Acoustic-wave-lumped-element Resonators Research Articles

Maximum Bandwidth Analysis With a Universal AWLR‐Based Filter Structure

ABSTRACTThe bandwidth enhancement of the acoustic‐wave‐lumped‐element resonator (AWLR)‐based filter has been investigated for a long time. However, few researches are focused on the maximum bandwidth analysis. Based on a universal circuit structure of the AWLR‐based filter, we proposed the maximum bandwidth analysis of the hybrid filter. In the analysis process, the universal filter structure can be transformed into a novel AWLR‐based filter structure by incidentally excluding most of the other structures (including the reported hybrid filter structures) under the defined electrical characteristics. The novel hybrid filter obtains the maximum fractional bandwidth (FBW). Moreover, the proposed calculation results are better than the simulation results without artificial intervention. They are 2.12kt2 and 1.63kt2, respectively. kt2 is the electromechanical coupling coefficient of the acoustic wave resonator (AWR). Third‐order AWLR‐based bandpass filters with five‐type circuit structures have been manufactured and measured. They include the proposed novel AWLR‐based filter and the hybrid filter with the reported structure based on the universal AWLR‐based filter structure. The experimental results indicate that the proposed novel AWLR‐based filter has the maximum FBW.

A methodology for enhancing bandwidth with keeping steepness of the ladder filters exploiting acoustic‐wave‐lumped‐element resonators

AbstractA methodology to acquire a wide bandwidth and steep transition of a hybrid ladder filter with the acoustic‐wave‐lumped‐element‐resonators (AWLRs) structure is proposed. Different from previous works, a unit of the new AWLR structure with two lumped elements respectively in series and shunt is proposed. It can change both the series‐resonant frequency and the parallel‐resonant frequency of the acoustic wave (AW) resonator to enhance the bandwidth of the hybrid ladder filter. For the ladder structure, all the AW resonators possess the same resonant frequency and impedance. This ladder structure is also used to keep the steep transition of the hybrid filter. For the validations, two‐stage and four‐stage AWLRs‐based hybrid filters are designed, manufactured, and tested. They include (1) a two‐stage filter with an insertion loss (IL) of 0.9 dB, a fractional bandwidth (FBW) of 1.90kt2 (kt2 is the electromechanical coupling coefficient), a center frequency of 1249.3 MHz, and (2) a four‐stage filter with the IL of 1.3 dB, the FBW of 1.86kt2, the SF30 dB/3 dB (shape factor defined as 30‐dB bandwidth divided by 3‐dB bandwidth) of 2.52, the figure of merit (defined as FBW divided by SF30 dB/3 dB) is 0.74kt2 which is the best result, a center frequency of 1249.4 MHz.

Reconfigurable All-Pass-to-Bandstop Acoustic-Wave-Lumped-Element Resonator Filters

Acoustic-wave lumped-element resonator (AWLR)based filters with reconfigurable all-pass-to-bandstop transfer function are reported. They are based on in-series cascaded resonant stages that comprise two parallel RF signal paths, namely: 1) a thru-line and 2) an AWLR-based resonant section. Reconfigurability is achieved by altering the phase of the thruline through varactors. In this manner, the filter response can be reconfigured from an all-pass to an enhanced fractional bandwidth (FBW)-i.e., wider than the electromechanical coupling coefficient k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> -bandstop one. For practical validation, two prototypes were designed and tested. They include 1) a single-stage prototype with insertion loss (IL) <; 0.38 dB in the all-pass mode and a bandstop mode with a center frequency of 917.5 MHz, an FBW of 0.29% (2.4 k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ), and a stopband isolation of 21.2 dB and 2) a two-stage prototype that exhibits an all-pass mode with IL <; 0.67 dB and a bandstop mode centered at 916.9 MHz, with an FBW of 0.47% (3.8 k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">v</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) and a tunable isolation between 20 and 43 dB.

