Ultra compact band pass filters implemented through complementary spiral resonators (CSRs)

In this work, it is shown that complementary spiral resonators (CSRs) etched in the ground plane of a microstrip line are useful for the implementation of band pass filters with very small dimensions and wide stop bands. By combining the CSRs with two series capacitive gaps and inductive vias, two transmission zeros arise. By properly allocating such transmission zeros at both sides of the pass band of interest, the out-of band rejection and frequency selectivity of the filters can be controlled. A prototype device example is reported to illustrate the performance and size of these CSR-based filters. This is an order-three Chebyshev band pass filter centered at f_o=1.1GHz with 10% fractional bandwidth. Measured in-band losses (optimum value) are IL=-4.42dB, in-band return losses are better than 9dB, and stop band rejection is better than 55dB up to 2.57GHz. Filter area is as small as 23.2 mm times 8.9 mm, that is 0.22 lambda times 0.08 lambda, where lambda is the wavelength (at the central filter frequency) of a 50Omega microstrip line in the considered substrate. The key novel and advantageous aspect of the reported filters is their small size, which is related to the small electrical size of CSRs. Another advantage is frequency selectivity, which is due to the presence of a transmission zero at both edges of the pass band. The limitative aspect is their relatively small unloaded quality factor (Q_u), this being attributed to the small dimensions of the considered resonators. These CSR-based devices can be of interest in those applications where size and frequency selectivity are the most severe requirements.