Ultra-Wideband balanced filtering attenuator with ultra-large tunable attenuation range and switch-OFF ability

In this paper, an ultra-wideband balanced filtering attenuator based on a double-sided parallel-strip line (DSPSL) structure is proposed, achieving an ultra-large tunable attenuation range and switch-OFF ability. To achieve the ultra-wideband differential-mode (DM) characteristic, an unequal-width three-coupled line structure is employed. The wide tunable DM attenuation range benefits from the variable resistances loaded between the end of the three-coupled lines and ground. To further enhance the DM attenuation range, L-shaped attenuation networks are introduced, so that the two attenuation networks operate cooperatively and the tunable range is considerably extended. By properly adjusting the attenuation networks, the energy transmission can be almost completely suppressed, thereby realizing a switch-OFF function. Furthermore, the DSPSL balanced feed structure is employed to achieve intrinsic broadband common-mode (CM) suppression. Consequently, ultra-wideband DM filtering, an ultra-large tunable attenuation range, broadband CM suppression and switch-OFF ability are achieved simultaneously in a single balanced component. To demonstrate the concept, a prototype of the filtering attenuator is fabricated and measured. It has a center frequency of 1.68 GHz and offers adjustable DM attenuation ranging from 2.7 to 50.9 dB. In addition, the device provides switch-OFF ability with in-band suppression of 58–72 dB. The measured results show good agreement with the simulations.