A novel ring resonator bandpass filter with reconfigurable bandwidth with central frequency at 2.4 GHz is demonstrated. Theoretical analysis for computing the resonant frequencies is shown, and the design approach of implementing π-section stubs connected to the ring to obtain the bandwidth reconfigurability is explained. The use of reconfigurable π-section allows the alteration of the stub's effective width and therefore the filter's bandwidth reconfigurability. p-i-n diodes are used as switching elements to achieve the narrowband/wideband response. Coupled line sections are used for suppressing the higher modes by generating out-of-band transmission zeros, resulting in a significantly enhanced out-of-band rejection. Measurements indicate that the 3-dB fractional bandwidth can be switched from 58.5% to 75% at a fixed center frequency of 2.4 GHz with an insertion loss better than 1.1 dB. Moreover, the -20-dB stopband performance is extended to 2.7f₀.

A reconfigurable bandpass filter (BPF) with constant centre frequency and wide tuning range of bandwidth is presented. The proposed filter is based on the novel configuration of two modified asymmetrical π-sections with folded shorted stubs loaded on a ring resonator. The novel topology allows the implementation of three reconfigurable passband states. A detailed theoretical analysis for calculating the transmission zeros using ABCD parameters is shown, and the design approach of implementing the random offset -x-, asymmetrical π-sections with the folded stubs to get the bandwidth reconfigurability is thoroughly explained. PIN diodes are utilized as switching elements to achieve narrowband/mediumband/wideband response. The measured results indicate an absolute bandwidth tuning range of 1.3 GHz, whereas the -3 dB fractional bandwidth (FBW) can be varied from 37% to 92% while the centre frequency remains constant at 2.4 GHz. The fabricated prototype reconfigurable BPF demonstrates one of the widest 3-dB FBW tuning ranges and one of the highest absolute tuning ranges with a sharp-skirt characteristic. It has also more than 20 dB attenuation in the upper stopband which is extended to 2.7 times the centre frequency.

A novel dynamically reconfigurable bandpass filter (BPF) employing stepped-impedance resonators (SIRs) that can operate as either a single-band or a dual-band filter is demonstrated. The reconfigurable BPF uses four positive-intrinsic-negative (p-i-n) diodes as switching elements. With the four p-i-n diodes in the ``OFF'' state, the filter behaves as a low-loss (0.85 dB) single-band BPF with a passband around 2.45 GHz. The reconfigurable SIR filter can operate as a dual-band BPF with two center frequencies at 1.6 and 2.45 GHz. The diodes are dynamically set to the ``ON'' state in the presence of a 1.6-GHz RF signal that is received by an RF triggered power management unit (PMU), integrated on the back side of the microstrip filter in a dual-layer architecture. The RF triggered PMU consists of a planar inverted-F antenna, a high-efficiency voltage doubler rectifier (47% at -13 dBm) and an active dc-to-dc power booster. The rectified output voltage is used as the enabling voltage for the dc-to-dc power booster. This, in turn, provides the required dc power for the diodes biasing. The filter switches from single-band to dual-band when a wireless input RF signal (>-13 dBm) is received by the RF triggered PMU antenna.

This paper demonstrates a new technique for designing high performance reconfigurable bandpass-to-bandstop filters by employing a ring resonator and a selective feeding scheme integrated with single-pole double-throw switches (SPDT). The transformation from bandpass-to-bandstop mode and vice-versa is achieved by connecting or disconnecting two $\lambda _{g}$ /4 open-circuited stubs on the ring using PIN diodes. SPDT switches are employed for electronic switching between two different feeding line sections. In the bandpass state the resonator presents two transmission zeros near the edges of the passband and four attenuation poles inside the passband, enhancing the filter’s performance, thus achieving excellent sharp rejection with high roll-off-rate ( $ROR_{20dB}$ ). On the other hand, high stopband rejection with wide bandwidth, good return loss and good skirt-band attenuation rates are achieved in the bandstop state. Even- and odd-mode analysis is adopted and closed-form expressions are derived to describe the filter’s behaviour. To verify the validity of the proposed design, a prototype filter was fabricated and measured. In measurement, a 65% 3-dB bandwidth bandpass filter (BPF) with an insertion loss of 0.86 dB was switched to a 70% 20-dB bandwidth bandstop filter (BSF) with more than 40 dB stopband rejection.