Symmetrical Quasi-Reflectionless SAW-Based Bandpass Filters With Tunable Bandwidth

Surface acoustic wave (SAW)-based bandpass filters (BPFs) with symmetrical quasi-reflectionless characteristics and continuously tunable bandwidth (BW) are reported. They are based on in-series-cascaded symmetrical quasi-reflectionless acoustic-wave (AW)-lumped-element resonator (AWLR)-based networks whose input and output ports are connected to resistively terminated AWLR-based bandstop filter (BSF) sections. The proposed concept is presented through the coupling routing diagram (CRD) formalism that allows their design using coupled-resonator-based synthesis. It is shown that by incorporating variable reactance elements in their BPF sections, continuous-type BW tuning can be realized. In addition, the fractional BW (FBW) of the filter can be designed to be wider than the electromechanical coupling coefficient $k_{t}^{2}$ of its SAW resonators. This allows enhanced-FBW passbands to be obtained. For proof-of-concept validation, two prototypes were designed and measured. They include a single-stage prototype with 1.9:1 continuously tunable BW and a static two-stage with effective quality factor > 6500, FBW of $1.2k_{t}^{2}$ , and return loss >15 dB throughout its passband and stopband ranges.

Multiband Acoustic-Wave-Lumped-Element Resonator-Based Bandpass-to-Bandstop Filters

A new class of multiband acoustic-wave resonator-based RF filters with reconfigurable bandpass-to-bandstop (BP-to-BS) transfer function characteristics is reported. They are based on a hybrid integration scheme in which multiresonant acoustic-wave-lumped-element-resonators (AWLRs) are combined with five static and one switched impedance inverters. Each multiresonant AWLR controls the number of bands and their center frequencies. Switching between the bandpass and bandstop modes of operation is achieved by changing the coupling path between its input/output ports through an RF-switched impedance inverter. For practical validation purposes, two prototypes were designed, manufactured, and tested. They include a single-band BP-to-BS filter at 418 MHz and a dual-band BP-to-BS filter with bands centered at 418 and 433.9 MHz.

Hybrid Bandpass-Absorptive-Bandstop Magnetically Coupled Acoustic-Wave-Lumped-Element-Resonator Filters

This letter presents a novel advancement in acoustic-wave-lumped-element-resonator (AWLR) filters, which improves transmission zero isolation by over 40 dB without adding any additional components. By spacing and orienting the AWLR-lumped inductors for intentional magnetic coupling, a hybrid bandpass-absorptive-bandstop filter is generated. The filter is demonstrated in three configurations using aluminum nitride contour-mode resonators and off-the-shelf lumped elements. Measured performance of as low as 3.2-dB insertion loss, bandwidth of 0.87%, and transmission zero notch depth greater than 60 dB is shown.

Constant In-Band Group-Delay Acoustic-Wave-Lumped-Element-Resonator-Based Bandpass Filters and Diplexers

The RF design of acoustic-wave (AW)-resonator-based bandpass filters (BPFs) with constant in-band group delay ( $\tau _{g}$ ) and analog transfer-function reconfigurability is presented. The proposed filter concept is based on $N$ identical acoustic-wave-lumped-element resonators (AWLRs) that are electromagnetically coupled through impedance inverters and result in quasi-elliptic-type transfer functions shaped by $N$ poles and $2N$ transmission zeros (TZs). Unlike conventional ladder- or lattice-type AW-resonator filter configurations, they facilitate: 1) flat in-band $\tau _{g}$ that does not depend on the electromechanical coupling coefficient ( $k_{t}^{2}$ ) of its constituent AW resonators and does not require the incorporation of lossy elements; 2) passbands with fractional bandwidths (FBWs) that can exceed $k_{t}^{2}$ ; and 3) continuous analog-type FBW tuning. The operating principles of the devised concept are presented through two different filter architectures that, respectively, present advantages in terms of size and FBW tuning and are subsequently extended to the design of RF diplexers. They are experimentally verified through the following prototypes: 1) a three-pole/six-TZ BPF that is centered at 418 MHz and exhibits bandwidth (BW) of 0.3 MHz, minimum in-band insertion loss of 2.1 dB (i.e., effective quality factor $Q_{\mathrm {eff}}$ of 9000), and in-band $\tau _{g}$ between $1.78 \pm 0.02~ \mu \text{s}$ ; 2) a two-pole/four-TZ BPF with 2.4:1 BW tuning ratio, flat $\tau _{g}$ , and $Q_{\mathrm {eff}}> 9000$ ; and 3) a flat $\tau _{g}$ diplexer with two transmission bands centered at 418 and 433.9 MHz.

