External Quality Factor Research Articles

A 135–150-GHz Frequency Tripler With Waveguide Filter Matching

The design and performance of a WR-5 band 135–150-GHz Schottky diode-based frequency tripler which uses waveguide resonator filters for low loss impedance matching is presented in this paper. The filters used in this paper provide filtering, impedance matching, and microstrip (MS) to waveguide transitions in one structure. The matching optimization is achieved by scaling the external quality factors and adjusting the resonance frequency of the filter cavities. This approach transfers most of the tripler’s matching networks from MS circuitry to lower loss rectangular waveguide resonators. This is desirable and useful in particular for submillimeter wave and terahertz frequencies. The device presented is a 47.5 to 142.5 GHz bias-less frequency tripler with a 15-GHz output bandwidth. The tripler was measured to have a conversion loss of 13.1–14 dB across the band, at an input power of 17 dBm. The measured $S_{11}$ at the input port is better than 15 dB and all the reflection zeros from the filter resonances are distinct. The good agreement between measurements and simulations verifies the accuracy of the filter-based design approach.

Reconfigurable Differential Filter With Constant Absolute Bandwidth and High Common-Mode Suppression

A reconfigurable differential filter with constant absolute bandwidth (ABW) and high common mode (CM) suppression is proposed in this letter. The design is aiming to achieve the constant ABW under the differential mode (DM) by choosing the loading place of the varactors on the resonators and adjusting the external quality factor. Under the DM and CM bisection, the frequency of the CM differs from that of the DM, which results in the wide CM suppression without loading extra elements. The transmission zeros are introduced by source–load coupling, which lead to high selectivity. The simulated and measured results are presented with a good agreement to verify the theory and design.

Implementation of Synthetic Material in Dielectric Resonator-Based Filtering Antennas

Design of a broadband filtering antenna, by utilizing a dielectric resonator (DR) also as the radiator, is proposed and experimentally verified in this communication. The relationship between the parameters of a DR and its external quality factor (Q <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">EXT</sub> ) is explored. Results obtained from this exploration indicate that a broad bandwidth of 15.5% is achievable for a filtering antenna (second order, RL <; 15 dB) using a DR as the radiator. In order to achieve the Q <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">EXT</sub> values required in different design specifications, the utilization of synthetic material with air-filled holes is proposed to continuously adjust Q <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">EXT</sub> of a DR antenna. The above-mentioned two steps also enable the synthetic DR to not only act as a radiator but also the last resonator of a bandpass filter. A second-order prototype is demonstrated at X-band with a center frequency of 11 GHz and a bandwidth of 950 MHz. For both S-parameter and far-field results, a good correspondence is achieved between simulation and measurement.

Frequency Tuning of a Magnetron of the 3-mm Wavelength Range Using an Additional Output

The frequency tuning of a magnetron that operates on a spatial harmonic of the π/2–1 oscillation type was investigated. The mechanisms that limit the frequency-tuning range upon a change in the electric length of an additional (second) energy output were clarified and the possibilities of creating preferable conditions for exciting an operating oscillation mode by changing the external quality factor via rotation of the electromagnetic-field structure were determined. Based on calculations, experimental prototypes of magnetrons were manufactured and investigated. Long-term stable oscillations with a power of up to 11.5 kW were achieved. The tuning range using the second output is 300 MHz with a decrease in the output power down to 68% relative to the maximum power. The tuning range is up to 600 MHz at a power level of ~30% of the maximum and a reduced anode voltage.

Microstrip Switchable and Fully Tunable Bandpass Filter With Continuous Frequency Tuning Range

A compact switchable and fully tunable bandpass filter (BPF) with continuous frequency tuning range has been presented in this letter. Since the BPF is formed by two varactor-tuned switchable resonators, which can be switched to higher and lower resonant modes, a very wide frequency tuning range can be achieved. Moreover, the external quality factors and coupling coefficients can be well controlled over the entire tuning range. For demonstration, a second-order BPF with frequency tuning range from 1.1 to 2.1 GHz (fractional tuning range: 63%) and constant absolute bandwidth of 40 MHz has been designed and fabricated with microstrip technology. The prototype circuit size of the BPF is only about $0.06\,\,\lambda {g} \times 0.27\,\, \lambda {g}$ .

