Microwave Filter Design: Coupled Line Filter

ABSTRACT MICROWAVE FILTER DESIGN: COUPLED LINE FILTER by  Michael S. Flanner 2011 Master of Science in Electrical and Computer Engineering Electronic Engineering Option California State University, Chico Spring 2011 Radio Frequency (RF) filters operating in the microwave frequency range are needed for applications including wireless and satellite communications as well as military applications. These applications demand high performance filters that will contribute as little as possible to a system's size and cost. Advances in materials used to construct these filters have played a significant part in meeting these demands. Planar and dielectric resonator filters are among the filter types which benefit from higher quality dielectric materials. Planar, or printed circuit board (PCB) based filters are popular and relatively practical to design. This paper presents the design and test of a planar coupled line filter constructed from relatively high quality dielectric material. The bandpass coupled line filter presented here is specified to have a midband at 1.69GHz and bandwidth of 0.169GHz. Passband insertion and return loss is specified to be 5dB and 10dB respectively. The design was derived from standard filter design theory and formula available in the literature. An optimized computer aided (CAD) design was also generated for comparison. The 'Microwave Office' design software was provided by Applied Wave Research Inc., operating with an educational license. Both formula and simulation based designs had nearly identical physical structure and performance under simulation. A prototype of the design was manufactured and tested on the bench using an Agilent 8714ES RF Network Analyzer (3.0GHz). The measured passband insertion loss was 3.6dB, meeting the specified goal and consistent with expected response based on simulation. Placement of the filter's midband was offset from the expected value. This was most likely a result of wide tolerance in the dielectric permittivity specified for the PCB substrate. Also the filter's bandwidth was wider than expected. Possible causes might have been the test equipment calibration, impedance mismatch amongst the measurement system's cables and adapters, or trace impedance errors resulting from structural defects in the etched microstrip lines. A coupled line RF filter was designed and working prototypes of the design performed well. Passband insertion loss measured well within the target specification. The filter owes its low signal loss to refined formula published in the literature as well as to the availability of the high quality PCB material sampled by Rogers Corporation. AWR's simulation software was also helpful in the design process. The simulation software saved design time and effort by allowing pre-production verification of the design. The software also allowed various design iterations to be explored much more quickly than would have otherwise been possible. Differences between simulated and prototype performance likely resulted from material specification tolerance, possible errors in transmission line trace structure, as well as other possibly measurement related factors. Measured passband insertion loss was less than 3.6dB, exceeding the target spec of 5dB maximum. Measured midband was 1.61GHz, showing an offset from the target 1.69GHz. Measured bandwidth was 206kHz, which was wider than the target 169kHz. Performance could be further improved with refinements to the design and the construction material. The design equations could be further optimized by taking into account field fringing effects and by obtaining precise material specs from the PCB manufacturer. Redoing the filter on substrate with lower specified dissipation factor (i.e. higher quality factor) could further reduce signal loss. The material used here had a quality factor approximately double that of standard FR-4 board material. Even higher quality material is available. The result should be an excellent and practical filter which can be designed without the need for expensive CAD tools