Switched‐bandwidth SAW‐based bandpass filters with flat group delay

Tunable surface-acoustic-wave-(SAW)-based bandpass filters (BPFs) with flat in-band group delay (Tg) and variable passband bandwidth (BW) are reported. They are based on N hybrid acoustic-wavelumped-element-resonator (AWLR) modules shaped by one lumpedelement resonator and K RF-switched SAW resonators that are arranged in a Gaussian-type source-to-load impedance-inverter network. Fractional bandwidth (FBW) tuning is achieved by reconfiguring the number of SAW resonators within the AWLR modules. Major advantages of the proposed filter concept when compared to conventional SAW BPFs are as follows: (i) they do not depend on the electromechanical coupling coefficient (k t 2 ) of the SAW resonators - FBW>k t 2 can be realised -, (ii) they do not require lossy elements or SAW resonators with different frequencies, and (iii) they can be tuned. For experimental-validation purposes, a 433.9 MHz two-pole/fourtransmission-zero prototype was built and measured. It exhibited flatin-band-T g passbands with discretely-tunable BW between 0.18 and 0.45 MHz (i.e. 2.5:1 tuning ratio).

Single and Multiband Acoustic-Wave-Lumped- Element-Resonator (AWLR) Bandpass Filters With Reconfigurable Transfer Function

New types of single- and multiband acoustic-wave resonator-based bandpass filters (BPFs) with continuously variable transfer function in terms of bandwidth (BW), selectivity, and out-of-band isolation (IS) are presented. They are based on single-/multiband resonant branches that are shaped by high-quality-factor ( $Q$ ) acoustic-wave-lumped-element resonators (AWLRs) and are connected in parallel to an all-pass network through variable impedance inverters. With the proposed configuration, quasi-elliptic-type passbands with wide fractional BWs (FBWs)—i.e., larger than the electromechanical coupling coefficient $k_{t}^{\mathrm {2}}$ of its constituent acoustic-wave resonator—can be realized and continuously tuned while preserving the high- $Q$ characteristics of its acoustic-wave resonators. Furthermore, by reconfiguring the location of the transmission zeros (TZs) of the AWLRs, tunable out-of-band IS profiles and asymmetrical passband BWs can be created. The operating principles of the devised spectrally agile filter concept are demonstrated through coupling-matrix-based synthesis and linear-circuit simulations. They are experimentally validated at the UHF band through the realization of: 1) a three-pole/six-TZ single-band prototype at 418 MHz with tunable BW between 0.16 and 0.49 MHz (FBW: 0.5– $1.5k_{t}^{\mathrm {2}})$ for an insertion loss (IL) between 3.3 and 1.2 dB (i.e., effective $Q >10$ 000), reconfigurable order (first-to-third-order states), out-of-band IS controllability in terms of BW (18–26.5 MHz at the upper stopband) and magnitude (23–46 dB at 426 MHz), and an intrinsically selectable all-pass mode and 2) a six-pole/nine-TZ dual-band BPF with two transmission bands located at 418 and 433.9 MHz. They, respectively, feature symmetrical and asymmetrical BW tuning between 0.3–0.54 and 0.6–1.23 MHz with in-band IL <1.7 dB for all tunable states.

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.

High- &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$Q$&lt;/tex-math&gt;&lt;/inline-formula&gt; Bandstop Filters Exploiting Acoustic-Wave-Lumped-Element Resonators (AWLRs)

This brief focuses on the radio frequency (RF) design of novel high-quality-factor $(Q)$ reflective-type bandstop filters (BSFs) with narrow bandwidth (BW), small physical size, and large stopband attenuation levels. They are realized by merging acoustic-wave-lumped-element resonators (AWLRs) and lumped-element impedance inverters in a single filter architecture. In this manner, deep stopband notches that correspond to resonators with effective quality factors $(Q\mbox{s}_\mathrm{eff})$ on the order of 1000 are created. Furthermore, the obtained rejection bands exhibit fractional BWs that are no longer restricted by the electromechanical coupling coefficient ${k_t^2}$ of the constituent acoustic wave (AW) resonators as opposed to the traditional all-AW filters. Experimental prototypes of first- and third-order frequency-static BSFs were built and tested at 418 MHz to validate the proposed concept. Measured stopbands with 3-dB BWs from 0.2 to 0.5 MHz, maximum stopband attenuation in the range of 15–43 dB, and passband insertion loss between 0.37 and 1.6 dB are reported. Moreover, an electronically switchable BSF with dynamic notch allocation is presented as a practical demonstrator device for switchable RF-interference suppression. A design methodology that relates to coupled-resonator filter formalism is also addressed to facilitate the systematic theoretical synthesis of the engineered AWLR-based reflective-type BSFs.