Trimming Mechanism of External Quality Factor in HTS Bandpass Filter Using Bulk Resonators

We investigated the usability of a trimming mechanism for varying an external quality factor (Q e ) in a hightemperature superconducting bandpass filter (BPF) using bulk resonators at around 5 GHz. The proposed trimming mechanism was composed of air space between an input port and the first resonator in a Cu cavity, a center conductor of the coaxial through the space, and a dielectric or metallic trimming rod. We decided the geometrical dimensions of the trimming mechanism and estimated the Q e by varying the height of rod (h Q ) using an electromagnetic simulator. These simulated results indicate that the proposed mechanism could function for theoretical Q e values up to those of 8-pole BPF. We then fabricated the trimming mechanism for Q e and measured the Q e by using a vector network analyzer. The measured data almost matched the simulated data. Next, we fabricated a 5-pole BPF with the trimming mechanisms for Q e and obtained good filtering responses.

Substrate-Integrated Waveguide Triple-Band Bandpass Filters Using Triple-Mode Cavities

Substrate-integrated waveguide (SIW) triple-mode triple-band bandpass filters (BPFs) are synthesized, designed, and demonstrated for the first time based on a novel type of SIW triple-mode square cavities perturbed by centered cross-shaped metallized via holes. The resonant characteristics of the cavity are analyzed first to show the range of the realizable frequency ratios, and the design parameters corresponding to the specifications, i.e., the external quality factors and the internal coupling coefficients for the three passbands, can be specified flexibly within certain ranges by exploiting an advanced triple-mode coupling controlling technique. Two SIW triple-mode triple-band BPFs, including a third-order direct-coupled one operating at 13, 14, and 15 GHz and a fourth-order cross-coupled one operating at 11, 12, and 13 GHz, are synthesized, designed, fabricated, and measured for demonstration. Compact circuit sizes and excellent filtering performances have been achieved for the two prototypes.

Compact High-Pole HTS Triband Bandpass Filters Using a New Feeding Structure

We have designed a compact high-pole high-temperature superconducting triband bandpass filter (TBPF) using a new feeding structure. It combines an eight-pole dual-band bandpass filter (DBPF) that uses stub-loaded meander line open-loop resonators with two types of waveguides and an eight-pole single-band bandpass filter (SBPF) that uses meander line open-loop resonators. The DBPF forms the first and third passbands, and the SBPF forms the second one. Using two types of waveguides enables flexible control of the coupling coefficient of the two passbands, and the proposed feeding structure enables the external quality factor of each passband to be controlled. The TBPF was designed and analyzed using an electromagnetic simulator based on the moment method. The center frequencies of the three passbands were 1.5, 2.0, and 2.5 GHz, respectively, with fractional bandwidths of 2.0%. The TBPF was fabricated using double-sided YBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Oy thin film on a 34.0 × 33.0 × 0.5 mm Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> substrate. The measured frequency responses of the filter match the simulated ones well. This filter is advantageous in terms of size miniaturization and high selectivity.

A Design Method of Multimode Multiband Bandpass Filters

In this paper, a general design method for multiband filters is proposed. First, the frequency and element transformations from a low-pass prototype filter to a practical multiband bandpass filter is derived. Afterwards, formulas for extracting the coupling coefficient $k$ between coupled multimode resonators and the external quality factor $Q_{e}$ are also obtained. Thereafter, the design procedure of coupled multiresonator multiband filters is similar to that of single-band filters. A tri-band microstrip filter with tri-mode resonators is successfully designed with the proposed method and fabricated, which validates this theory.

