MIC FILTERS

Abstract

One of the most common types of MIC bandpass filters is the parallel-coupled filter. When such filters are implemented using non-homogeneous media, the difference in the even- and odd-mode phase velocities have a detrimental effect on their performance. These effects in the Microstrip parallel-coupled filter were studied and found to be mainly the emergence of a spurious passband at 2 times the frequency of the main passband which in turn resulted in a lower upper-stopband rejection. A method of phase velocity compensation using wiggly coupled lines was then applied and verified through measurements to be effective in suppressing the spurious passband. Next, the usage of the CPW technology in parallel-coupled filters was investigated. Parallel-coupled lines in CPW have even- and odd-mode phase velocities which are closer to each other than in Microstrip coupled lines and so parallel-coupled CPW filters were expected to perform better. From the measured performance of a parallel-coupled CPW filter, it was observed that a spurious passband at 2 times the main passband frequency is still present but it is much narrower and more attenuated than that found in the Microstrip filter. Parallel-coupled CPW filters can also be constructed with coupled lines whose free ends are shorted to ground. However, the dimensions of the coupled lines in these filters have to satisfy certain restrictive conditions before they can be directly cascaded to one another. To avoid such restrictions, it was suggested that a modified topology with a small section of transmission line between the coupled line sections be used. A CAD based methodology was used to design the filter with the new topology. The simulated results corresponded to the filter specifications thus verifying the validity of the modified topology and design methodology used. Loop directional filters were investigated next. A simple analytical formula for analysing the filter was derived and verified. The effect of unequal phase velocities in Microstrip loop filters was investigated and the usage of wiggly coupled lines was found to improve the upper-stopband performance of the filter. Next, a synthesis method for multiple loop directional filters was investigated and found to be very accurate if ideal components were used. When Microstrip components were used, the performance hardly corresponded to specifications. Usage of wiggly coupled lines in these multiple loop Microstrip filters is expected to bring their performance closer to design specifications. One other way of avoiding the detrimental effects associated with phase velocity differences is to use end-coupled half-wavelength resonators. However, realising end-coupled half-wavelength-resonator filters with wide bandwidth has always been a problem. And even if realised successfully, the filters would suffer from a low upper-stopband rejection because of an emerging second passband at 2 times the centre-frequency. Thus it was seen that a suitable alternative to the parallel-coupled filter should not only allow the realisation of wide bandwidths but must also exhibit comparable stopband characteristics. The final type of filter investigated in this work, the end-coupled quarter-wavelength-resonator CPW filter, satisfied both these requirements. While being smaller in size than half-wavelength-resonator filters, this new filter also requires a looser coupling between the resonators which allows the realisation of very wide bandwidth filters. It has a second passband at 3 times the main passband frequency, hence exhibiting a very good upper-stopband rejection. The superior performance of the filter was verified through the measured results of two filters with bandwidths of 5% and 50% at a centre-frequency of 10 GHz.