CROSS COUPLED PLANAR OPEN LOOP RESONATOR BANDPASS FILTERS WITH WIDER STOPBAND CHARACTERISTICS

Abstract

The RF/Microwave filters with high selectivity and low insertion loss have various applications in Communication and Satellite systems. These systems place some typical demands of lowest-possible insertion loss, sharp cutoff slopes and flatness of group delay. All these demands can be met by cavity resonators or dielectric resonator at the cost of larger size. Cross- coupled microstrip based filters provide an attractive solution to fulfill the requirements with smaller size and low cost. The techniques and methodology to develop the cross-coupled bandpass filter were studied. These filters are arranged in such a way that they establish the direct coupling between the adjacent resonators and cross-coupling between non-adjacent resonator. Cross-coupled filters provide an improved skirt selectivity and group delay flattening. The main thesis objective was to design and develop cross-coupled resonator bandpass filter. With this objective, a 4-pole cross-coupled resonator bandpass filter centered at 2.6GHz were designed and fabricated. Both simulation and testing of the circuit was undertaken to verify the design. The measured results show the sharp roll-off with insertion loss of 1.86dB and 3-dB bandwidth of 60MHz. However as usual with such filters, testing results show passband harmonics occurred at 2f0 with insertion loss of 6.5dB where f0 is the centre frequency. It is desirable to eliminate the interference by keeping out-of-band signals for wireless communication receivers. In this regard, stepped impedance resonator or slow wave structures can be used to suppress the harmonics to avoid channel interferences. The microstrip slow-wave open loop resonators filters are not only compact in size due to slow-wave effect, but also have a wider upper stopband resulting from the dispersion effect. The work is further extended to design a crosscoupled filters centered at 1.3GHz using the slow wave structure. The measured results show the 3-dB bandwidth of 55MHz, insertion loss of 2.7dB of filter. The results clearly verified the suppression of second harmonics to 44dBs.