Absorptive Bandstop Filter Research Articles

Balanced BPF With Dual-Port Quasi-Reflectionless Characteristic and Selectivity Enhancement

A balanced microstrip bandpass filter (BPF) with quasi-reflectionless (QR) characteristics on both input and output ports and good common-mode (CM) rejection is proposed in this brief. The presented QR BPF consists of a Pi-shaped BPF and two absorptive bandstop filters (ABSFs). Good QR performance is achieved by the complementary duplexer architecture formed with these two parts. The balanced microstrip-to-slotline (MSTS) transition structures are used to achieve exceptional CM rejection without deteriorating differential-mode (DM) transmission. The in-band energy is transmitted by the BPF section, where the ABSFs appear as open circuits. The out-of-band energy is absorbed by a pair of loaded resistors in the bandstop section. Through detailed analysis, it shows that by loading the ABSF structures at both ends of the BPF section, the DM passband selectivity is improved and six quasi-transmission poles (Quasi-TPs) are generated on both sides of the DM passband to eliminate out-of-band reflections. To verify the feasibility of the proposed model, a prototype was designed and fabricated, whose simulations and measurements are in good agreement.

Miniature Dual-Band Absorptive Bandstop Filters With Improved Passband Performance

In this paper, a miniature dual-band absorptive bandstop filter (ABSF) design with improved lower passband performance is proposed. Specifically, a stepped-impedance short-circuited stub is introduced to a bridged-T coil-based dual-band ABSF design, such that the insertion loss around the desired passband center frequency can be largely reduced while the absorptive stopband performance remains intact. This also helps equalize the lower passband insertion loss response and achieve a flatter passband, which is especially advantageous when cascading multiple ABSFs for improving the stopband rejection or increasing the number of stopbands. Closed-form design formulas are established to facilitate the synthesis of proposed dual-band ABSF. As an example, a 4.8/9.6-GHz dual-band ABSF in GaAs is realized with a measured insertion loss within 0.85± 0.12 dB from dc to 2.6 GHz. The circuit size is only 1.6 mm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times $ </tex-math></inline-formula> 1.84 mm, and the corresponding electrical size is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.013\lambda _{0} \times 0.015\lambda _{0}$ </tex-math></inline-formula> at the designated passband center frequency of 2.4 GHz. Application of the proposed dual-band ABSF for the passband performance improvement of cascaded ABSF designs is also demonstrated, and good performance is obtained.

Ultrawideband Antipodal Tapered Slot Antenna With Reflectionless Notched Band

In this letter, an ultrawideband (UWB) antipodal tapered slot antenna with reflectionless notched band is proposed and investigated. The proposed antenna uses an integrated absorptive bandstop filter, consisting of two quarter-wavelength open-circuit stubs and a 50 Ω resistor, to form a reflectionless notched band. Consequently, the input energy at the notched frequency is absorbed by the resistor rather than reflected. An antenna prototype is designed and fabricated to validate the design. Both simulated and measured results present that the realized gain drops over 40 dB at 5.5 GHz, and the reflection coefficient is below –10 dB from 3.1 to 10.6 GHz. Besides, a low ringing level of –23.4 dB of the measured reception pulse shows a good time-domain performance. Therefore, the proposed antenna could be applied in UWB short-pulse system to inhibit the 5 GHz WLAN interference.

Quasi-Reflectionless Double-Sided Parallel-Strip Line Bandpass Filter With Enhanced Selectivity

A novel quasi-reflectionless bandpass filter (BPF) using double-sided parallel-strip line (DSPSL) is proposed in this brief. It is composed of a BPF and two symmetric absorptive bandstop filters (ABSFs) terminated by a pair of resistors that can dissipate the input untransmitted signal energy. Through detailed analysis, the high characteristic impedance is required for the open-circuited stubs of the ABSFs to enhance the selectivity. This cannot be realized in traditional microstrip designs, but it can be obtained easily by offsetting the DSPSL. Meanwhile, the BPF with T-shaped structure consists of a half-wavelength ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda /2$ </tex-math></inline-formula> ) DSPSL with a centrally-loaded <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda /2$ </tex-math></inline-formula> stub, which answers for generating two transmission zeros (TZs) to achieve better out-of-band suppression. For demonstration, a prototype is designed, fabricated and measured, and the simulated and measured results with good agreement are presented.

