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This article discusses a rigorous and straightforward method to synthesize and design a transmission-line-based reflectionless bandstop filter with high performance in terms of reflectionless range. Our design strategy targets to allow a reflectionless bandstop filter to feature three distinguished advantages that are incomparable to others. First, the presented filter structure is capable of producing an exceptional broadband impedance matching performance. Second, it can be designed to have an extended upper passband. Third, an ${N}$ th-order filter has ${N}$ coupling structures and each of them is used twice in design. Hence, it is required to design and tune only ${N}$ coupling structures, whereas other design approaches use more than ${N}$ distinct coupling structures. Although reflectionless filters having one of the aforementioned features have been reported, one exhibiting all the characteristics has never been reported to date. Fabricated filter examples fully validate the design theory. [ABSTRACT FROM AUTHOR]

This article presents a rigorous design method for a one-port-reflectionless filter with a Chebyshev response. In particular, we, for the first time, present inverter-coupled resonator filter topologies capable of having a Chebyshev response with zero reflection at all frequencies in theory. This work demonstrates a systematic approach for filter synthesis to find the normalized coupling (inverter) values without employing optimization methods or heuristic approaches. In addition, it provides closed-form design formulas in terms of a target frequency for the design of a one-port-reflectionless filter of coupled-line structure. We also present an exact approach to increase the reflectionless range of a distributed-element filter. Distributed-element bandpass filter structures formulated by this work are capable of producing a predefined canonical transmission response and a reflectionless feature over the entire frequency range at the same time, which has never been reported up to date. The presented topologies, synthesis results, and design methods have been applied to the design of second- and third-order filters for demonstration. [ABSTRACT FROM AUTHOR]

In this article, we present a novel generic reflectionless filter topology that can produce an arbitrary predefined transmission response while having the perfect impedance matching at both ports. A detailed procedure for synthesizing reflectionless filters using our topology is also given in this article. One of the unique features of our reflectionless filter topology is that the even- and odd-mode half circuits are not correlated with each other, which makes the filter synthesis straightforward. Hence, the synthesis procedure does not necessitate using either an optimization technique or a numerical method. This article also presents a methodology for transforming a generic topology into a practical one with a reduced number of elements. The presented filter topology and design parameters have been verified by designing lowpass and bandpass filters. [ABSTRACT FROM AUTHOR]

In this paper, we present a reconfigurable filter capable of having three different responses, such as bandstop, bandpass, and all-pass responses. For designing the tri-function filter, we have developed a new topology containing coupled resonators and a switch-embedded transmission line running from the input to output ports. A bandpass response can be obtained by opening the switches embedded in the transmission line. On the other hand, closing the switches produces bandstop and all-pass responses. The details of the coupling structure design satisfying three sets of the coupling coefficients for the three responses are presented. In addition, we also discuss in detail an approach for incorporating the measured parameters of the nonideal switch in our filter design. For demonstration, a frequency-tunable tri-function filter has been designed using substrate-integrated waveguide resonators. It is shown that the designed filter is able to exhibit three responses over a 1.2:1 frequency tuning range. [ABSTRACT FROM PUBLISHER]

This paper presents a new class of filter responses called pseudo-Butterworth filter response. A detailed mathematical analysis on the pseudo-Butterworth response has been carried out. In addition, the comparison between the typical complete Butterworth response and the new pseudo-Butterworth response is provided. The theory for the pseudo-Butterworth filter response indicates that bandwidth tuning maintaining excellent return loss performance can be executed adjusting not all coupling structures of a filter. As an example, a bandwidth tuning method for a second-order filter is presented. For verifying the presented bandwidth tuning theory, a second-order substrate-integrated waveguide (SIW) resonator filter has been designed, fabricated, and measured. As an example of applications to higher order filters, a theory for the fourth-order pseudo-Butterworth responses and a bandwidth tuning method for the fourth-order filters have been developed. A fourth-order filter has also been fabricated and measured for verifying the presented bandwidth tuning technique. [ABSTRACT FROM AUTHOR]

This paper presents a new lumped-element absorptive bandstop filter topology. It exhibits infinite attenuation at the center frequency and no reflection regardless of the quality factor ( $Q$ -factor) of the resonators. The new topology exhibits the absorptive characteristic with extremely low- $Q$ resonators, whereas an absorptive bandstop filter topology in the literature cannot be used for such resonators. This superiority of the new filter topology is verified by comparing it with the topology in the literature. The filter designed using the new topology can change its center frequency by tuning only the capacitor values of the resonators. A 1.6:1 frequency tuning range maintaining the absorptive characteristic has been obtained by measurement. [ABSTRACT FROM PUBLISHER]