Wideband Diplexer Design and Optimization Based on Back-to-Back Structured Common Port

A “back-to-back” structured common port is proposed with high isolation obtained between the two channels. This port has the advantage of being able to easily achieve a low external quality factor ( $Q_{e}$ ). By introducing this type of port, a “back-to-back” structured diplexer with wide bandwidth is designed and fabricated. The bandwidths of each passband are 9.5% and 13.6%. Each channel filter is optimized by using the Cauchy method. Guided by a real-time Cauchy extracting system, the diplexer is tuned. The measured reflection coefficient in the passbands is below −23 dB, and the average insertion losses are 0.7 and 0.55 dB. The measured isolation between the two passbands is higher than 34 dB. An agreement is found between measured and simulated results.

Design of an Ultracompact On-Chip Bandpass Filter Using Mutual Coupling Technique

In this paper, design of an ultracompact bandpass filter (BPF) in GaAs technology without compromising its electrical performance is investigated by means of both theoretical analysis and electromagnetic simulation. In particular, the relationship between the external quality factor and the coupling coefficient of the second-order BPF is formulized to better understand the principle of the mutual coupling effect. To prove the concept, the designed filter is implemented in a commercial 0.1- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> GaAs technology. A step-by-step design guideline is elaborated. The BPF has not only the merits of ultracompactness, but also remarkable insertion loss (IL) compared with other state-of-the-art on-chip designs. The measurement results show that the 1-dB bandwidth of the BPF is from 28 to 36 GHz, while the IL is less than 1 dB at 29.5 GHz. In addition, more than 40-dB rejection is achieved from 56 to 69 GHz. The size of the filter is only <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$230 \times 280\,\,\mu \text{m}^{\mathrm {\mathbf {2}}}$ </tex-math></inline-formula> , excluding the pads, which is equivalent to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.074 \times 0.09\,\,\lambda \text{g}^{\mathrm {\mathbf {2}}}$ </tex-math></inline-formula> at 28 GHz. To the best of our knowledge, the proposed design is known to be the most compact one in the open literature using GaAs technologies.

Quality factor of a transmission line coupled coplanar waveguide resonator

We investigate analytically the coupling of a coplanar waveguide resonator to a coplanar waveguide feedline. Using a conformal mapping technique we obtain an expression for the characteristic mode impedances and coupling coefficients of an asymmetric multi-conductor transmission line. Leading order terms for the external quality factor and frequency shift are calculated. The obtained analytical results are relevant for designing circuit-QED quantum systems and frequency division multiplexing of superconducting bolometers, detectors and similar microwave-range multi-pixel devices.

Synthesis Applied 4th-Order Constant Absolute Bandwidth Frequency-Agile Bandpass Filter With Cross-Coupling

A novel 4th-order cross-coupled electrical tunable bandpass filter with constant absolute bandwidth and high frequency selectivity is proposed. The electric coupling coefficients and the magnetic coupling coefficients of the cross-coupled resonators and the external quality factor of the tunable filter are investigated. The design curves of coupling coefficients and external quality factor versus the resonant frequency are synthesized for the cross-coupled tunable filter. Tunable resonator with separate electric dominant and magnetic dominant coupling regions is designed to build the cross-coupled tunable filter topology. Theoretical analysis is derived to estimate the coupling coefficients and the performance of the proposed filter. A prototype is fabricated and measured. The measurement shows that the filter has a −3 dB bandwidth of 117 ± 3MHz from 0.95 to 1.48 GHz. Two cross-coupled transmission zeros beside the passband are generated, and the frequency selectivity is enhanced. The −30 dB/−3 dB bandwidth shape factor is 1.83~1.93.