4-Pole Tunable Absorptive Bandstop Filters Using Folded Coupled-Lines With an Inductor

The ever-growing field of wireless communication requires efficient radio frequency transceivers with tunable absorptive bandstop filters that can prevent the interference of out-of-band and reflected signals. This paper presents 4-pole tunable absorptive bandstop filters using folded coupled-lines with an inductor. A 4-pole filter transfer function is achieved to obtain an absorptive bandstop filter coupling diagram with 45° electrical-length transmission-line source/load coupling. The folded-loaded coupled-lines with an inductor are proposed to obtain the control for the required coupling coefficients based on the filter transfer functions. The required quality factor extracted through the transfer function is accomplished using a distributed resonator loaded with asymmetrical varactors without auxiliary circuits, such as an impedance-matching network. Two types of filters were designed and built on a substrate with ε <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> = 3.42 and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">h</i> = 30 mil. The center frequency tuning was performed using silicon varactors. The first filter shows a return loss of 16.2–38 dB and a rejection level of 41–48 dB in the tuning range 0.83–1.09 GHz. The second filter results in a measured return loss of 14–34 dB and a rejection level of 40.1–51 dB at 1.16–1.47 GHz. 4-pole tunable absorptive bandstop filters with high attenuation and reflectionless responses were achieved which have applications in cognitive radios and carrier aggregation systems.

Combination of absorptive notch filter and tunable dual‐band conventional notch filter

AbstractIn this paper, a new kind of absorptive bandstop filter composing three series‐coupled lines is designed, investigated, and fabricated. The proposed circuit is indeed a combination of a dual‐band conventional notch filter and a single‐band absorptive filter generating two conventional and an absorptive notch. More precisely, the circuit introduces a fixed absorptive notch at 4.22 GHz and two electronically tunable (approximately 20%) conventional notches at 1.62–2 and 4.71–5.75 GHz, respectively. Besides, the FBWs of the first conventional notch, the second one, and the absorptive notch are 5.5 ± 1, 3.5 ± 1, and 3 ± 1%, respectively. The instantaneous surface current density of the circuit is depicted to investigate the generation of notches. The circuit has a compact size of 44.7 × 11.85 mm2 equaling to 0.489 × 0.129 ; moreover, it can successfully absorb more than 98% of the input power at its absorptive notch.

Scattering Suppression of Closely-Packed Cross-Band Patch Antennas Using Absorptive/Reflectionless Filtering Techniques

This paper presents a novel method for cross-band scattering cancelation in closely-spaced antennas inspired by absorptive/reflectionless filtering principles. Prior-art approaches that combine a bandpass-filter (BPF) unit and an antenna in multiple-input multiple-output (MIMO) systems can improve cross-band isolation between the antenna ports. However, such procedure is ineffective in suppressing cross-band scattering, which leads to radiation-pattern distortion. This is because in closely-packed antennas each radiating element behaves as an external source for the others, so that it induces currents into them. To overcome this problem, a method originating from absorptive/reflectionless filtering techniques is reported here and applied to a dual-antenna scenario. In complementary-diplexer-based reflectionless BPFs, an absorptive bandstop-filter-(BSF)-type branch loaded by a resistor is utilized to absorb the out-of-band RF power not transmitted by the main BPF channel. By using this concept in a first patch-type antenna and owing to the reciprocity principle, the RF coupled energy or induced currents in the first antenna due to the surrounding antenna can be absorbed. Thus, radiation-pattern distortion for the surrounding antenna is avoided. Equivalent-circuit, simulated, and measured results are provided to show the effectiveness of this engineered scattering-suppression method, which is believed to be promising for application in cross-band multi-antenna systems.

Reflectionless Filtering 90° Coupler Using Stacked Cross Coupled-Line and Loaded Cross-Stub

In this letter, a wideband reflectionless filtering 90° coupler with a wide stopband is proposed, which consists of two parallel connected channels. The main channel is implemented by the stacked cross coupled-lines, which can not only introduce a filtering 90° coupler with a compact size but also achieve a wide-stopband response. Meanwhile, the stepped-impedance microstrip line, loaded cross-stub, and resistor are used to construct the absorptive bandstop filter in the auxiliary channel, which absorbs the reflected signal from the filtering coupler to obtain the wideband reflectionless operation. To verify the aforementioned mechanism, a wideband reflectionless filtering 90° coupler operating at 1.47–2.53 GHz is implemented and fabricated. The measured results demonstrate that the proposed coupler has the merits of low passband insertion loss, low amplitude and phase imbalances, wide stopband, and wide reflectionless range.

