Microwave Design Notes - Coupling and Frequency Tuning

Means of Coupling to the Dielectric Resonator

There are two typical techniques for coupling into the field radiating from a dielectric resonator.

Cavity Method

The most commonly used technique for coupling into a dielectric resonator is through the use of coaxial couplings which are mounted through holes in the cavity wall. Cavity design must take into account characteristics of the dielectric resonator; however, in post production, tuning elements may also effect the coupling and temperature stability of the design. Many filter designs require multiple resonators to be coupled together in one housing, this can be achieved by continuing the coaxial loop design in cavities positioned directly next to each other. This allows propagation of the field between the individual cavities and can produce a filter band, rather than an individual resonator response.

Resonators used in cavity filters are commonly mounted on a low ceramic support (D6 or D9).

Substrate Method (Micro Stripline coupled)

The resonator is typically mounted onto the surface of the substrate using a thermally cured epoxy adhesive. In order for the resonator to be effectively coupled when using Micro Stripline it should be placed adjacent to the Stripline without making any direct contact (with the line).

In order to maintain a stable frequency response of the resonator (with temperature variation) it is advisable to ‘lift’ the resonator off the substrate by using a low ceramic stand off (D6) or by using a monolith solution (resonator with integral stand off). Typically for a 10 GHz DR, the stand off height should be approximately 1.5 mm.

Frequency Tuning

The ceramic dielectric resonator will often be machined to a thickness in order to achieve a specific resonant frequency. However when the dielectric resonator is mounted into a cavity filter it may be necessary to fine tune the frequency by perturbing the fringe fields around the resonator. The fine tuning can be achieved by mechanically moving a ceramic (or metal) disc closer to the resonator.

There is a limit to the frequency shift (tuning) achievable by this method before it adversely effects Q and temperature stability - this limit is often found to be no more than 5% of resonant frequency.

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