Resonators are devices that exhibit resonant behavior associated with a very high reactive peak at the resonant frequency. They are used in applications where narrow oscillation or filtering characteristics are required. This includes oscillator circuits and band pass filter sections. Resonators are based upon a number of different topologies .that includes: parallel LC tank circuits, crystal resonators, ceramic resonators and SAW resonators.

Parallel LC tank circuits are comprised of parallel LC circuit that is charge pumped by an amplifier configured as  positive feedback. The broadband noise energy is circulated by the feedback and filtered until oscillation is occurring at the characteristic resonant impedance of the tank. Popular parallel LC tank circuits are Hartley, Colpitts and Clapp oscillators.  They are highly tunable  using fewer components compared to other types or resonators, but lack accuracy and are extremely  dependent on component value precision.

Crystal resonators are the most popular technology today. They are used in crystal oscillators (XCO), voltage controlled oscillators (VCO), temperature controlled crystal oscillators (TCXO) and oven controlled crystal oscillators (OXCO). They are usually constructed with specially cut and doped quartz crystals that exhibit the piezoelectric effect. They range from tens of kHz to several hundred MHz. VCOs vary a bias voltage across the crystal resonator to pull the frequency either side of the center frequency. This change in frequency is reflected in the specification of the oscillator’s ability to pull. TCXOs use temperature compensation to tweak the resonant frequency to cancel out temperature dependent frequency characteristics of the crystal. OXCOs also add an oven and temperature sensing to maintain the crystal at a particular temperature for improved resonant frequency stability. Ceramic resonators are similar to crystals in that they are based upon piezoelectric material. This is usually lead zirconium titanate (PZT).

Crystal filters are used in RF receivers to narrow band pass filter intermediate frequencies (IF). They include crystal resonators as a component of the band pass filter. They are typically 12.5kHz or 25kHz wide to correspond with the bandwidths of private mobile radio channels. They filter out adjacent channels, providing selectivity to the receiver. It is important for a crystal filter to have a low insertion loss, high stopband attenuation and low passband ripple. Ceramic filters are also used in receivers in the same way as crystal filters. They have lower Q than crystal filters and were historically only available from several kHz up to 10.7MHz, but recent developments have made devices available at UHF frequencies and above.

SAW (Surface Acoustic Wave) resonators use stress induced mechanical acoustic waves on the surface of piezoelectric material. They are used in: keyless entry remote controls, GPS receivers, cellphone filters and many other applications. One particularly unusual application is in circuits that ‘smell’ disease markers for ovarian cancer. Their highly selective filter characteristic is exploited in devices that detect plastic explosives. SAW resonators are used in probes that detect E.coli, clostridium pathogens and even salmonella.