Dielectric resonating cavities are components of filters, reflection-type amplifiers, and oscillators. A dielectric resonating cavity refers to a space bounded by an electrically conducting surface in which oscillating electromagnetic energy is stored. Resonating cavities are typically rectangular or cylindrical in shape with conducting side walls and an input and output couple for electromagnetic energy. Dielectric blocks or pucks may be positioned in the cavity to provide a desired resonant frequency of the resonating cavity (i.e., the cavity resonant frequency). The desired cavity resonant frequency determines the frequency characteristics of the electromagnetic energy output by the cavity.
The cavity resonant frequency is determined by the resonant mode and dimensions of the resonating cavity and the electric permittivity of the dielectric block or puck located in the cavity. The cavity resonant frequency can vary in response to thermal expansion/contraction of the resonating cavity, thermally induced fluctuations in the electric permittivity of the dielectric block or puck, and/or dimensional tolerances of the resonating cavity and its placement in the circuit.
One method for fine tuning a cavity in response to fluctuations in the cavity resonant frequency is to use a metal or dielectric material to selectively perturb the electromagnetic energy distribution in the resonating cavity. Typically, this is accomplished either by manually or mechanically turning a number of tuning screws in the cavity or by altering the position or shape of the dielectric block or puck in the cavity. This method can have a slow tuning speed, a low degree of tuning precision, and, for mechanical tuning, a high rate of mechanical problems.
Another method for fine tuning a cavity is to alter the permeability of a ferromagnetic or ferrimagnetic material, such as yttrium iron garnet, located in the cavity. The permeability is controlled by controlling the strength of a magnetic field applied to the material. This method can have a slow tuning speed, a high hysteresis loss (especially at frequencies used for cellular and Personal Communications Systems (PCS) wireless system), and a permeability that is strongly dependent upon temperature fluctuations. An additional problem which limits the use of ferrite tuning is that the magnetic field used to tune a first cavity often has an adverse effect on other adjacent cavities located in close proximity to the first cavity.
Yet another method for fine tuning a cavity is to couple a semiconductor varactor to the electromagnetic energy in the cavity. Altering the capacitance of the varactor results in a change in the cavity's resonant frequency. Semiconductor varactors are rarely used at microwave or higher frequencies because such varactors can result in a high insertion loss and generate spurious signals at undesired frequencies. In signal transmission applications, the voltage and/or current breakdown strengths of semiconducting varactors can be exceeded when the power level of the cavity exceeds approximately one milliwatt. Filters used for signal transmission typically operate in the 1 to 800 watt range.
Another method for fine tuning a cavity is to alter the capacitance of a varactor diode coupled to the cavity via a coupling loop. The diode capacitance is varied by varying the d.c. voltage applied to the diode, which changes the width of the charge depletion layer in a semiconductor. At microwave and millimeter frequencies, the diode and coupling loop can produce high microwave attenuation due to the series resistance of the semiconductor areas adjacent to the charge-depleted portion of the semiconductor. The high attenuation can result in an undesirably low Q, and thus unacceptably high loss of the electromagnetic energy input into the cavity.