Balanced filter with parallel resonances for very wide band common mode rejection

Abstract

A new type of balanced filter with a very wide common mode (CM) free band is presented. In this design, the filter’s symmetry line lies parallel to the resonator’s current flow. Therefore, when a differential signal is excited, a virtual open circuit is forced all along this symmetry line, which results in the excitation of parallel resonances on the resonators. However, for CM operation, a short circuit is forced all along the resonator, therefore eliminating any parallel CM resonances. The filter was designed with Chebyshev response using four poles at a differential frequency of 1.11 GHz. Its fractional bandwidth is 5%. It was fabricated on a substrate with ε_r = 10.2 and a thickness of 1.9 mm. Experimental and simulated results on the test filter show good agreement. The first CM resonance is excited at about 10 times the desired center frequency. This is due to orthogonal resonances of the resonator.

Keywords:

• index terms – balanced filters
• common mode
• differential mode

Disclosure statement

No potential conflict of interest was reported by the authors.