In this paper, we provide a new third-order bandstop filter (BSF) topology with attenuation control capability. The proposed BSF topology contains inter-resonator coupling structures and it can duplicate the frequency responses of the conventional BSF topology, which does not have inter-resonator couplings. In addition, the proposed topology can control the attenuation level by adjusting the resonant frequency of each resonator, and this characteristic cannot be obtained from the conventional one. Two BSFs utilizing the conventional and proposed topologies are designed, fabricated, and measured to support the presented theory. Two filters have been designed and fabricated employing frequency tunable substrate integrated cavity resonators. The measurement verifies that the BSF utilizing the presented topology is capable of adjusting the attenuation level as well as reproducing the conventional bandstop responses. [ABSTRACT FROM PUBLISHER]

A tunable highly integrated bandpass–bandstop filter (BSF) cascade that is capable of dynamically relocating its transmission zeros arbitrarily close to its passband without causing parasitic resonances and additional mismatch loss is demonstrated in this paper. An arbitrary phase BSF that enables a compact cascade topology and an extension of the classical coupling matrix synthesis for filters terminated with a complex impedance are the key advantages of the proposed bandpass–BSF cascade. Independent and simultaneous tuning of the passband center frequency, bandwidth, and transmission zeros are possible in this topology. The proposed cascade synthesis approach is experimentally validated using commercially available lumped elements. In these implementations the transmission zeros can be placed on the skirt of the bandpass filter and can be used to create high isolation (50-dB notch 4.7% away from the passband edge), increase the selectivity of the passband, and tune the fractional bandwidth from 3% to 9% without additional mismatch loss. [ABSTRACT FROM AUTHOR]

In this letter, we present a third-order frequency-agile substrate-integrated waveguide (SIW) filter with a pair of transmission zeros. In addition, a relationship between the symmetric and asymmetric topologies for having same frequency responses is derived in this letter so that the coupling values of the asymmetric filter topology can be directly obtained from those of the symmetric one. For practical implementation, we also present a new methodology for physical implementation of a source-to-multiresonator coupling for frequency-tunable SIW resonator structure. The filter has been fabricated and measured for verifying the presented filter topology. [ABSTRACT FROM PUBLISHER]

Dual-mode and/or dual-band microwave filters often employ high quality factor (Q), physically large, and frequency static cavity resonators or low Q, compact, and tunable planar resonators. While each resonator type has advantages, choosing a dual-mode and/or dual-band resonator type is often limited by these extremes. In this paper, a new dual-mode and/or dual-band resonator is shown with Q (360–400) that is higher than that of planar resonators while being frequency tunable (6.7% tuning range) and compact relative to standard cavity resonators. In addition, both degenerate modes of the resonator are tunable using a single actuator. The resonator is used in a single-resonator two-pole filter design and a double-resonator dual-band filter design. An analytical model is developed and design techniques are given for both designs. Measured results confirm that the proposed resonator fits between the design spaces of established dual-mode and/or dual-band resonator types and could find application in systems that require a combination of relatively high Q, tuning capability, and ease of integration. [ABSTRACT FROM PUBLISHER]

The theory of a new type of bandstop-to-all-pass reconfigurable filter is developed in this work. A bandstop filter structure with both source-to-load and inter-resonator coupling is implemented. The synthesis equations are manipulated such that the signals in the filter's resonators and source-to-load transmission line can be made to constructively or destructively interfere at the output port through tuning of the resonant frequency of the filter's resonators. The relationship between resonator quality factor, filter bandwidth, and the all-pass response state is shown for the first time. The theory is proven through fabrication of a bandstop-to-all-pass filter with resonator unloaded quality factors greater than 500. Measured results show that the filter can continuously tune from insertion loss of 2.1 dB in the all-pass state to insertion loss of 69 dB in the bandstop state at the center frequency of the filter. Analog tuning of the attenuation level is also shown. The capability to switch from an all-pass to a variable-attenuation bandstop response enables a spectrally aware system to operate over wide bandwidths when interference levels are low and to dynamically add bandstop responses when interference affects its performance or signal equalization is required. [ABSTRACT FROM PUBLISHER]

This paper presents a cascade of tunable bandstop filters with a wide spurious-free upper passband, which is completely spanned by the tuning range of the notch responses. A collection of resonators is shown to be able to provide bandstop filter responses over a 7.8 to 1 tuning range. By using spurious-free upper passband aperture-coupled cavity bandstop filters, multiple resonators, each with octave tuning, can cover a multioctave frequency range in a cascade. It is shown that the upper passband of this type of filter is limited by the reactance of the coupling apertures, which produce an unwanted in-band resonance unless designed properly. The details of this design process are explained and used to design a six-resonator bandstop filter cascade that is able to provide a bandstop filter response with up to 55 dB attenuation over the continuous band of 0.85–6.6 GHz. Through dynamic allocation of the cascade circuit's transmission zeros, one-, two-, three-, and four-pole bandstop filter responses of variable bandwidth can be realized over different frequency ranges, offering numerous bandwidth-attenuation level tradeoff combinations. [ABSTRACT FROM PUBLISHER]