Broadband Circularly Polarized Filtering Antennas

This paper consists of two parts. The first part presents a review of the recent development in broadband circularly polarized filtering antennas. The second part presents a novel design of broadband integrated filtering antenna based on eighth-mode SIW (EMSIW) resonators for rectenna applications. This work has three main novel contributions. First, by adjusting the external quality factors and coupling coefficients of the resonators in this filtering antenna, optimum input impedance with a complex value can be realized within the filtering antenna. Thus there is no need for an external impedance matching network, which is usually required between the antenna and the rectifying circuits; Second, compared with traditional microstrip resonators, high-Q EMSIW cavities are used to increase antenna gain; third, the coupling gap between the EMSIW resonators also acts as the feeding structure of the radiator. So the feeding structures are all on the middle layer. The ground plane on the back side is a complete structure without any defects. This novel structure design improves front-to-back ratio to enhance the antenna receiving efficiency. To validate this method, two C-band circularly polarized integrated filtering antennas with an input impedance of 50 $\Omega $ and complex impedance are designed, simulated, and fabricated. The measured results show that the operating frequency bandwidth of the proposed antennas is more than 14.5% at C-band with the gain above 8 dBi. The 3-dB axial ratio bandwidth is larger than 8.5% and the front-to-back ratio is higher than 18 dB. Moreover, the proposed antenna with complex impedance is conjugate matched with the input impedance of a specific rectifying circuit at 5.8 GHz and harmonics suppression at the second-harmonic frequency is achieved.

Balanced-to-Balanced Microstrip Diplexer Based on Magnetically Coupled Resonators

Two balanced-to-balanced planar diplexers based on magnetically coupled microstrip resonators are proposed in this paper. For the first prototype, each channel/differential-output is composed of a second order single-band balanced bandpass filter based on open-loop resonators. For the second diplexer example, the filters composing the differential outputs are fourth order and are implemented by means of folded stepped-impedance resonators. The design procedure for the differential response is quite straightforward, since it is based on the use of the well-known external quality factor and coupling coefficients concepts. Common-mode is inherently rejected thanks to the benefits of magnetic coupling, which precludes common-mode transmission over a wide frequency range. The proposed structure also offers a high level of channel-to-channel isolation. To demonstrate the usefulness of the proposed idea, the two prototypes are simulated, fabricated, and measured. Good differential-mode and common-mode performance is observed in both examples. Simulations and measurements show good agreement.

A Tunable Dual-Band Bandpass-to-Bandstop Filter Using p-i-n Diodes and Varactors

In this paper, a tunable dual-band bandpass-to-bandstop filter and its analysis are proposed. The bandpass-to-bandstop transformation could be obtained by changing the state of the p-i-n diode. Using two sets of half-wavelength resonators loaded with varactors, the center frequency of each band can be tuned independently by adjusting the varactors. In order to achieve the controlling of the external quality factors ( ${Q} _{e}$ ) for the bandpass mode, the feeding lines are loaded with two varactors and wide tuning range of ${Q} _{e}$ can be obtained. In the bandpass mode, the lower passband tuning range is from 1.7 to 2.2 GHz (25.6% tuning range) with return loss better than 10.5 dB, and the tuning range of higher passband is from 2.2 to 2.7 GHz (20.4% tuning range) with return loss better than 12.9 dB. For the bandstop mode, the lower stopband tuning range is from 1.7 to 2.3 GHz (30% tuning range) and the higher stopband tuning range is from 2.3 to 2.9 GHz (23.1% tuning range), while the rejection level is higher than 16.4 dB in the stopband.

