Low-Loss Microstrip Filters Developed by Frequency Scaling

Microstrip filters are used in frequency converters, frequency multipliers, and branching networks for RF systems. Microstrip filters have been built in the past with photo-etched conductor patterns on ceramic substrates such as alumina, sapphire, and beryllia. 1 - 2 These materials arc useful in microwave integrated circuits at a few gigahertz where a desirable size reduction relative to free-space wavelength is achieved because of the high dielectric constant of the substrate. At higher frequencies, size reduction is a disadvantage because the circuit elements are approaching the limitations of photolithographic reproduction. A suitable substrate material for use at higher frequencies is clear fused silica. Its relative dielectric constant over a wide frequency range is 3.82 and its dielectric Q is 3,000 or higher at all frequencies up to 50 GHz. The purpose of this paper is to show that optimized microstrip filters at microwave and millimeter-wave frequencies can be built by linear reduction of a low-frequency model. The flow chart of this procedure is shown in Fig. 1. The oversize microstrip filter is designed, built, and tested at a few hundred megahertz. All circuit dimensions 1919