A tunable, substrate integrated, high Q bandpass–bandstop filter cascade is demonstrated that is capable of providing up to 100 dB of isolation between two dynamically selectable frequencies of interest. Evanescent-mode cavity filters are used in the cascade to allow high quality factors and wide tuning ranges. It is shown that it is theoretically possible to tune the cascade circuit's passband and transmission nulls independently over more than an octave in frequency while displaying good performance. General coupling matrix theory, ABCD matrix theory, and measurements are shown that describe the behavior of the filter cascade over the entire frequency tuning range. The high, dynamic isolation provided by cascade circuits has potential to be useful in concurrent transmit–receive systems, shared aperture systems, and spectral environments with strong co-site interference. [ABSTRACT FROM AUTHOR]

This paper describes an analytic design procedure for microstrip tunable filters. Step-impedance resonators are employed and loaded with varactors for achieving agility in the filter response. Fixed lumped capacitors are utilized as admittance inverters in order to minimize the number of varactors in the filter. An analytic approach for filter design makes it possible to achieve a tunable filter showing the same frequency response when the center frequency is adjusted. A two-pole microstrip tunable filter whose center frequency can be adjusted from 1.1 to 1.5 GHz is designed to demonstrate the validity of design theory. A prototype tunable filter operating from 2.1 to 2.7 GHz is also designed and measured. A good agreement between the measured and simulated results is shown. Finally, three- and four-pole tunable filters are designed to show straightforward application of the presented design method to higher order tunable filter design. [ABSTRACT FROM AUTHOR]

This paper introduces a synthesis method for triple-passband microwave filters. A frequency transformation is developed for finding the locations of poles and zeros of the triple-pass-band filter. The poles and zeros obtained as such are optimized to achieve a transfer function with a reduced number of transmission zeros in order to reduce the number of cross-couplings. Six- and 12-pole triple-passband filters are synthesized for validation of this proposed method. A 12-pole triple-passband filter is fabricated with a microstripline structure and shown to provide good agreement between synthesis and measurement results. Finally, the frequency transformation for asymmetric triple-passband filters is briefly discussed. [ABSTRACT FROM AUTHOR]

In this letter, we present a design method for K-band substrate-integrated waveguide (SIW) resonator filters utilizing TM02 mode. More importantly, a methodology for suppressing an unwanted higher-order mode (TM11) close to the passband is demonstrated. It is shown that suppressing the unwanted mode give rise to the improved stopband performance. In addition, the resonator used in this filter design has capability of adjusting the resonant frequency, and this allows for compensating the fabrication error. Verification of the presented design method has been carried out by fabricating and measuring the filter. [ABSTRACT FROM PUBLISHER]

This letter proposes a new design method for a bandstop filter with two stopbands. Contrary to a conventional method, the presented method allows for achieving two stopbands without increasing the number of the couplings between resonators and the transmission line connecting the input and output ports. In addition, a filter designed using the presented method can not only have different center frequencies but also have different bandwidths by adjusting the resonant frequencies of the resonators. For verification of the proposed approach a dual-stopband filter using four tunable substrate-integrated waveguide resonators has been designed and measured. [ABSTRACT FROM PUBLISHER]

This letter presents a rigorous extension of the concept for the bandstop filter (BSF) with inter-resonator couplings to a higher order filter. We show that a BSF topology with inter-resonator couplings can exhibit the frequency response that a conventional BSF topology without inter-resonator couplings generates. Equivalence between the coupling matrices of the two topologies are shown. As an example, this letter shows that fourth-order BSFs with and without inter-resonator couplings can exhibit identical frequency responses. [ABSTRACT FROM PUBLISHER]

In the above-named letter [ibid., vol. 23, no. 8, pp. 403?405, Aug. 2013], incorrect versions of Figs. 4, 5, and 6(b) were published. The corrected figures are presented here. The published Figs. 4 and 5 show incorrect slot locations and two out of four slots can be interpreted as negative coupling structures, which does not support the presented theory. The correct version of the filter has the slots with the required positive values and its measured result is presented here [Fig. 6(b)]. To conclude this letter, the authors would like to point out that the above errors do not affect the presented theory in [1]. [ABSTRACT FROM PUBLISHER]