resonators are used in voltage-controlled oscillators (VCOs), coaxial-resonator oscillators (CROs), and filter components. These resonators are used as components in oscillators, bandpass/bandstop filters, and electromagnetic-interference (EMI) filtering.

Dielectric Resonators

A dielectric resonator is a piece of nonconductive material, usually ceramic, designed to function as a resonator in the microwave and millimeter wave bands. The EM waves are confined inside the resonator by the rapid change in permittivity at the surface and bounce back and forth between the sides. At the resonant frequencies, the EM energy forms standing waves in the resonator, oscillating with large amplitudes. This resonant frequency is determined by the physical dimensions of the resonator and the dielectric constant of the material. Dielectric resonators can replace resonant cavities in components, such as filters and oscillators, and have a high εr value that provides an advantage in compact design. This type of resonator confines the EM fields which allows for only a small amount of loss while providing a high Q factor to be achieved. The main use for this type of resonator is in millimeter-wave electronic oscillators to control the frequency of the radio waves or as bandpass filters and antennas.