What are Tunable Resonators, and What do They Have to do With Filters and Oscillators? Part 2
In last week’s post we defined and explored the different types of tunable resonators that are available. This week we take a look at more resonator types and discuss the pros and cons of each.
Quartz crystals are used as high-quality electromechanical resonators due to their piezoelectric properties, that is, their ability to generate an electric charge in response to a mechanical stress. When a piezoelectric material is placed under mechanical stress, the positive and negative charge centers shift resulting in an external electrical field and, when reversed, an outer electrical field either stretches or compresses the piezoelectric material. In a quartz crystal resonator, this process begins with a thin slice of quartz cut to specific dimensions and is then placed between two electrodes. The cut determines the resonator physical and electrical parameters and is classified by the cut from the original crystal. For example, the AT crystal cut is used for electronic instruments where oscillators are required to run in the range 500 kHz to around 300 MHz. The BT cut is another example, similar to the AT cut, that provides repeatable characteristics with a frequency constant 2.536 MHz/mm. While the temperature stability characteristics are not as good as the AT cut, it can be used for higher frequency operation due to its higher frequency constant.
Although less accurate than quartz crystals, ceramic resonators are an alternative to the more costly crystal resonators. Like crystal resonators, ceramic resonators use the piezoelectric properties in the material, such as lead zirconium titanate (PZT). In this design, metal electrodes placed on the top and bottom of the ceramic substrate, a voltage is applied, and the substrate vibrates between the electrodes are energized, thus yielding a resonant frequency determined by the thickness of the material.
SAW and BAW Resonators
Much like crystal resonators, surface acoustic wave (SAW) resonators are components in oscillators found in higher-frequency applications. A SAW is made of an interdigital transducer with two grating reflectors that are placed on a piezoelectric material by a photolithographic process. This photolithographic process uses light to transfer a geometric pattern from a light-sensitive chemical photoresist, or resist, onto the substrate. The reflectors then form a resonant cavity that is coupled to the external circuit by the transducer. Applications include automotive remote-keyless-entry (RKE) devices, security systems, and garage door openers. Similarly, BAW resonators leverage bulk acoustic waves instead of surface acoustic waves to generate a resonant effect.
Yttrium iron garnet (YIG) resonators are used as components in the design of oscillators and filters. A YIG is a synthetic form of garnet with magnetic properties that exhibits a very high Q and low phase noise oscillations capable of achieving multi-octave bandwidths. In the sphere configuration, a single crystal of synthetic yttrium iron garnet acts as a resonator with the garnet mounted on a ceramic rod with a pair of small loops around the sphere couple fields where the loops are half-turns and positioned at right-angles to prevent direct EM coupling. One advantage of the YIG is that the garnet can be tuned over a wide frequency range by varying the strength of the magnetic field, anywhere from 3 GHz up to 50 GHz. YIG filters are usually made of several coupled stages where each stage is designed with a sphere and a pair of loops.