Compact Wideband Reflective/Absorptive Bandstop Filter With Multitransmission Zeros

Novel wideband reflective bandstop filters (RBSFs) and absorptive bandstop filters (ABSFs) with high stopband rejection (SR) and good frequency selectivity are proposed. Based on a coupled line (CL) with an open-circuited stub, a broad stopband response with three transmission zeros (TZs) is implemented in a compact circuit configuration. Two modified CL structures are loaded at the ports resulting in two additional TZs. In addition, a grounded resistor is introduced to absorb unwanted signals in the stopband. For demonstration, the five-zero RBSFs and three-zero ABSFs operating at 2 GHz are designed and built. The reflective filter shows a 35-dB SR with 79.5% relative bandwidth (RB). The absorptive filter with 69.5% RB of 24-dB SR achieves an all-passband 12.2-dB return loss. Good agreements between simulated and measured results are observed.

Theory and Design of Frequency-Tunable Absorptive Bandstop Filters

Absorptive bandstop filters are a relatively new class of bandstop filter, which are able to achieve very high levels of stopband rejection with relatively low-quality-factor resonators, in contrast to typical reflective bandstop filters, whose stopband rejections are limited by the quality factors of their resonators. This paper performs an in-depth theoretical and practical analysis of this class of filter, presenting design principles for reducing the sensitivity to process variation, increasing the tuning range over which the filters can operate with good performance, and addressing the practical non-ideal effects of implementation, such as frequency variation of couplings and quality factor. Four varactor-tuned microstrip bandstop filters are presented to verify the presented theory. They illustrate the design tradeoffs between selectivity and tuning range, choice of coupling topology and tuning range, and show the benefits and drawbacks of cascading stages to create higher-order filters.

Narrowband balanced absorptive bandstop filter integrated with wideband bandpass response

A novel narrowband balanced absorptive bandstop filter (BABSF) with wideband bandpass response is proposed, which consists of four coupled-line sections with open-circuited stubs, two half-wavelength transmission lines, and two ground resistors. Excellent differential-mode response with narrowband bandstop response and wideband bandpass property is realised for high frequency selectivity. Good common-mode suppression can be achieved, rejecting the interference noise. For demonstration, a BABSF prototype operating at 2 GHz is designed with good agreement between simulated and measured results.

Miniature Multi-Band Absorptive Bandstop Filter Designs Using Bridged-T Coils

In this paper, miniature absorptive bandstop filters (ABSFs) with one to three stopbands are demonstrated. First, the proposed single-band ABSF is realized by the introduction of one lossy resonator to a conventional stub bandstop filter, while the circuit size is largely reduced by employing bridged-T coils to replace the transmission line sections. Next, by employing the dual-band design of bridged-T coil, a miniature dual-band ABSF can be achieved. The proposed tri-band ABSF is then implemented through the cascade of the proposed single- and dual-band ABSFs. As a demonstration, three proposed single-, dual-, and triple-band ABSFs are implemented in a commercial GaAs pHEMT process. Much smaller circuit size than the conventional multi-band ABSFs and good measured performance are achieved.

Absorptive Bandstop Filter With Prescribed Negative Group Delay and Bandwidth

An absorptive bandstop filter is proposed and synthesized with prescribed negative group delay (NGD) and negative group delay bandwidth (NBW). It is realized with resistor-loaded coupled-line structures where the NGD can be controlled by the Q-factors of resonators. The resistor of the first section is determined for impedance matching, also providing a negative delay. A prototype is developed, following the proposed design procedure, with NGD = −6.5 ns and NBW= 105 MHz obtained. The proposed method is experimentally validated by the good agreement between the synthesized and measured results.

Novel Wideband Absorptive Bandstop Filters With Good Selectivity

This paper presents a new type of wideband absorptive bandstop filter (ABSF) design that is capable of absorbing the input power within the stopband at its Port 1. It is achieved by introducing one additional resistor to a conventional wideband stub bandstop filter. Suitable design procedures are established to facilitate the synthesis of the proposed wideband ABSFs with different filter orders and stopband bandwidths. To validate the proposed design method, microstrip ABSF design examples with a stopband center frequency $f_{0}$ of 2 GHz and 30-dB stopband bandwidths ranging from 12% to 42% are demonstrated. The measured stopband rejection at $f_{0}$ is larger than 60 dB for all design examples. In addition, good input return losses both in the passband and stopband are achieved, and the measured input return loss is better than 10 dB from dc to $2.5f_{0}$ . Larger than 90% of the input power within the stopband can thus be dissipated by the proposed ABSF. To the best of our knowledge, these are the ABSFs with the widest stopband bandwidth ever reported.

