Cascaded ring bandpass filters with high out-of-band rejection

This paper presents two microwave bandpass filter topologies which are based on two different ways of series-cascading two quarter-wavelength side-coupled ring resonators. In the first topology, a quarter-wavelength impedance inverter is used to connect the two rings. As each ring gives two poles, a 4^th order bandpass filter is obtained from the topology. The impedance inverter is replaced by a quarter-wavelength coupled line in the second topology. In this case, two half-wavelength resonators are formed at the interconnection between the rings and the coupled line, leading to the creation of an extra of two poles in the passband response. Thus, a 6^th order filter is obtained from the second topology. Both topologies are realized at 2 GHz center frequency using microstrips on epoxy-glass substrate. Measurement results are found to be in good agreement with the simulations with very high out-of-band rejection level.