Bimetal Temperature Compensation for Waveguide Microwave Filters
Keats, Brian Franklin
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Microwave communication devices have become ubiquitous in the past decade. As an increasing number of systems compete for spectrum, guard bands have shrunk to increase bandwidth efficiency. The frequency behaviour of microwave devices is affected by thermal expansion. In order to avoid interference with adjacent bands, microwave components must exhibit high temperature-stability in most communications applications. Thermally stable materials can be used to construct temperature-stable components. However, this approach requires an expensive mass and cost trade-off. Temperature compensated aluminum resonators and filters provide major advantages in cost and mass. This work proposes that a compensating tuning screw with a temperature-dependent effective length be constructed by mounting a bimetallic compensator at the end of a mounting screw. This so-called bimetal tuning-screw can be used to produce temperature-compensated resonators and filters. There are several advantages to this approach. Compensation can be tuned by adjusting the depth of the bimetal, simply by adjusting the mounting screw. Since there are no moving parts inside the cavity or filter, and the bimetal can be plated, there are no additional sources of passive intermodulation. Also, this design is simple to implement for waveguide designs in general. In order to compensate for temperature drift, it is useful to quantify uncompensated drift. Temperature drift for a lossless linearly expanding RF component is derived from Maxwell's equations. For the lossy case, it is demonstrated that the resulting formula is approximately true, and that the quality of this approximation is excellent for practical levels of temperature range and thermal expansion. Experimental results are provided that demonstrate bimetal compensation under uniform-temperature conditions for a single aluminum resonator. Measured drift of the compensated resonator is -0.38 ppm/°C, compared to -23 ppm/°C for an uncompensated resonator. Measured drift for a bimetal-compensated 4-pole filter prototype is 2.35 ppm/°C. A method for adjusting compensation for a filter is also provided. Multiphysics simulations are used to examine power handling for bimetal-compensated filters. It is demonstrated that power-handling can be improved by reducing the effective length of the compensator to improve heat conduction to the cavity or filter.