Dielectric Materials 
At Trans-Tech, we use the term dielectric constant whereas permittivity is cited in texts. In reality, dielectric constant isn't constant. It varies somewhat with the blend that's used to determine the ceramic's temperature coefficient, it varies slightly from lot to lot, and it changes perceptibly with temperature. We compensate for these effects by offering dielectric resonators (DRs) sized to frequency, and with "custom-tailored" temperature coefficients when necessary.
Dielectric microwave materials are commonly assigned a loss tangent to permit an estimate of signal losses. But ceramic dielectric resonators operate at a specific frequency, in a specified geometry, which allows direct measurement and specification of Qu, the unloaded quality factor. Qu is a fundamental resonator parameter which is particularly appropriate (and more useful than loss tangent) for filter and oscillator applications.
Ceramics don't age perceptibly. Any change in the resonant frequency of a DR over time can be attributed to change in the measurement cavity or measurement technique.
Ceramics don't absorb moisture noticeably, but moisture condensation on the surface of the DR can affect Qu. The Qu will recover when the moisture is driven off, for example, by self-heating of the DR in a transmitter filter.
The Qu of ceramic resonators can be degraded by finger oils, pencil marking, tape, and a host of other contaminants. Cleanliness is important.
Ceramics chip easily when they contact hard surfaces. Most tiny chips will not affect the electrical performance at all. Neither is surface roughness particularly important: there are no currents in a ceramic dielectric resonator, only stored energy in the form of fields. Smooth surfaces are desirable from the standpoint of avoiding trapped contaminants.
Ceramics are "born" in kilns at temperatures over 1,000 C add superscript degree symbol, 1,000[wordspace]degree/C. They can stand much higher temperatures than the electronic equipment they are used with, i.e., far in excess of soldering temperatures. But ceramics conduct heat much more slowly than metals. A large enough temperature gradient through a ceramic part can cause failure due to differential expansion: we call this thermal shock. Sudden application of heat on one side of a thick ceramic part invites fracture.
Adhesives used to mount DRs must be chosen carefully. Adhesives will always degrade a DR's Qu, but Trans-Tech has developed bonding systems to minimize Qu loss while guaranteeing bond strength. Refer to Application Note No. 2002, "Adhesives for Dielectric Resonators."
Trans-Tech Dielectric Materials include:
|
