Miniaturized bandpass filter based on double spiral stripline resonators

Abstract

A new miniature monolithic bandpass filter, which can be manufactured using multilayer standard all-PCB technology, is presented. Each resonator in the filter is formed by metal spirals of the left and right wist inserted into each other, in which one pair of adjacent ends is grounded, and the other is free. Spiral inductors have the form of identical rectangular frames and are designed on dielectric substrates, located on a multilayer structure strictly under each other with alternating turns of the left and right spirals. The design of the filter based on such multilayer double-spiral resonators is both small in size, and highly selective, which is proved by the measured characteristics of the fourth order filter fabricated at seven dielectric layers of RO4350B material. The filter has the central frequency of the passband f0 = 60 MHz, and the fractional bandwidth Δf / f0 = 18%, while the size of the device is of the device are only 34 × 16.5 × 4.3 mm3 (0.007λ0 × 0.003λ0 × 0.001λ0, where λ0 is the wavelength at the center frequency f0). The filter has a wide stopband, which extends up to the frequency of 16f0 at a level of –38 dB.