Coupling-Matrix-Based Design of High-&lt;formula formulatype="inline"&gt;&lt;tex Notation="TeX"&gt;$Q$&lt;/tex&gt;&lt;/formula&gt; Bandpass Filters Using Acoustic-Wave Lumped-Element Resonator (AWLR) Modules

This paper presents an original and simple coupling-matrix-based synthesis methodology for the design of a new class of bandpass filters (BPFs) that employ hybrid acoustic-wave-lumped-element resonator (AWLR) modules with improved out-of-band isolation (IS). The proposed BPFs feature quasi-elliptic-type frequency response—shaped by $N$ poles and $2N$ transmission zeros (TZs) for an $N$ th-order transfer function, compact physical size, and high effective quality factors $(Q_{\rm eff})$ of the order of 1000. Despite the use of acoustic wave (AW) resonators, passbands exhibiting fractional bandwidths (FBWs) that are no longer limited by the electromechanical coupling coefficient $(k_{t}^{2})$ of the constituent AW resonators are obtained. A coupling-matrix-based model of a multi-mode AW resonator is also reported. It facilitates the incorporation of high- and low-frequency spurious modes that are present in a realistic filter response so that they can be anticipated at the synthesis and simulation levels. For proof-of-concept validation purposes, two BPF prototypes at 418 MHz made up of commercially-available surface acoustic wave (SAW) resonators and surface mounted devices (SMD) were built and measured. They perform first- (one pole and two TZs) and second-order (two poles and four TZs) transfer functions with measured passband insertion losses (IL) between 2.4–5.4 dB, $Q_{\rm eff}$ between 1600–1900, 3-dB absolute bandwidths ranging from 0.52 to 1 MHz (i.e., 1.6–3.2 times $k_{t}^{2}$ ), and minimum IS levels between 25–46 dB.

Hybrid Acoustic-Wave-Lumped-Element Resonators (AWLRs) for High-&lt;formula formulatype="inline"&gt;&lt;tex Notation="TeX"&gt;$Q$&lt;/tex&gt; &lt;/formula&gt; Bandpass Filters With Quasi-Elliptic Frequency Response

A new class of bandpass filters with quasi-elliptic frequency response, small physical size, and effective quality factors $(Q{\bf s}_{\bf eff})$ of the order of 1000 are presented. They are based on novel hybrid acoustic-wave-lumped-element resonator (AWLR) modules that enable the realization of bandpass filters with fractional bandwidths (FBWs) that are much larger (0.91–5.1 $k_{t}^{2}$ ) than the electromechanical coupling coefficient $(k_{t}^{2})$ of conventional acoustic wave (AW) resonator filters that exhibit FBWs between 0.4–0.8 $k_{t}^{2}$ . In addition, they exhibit $Q{\bf s}_{\bf eff}$ that are 25–50 times larger than in traditional lumped-element filter architectures. Quasi-elliptic single-pole bandpass filters (one pole and two zeros) made up of commercially available one-port surface AW resonators and surface-mount-device components are manufactured and measured at 418 MHz. Various passbands with FBW between 0.07% and 0.4% and insertion loss below 1.25 dB (i.e., $Q{\bf s}_{\bf eff}$ in the range of 1525–5150) are demonstrated. Second-order bandpass filters (two poles and four zeros) with 0.24% FBW and less than 1.15 dB of insertion loss (i.e., $Q{\bf s}_{\bf eff}$ of 4000) are also shown to improve the passband selectivity and out-of-band rejection. These filters feature dynamic stopband characteristics by tuning the position of their transmission zeros. The operating principle, theoretical analysis, and design guidelines of the AWLR module, as well as a comparison with traditional all-AW ladder-type filters, are also reported. An original and rather simple $RLC$ -circuit equivalent of AW resonators that facilitates the incorporation of spurious modes into a simple Butterworth–Van-Dyke model for a more accurate synthesis of AWLR-based filters is extracted from measured two-port S-parameters.