A new lumped element bridged-T absorptive band-stop filter

Following a brief review of previous work on bandstop filters, the inadequacy of a recent work to obtain a perfect notch or perfect absorption at the notch frequency ?0 is demonstrated. A simple and elegant alternative solution, based on purely analytical arguments, is then presented. The resulting network is shown to achieve perfect matching as well as perfect absorption at the notch frequency and has several other advantages. A comparison has also been made with the conventional bridged-T band-stop filter.

Frequency-Tunable Low- $Q$ Lumped-Element Resonator Bandstop Filter With High Attenuation

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.

Quasi-Lumped Element Bridged-T Absorptive Bandstop Filter

In this letter, we present a new absorptive bandstop filter topology that is capable of operating in the microwave regime (C-Band) and is capable of creating large attenuation using resonators which are comprised of printed circuit elements and surface mount elements. This particular topology allows for a high level of miniaturization when compared to transmission line realizations. Simulated and measured results are presented and show good correlation. We demonstrate a prototype absorptive bandstop filter that operates within the C-band, achieves > 50 dB of rejection, and has a total size of 7 mm × 8.2 mm.

Acoustic-Wave-Lumped-Element-Resonator Filters With Equi-Ripple Absorptive Stopbands

Acoustic-wave-lumped-element-resonator (AWLR)-based absorptive bandstop filters (ABSFs) with enlarged-bandwidth (BW) equi-ripple stopbands are presented. They are based on reflection-type architectures whose reflective loads are shaped by $N$ identical series-cascaded AWLRs and $N$ static impedance inverters each. In this manner, equi-ripple stopbands whose BW is no longer limited by the electromechanical coupling coefficient $(k_{t}^{2})$ of the acoustic-wave (AW) resonator are obtained through the distribution of $2N$ transmission zeros (TZs). In contrast to conventional ABSF cascades, fewer inverters and identical AW resonators are employed. This results in benefits in terms of lower insertion loss (IL), reduced size, and higher fabrication robustness. A continuous-type tunable-isolation (IS) and BW-control mechanism by means of variable resonator resistance is reported as a novel contribution. Two-, four-, and six-TZ 418 MHz AWLR-based ABSF prototypes with low passband IL (0.96–1.14 dB), large levels of IS (up to 70 dB), and stopband BWs in the range of 0.03–0.11 MHz were manufactured and characterized as proof-of-concept demonstrators. Reconfigurable prototypes with variable levels of IS between 17–65 dB and BW tuning ratio of 1.33:1 were also measured.

Narrowband Coupled-Line Bandstop Filter With Absorptive Stopband

This paper presents a new type of absorptive bandstop filter (ABSF) design that is targeted for absorbing the local-oscillator-to-RF leakage of mixer in a heterodyne receiver. The proposed ABSF is designed to absorb the input power at its Port 1, and it is achieved by introducing one lossy resonator to a conventional narrowband coupled-line bandstop filter. Suitable equivalent circuit models and closed-form design equations are established to enable the direct synthesis of proposed ABSFs. Specifically, a proposed microstrip ABSF with a stopband center frequency $f_{0}$ of 2 GHz is demonstrated. The maximal stopband rejection is 34.8 dB with an input return loss of 27.7 dB, which implies that 99.8% of the input power is dissipated by the proposed ABSF.

Acoustic Wave Resonator-Based Absorptive Bandstop Filters With Ultra-Narrow Bandwidth

Ultra-narrowband absorptive bandstop filters (ABSFs) are presented in this letter. They are based on a hybrid RF-design approach in which high-unloaded quality-factor (Q) acoustic wave (AW) resonators and low-Q lumped-element components are effectively combined to create narrow <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$({&lt;} 0.1\hbox{\%})$</tex> </formula> fractional-bandwidth (FBW) stopbands that exhibit theoretically infinite attenuation and effective quality factor <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(Q_{eff}$</tex></formula> ) of the order of a thousand. The proposed ABSF design approach is experimentally validated with two manufactured prototypes at 418 MHz that consist of commercially-available surface acoustic wave (SAW) resonators and surface mounted devices (SMDs). Measured FBWs as narrow as 0.02% that correspond to a <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$Q_{eff}$</tex> </formula> of 10,000 and maximum stopband isolation of 68 dB validate the conceived ABSF design concept.