RF MODULE

Thermal Drift in a Microwave Filter
Introduction
Microwave filters serve to suppress unwanted frequencies in the output of microwave
transmitters. Amplifiers are in general nonlinear and produce harmonics that must be
suppressed using one or several narrow passband filters on the output. High frequency
stability of such filters can be hard to achieve because microwave systems may be
subject to thermal drift caused by high-power loads or harsh environmental conditions
like exposure to direct sunlight in the desert. Thus, system engineers need to estimate
the drift of the passband frequency that arises due to thermal expansion of a filter.
Model Definition
Figure 3-10 shows the filter geometry. It consists of a box with a cylindrical post
centered on one face. It is typically made of brass covered with a thin layer of silver to
minimize losses. The silver layer is sufficiently thin to have a negligible influence on the
device’s thermal and mechanical properties.
Figure 3-10: A microwave filter consists of a box-shaped thin metallic shell, typically made
of brass, that contains a cylindrical post. This configuration forms a closed air-filled
electromagnetic cavity between the box walls and the post.
The thermal expansion and the associated drift in eigenfrequency are caused by a
uniform increase of the temperature of the cavity walls. The thermal expansion is
readily computed using the Shell application mode from the Structural Mechanics
96 | CHAPTER 3: RF AND MICROWAVE MODELS