Compact cross-coupled filters for UMTS applications

Very compact planar resonators were developed for the 1950 MHz frequency band. Resonators are of the stepped-impedance (SIR) type and they are folded, in order to occupy a very small surface area. On a 0.635 mm height commercial substrate with a 10.8 dielectric constant, a resonator can be fitted into a square with an edge size of 5.8 mm. Small-size filters were designed by cross-coupling the compact resonators. The four-pole filters have an improved response due to the cross-couplings, which introduces transmission zeros on each side of the pass-band. The filters were manufactured and characterized. The measurement data are in good agreement with the electromagnetic field simulations. The developed filters are small-size and cost-effective and their technology is simple, without any via holes, air-bridges or lumped elements.