High isolation and miniaturized quad-channel diplexer using short stub-loaded resonators and distributed coupling feeding line

This paper presents a high isolation and miniaturized microstrip quad-channel diplexer. The quad-channel diplexer consists of a distributed coupling feeding line, two coupling feeding lines, and two pairs of short stub-loaded resonators combined by the pseudo-interdigital structure. Based on the even–odd mode method, the resonant modes of the short stub-loaded resonator are investigated. The mode splitting is created when a pair of short stub-loaded resonators are combined by a pseudo-interdigital structure. Each mode of the short stub-loaded resonator is split into two modes to form a two-order channel filter. The center frequencies of the quad-channel diplexer can be adjusted independently, while the bandwidths of channel filters can be tuned by the external quality factor and the coupling coefficient. Taking advantage of the distributed coupling feeding line, no extra matching network is needed and the loading effect is very small. For demonstration, a quad-channel diplexer working at 2.4, 3.15, 4.4, 5.8 GHz with better than 40 dB isolation is presented. The measured results agree well with the full-wave simulated results.

Design of Miniaturized Triplexers via Sharing a Single Triple-Mode Cavity Resonator

A novel approach for design of a miniaturized cavity triplexer is proposed in this paper by using triple-mode resonator (TMR) as a common feeder of three frequency channels in a triplexer. Three frequency channels are generated by three fundamental modes of a single rectangular triple-mode cavity resonator. Without installing any connection-oriented junction network, i.e., T-junction or star junction, only coupling slots are adopted herein to achieve required coupling coefficients and external quality factors toward the specified Chebyshev responses in the three passbands simultaneously. High isolation among these three separated bands can be effectively achieved owing to the modal orthogonality via sharing a common TMR structure as a feeder. Two triplexers are then designed and fabricated using varied topologies. The hybrid resonator triplexer method is presented as the first approach by using a TMR as a feeder to feed three sets of single-mode cavity bandpass filters. As the second approach, the triple-mode triplexer (TMT) method is developed by cascading a few TMRs to form a multistage higher order TMT. Finally, two fabricated triplexer prototypes are tested for experimental verification of the proposed design methodology. Good agreement between measurement and simulation is achieved.

Design of Dual-Mode Dual-Band Superconducting Filters

This paper presents a synthesis design method for superconducting dual-mode dual-band filters. A low-pass to dual-band bandpass frequency transformation is proposed based on lossless lumped-element dual-mode resonator circuits. Formulas for extracting the coupling coefficient $k$ and external quality factor $Q_e$ of dual-mode microstrip resonators are derived using the coupling matrix theory and the proposed frequency transformation formula. This makes the design process of dual-mode dual-band filters similar to that of the single bandpass filter. For the demonstration, four dual-band superconducting filters with different dual-mode resonator structures, such as stepped-impedance, stub-loaded, and embedded resonators are designed and fabricated. The measured results match well with the simulations.

High-Order Dual-Band Superconducting Bandpass Filter With Controllable Bandwidths and Multitransmission Zeros

In this paper, a compact dual-mode hairpin ring resonator (HRR) with two controllable resonances is proposed to design high-order dual-band high-temperature superconducting (HTS) bandpass filters. Its noncoupled dual-mode resonant characteristics and the mechanism for inherent transmission zero (TZ) production are studied. Moreover, a general dual-path coupling scheme is introduced for high-order coupling realization and applied to the third-order and eighth-order dual-band filter design. Also, two coupling structures for the adjacent HRRs with different orientations are proposed to excite TZs between the passbands for high selectivity. Besides, dual-parallel input–output feeding structure is adopted to tune the external quality factors of the dual-mode HRR individually so that the two passband are realized easily using high-order structure. For demonstration, an eighth-order dual-band HTS filter both operating at 1.9 and 2.6 GHz for mobile communications application are designed and analyzed. Compared with the third-order one, the selectivity of the eighth-order filter has at least 4.2 times improvement and five TZs are produced for high band-edge selectivity. Finally, the eighth-order dual-band filter is fabricated on MgO substrate with YBCO thin films. Measured results agree well with the simulations and show the excellent performance with 0.12-dB minimum insertion loss and better than −75-dB rejection level up to 5 GHz. In addition, the nonlinear characteristics of the fabricated HTS filter is observed by experiment, which shows the temperature dependence of filter center frequency performance and microwave insertion loss.