Li-Heng Zhou

Novel Frequency-Agile Bandpass Filter With Wide Tuning Range and Spurious Suppression

A new frequency-agile scheme for the loaded stepped-impedance resonator (SIR) is introduced, which can be employed for designing a wideband tunable bandpass filter (BPF). Two identical capacitors are added to the two open ends, and a capacitor is added to the center of the SIR. By properly controlling their respective values, the even-mode resonant frequency (feven) can be bidirectionally tuned, resulting in a wideband tuning range (approximately double those of the traditional tunable resonators). However, in such frequency range, the odd-mode resonant frequency (fodd) functions as a spurious resonance would appear. To overcome this, the feed line is attached to the center of the SIR where it is always short-circuited for fodd and its odd-order harmonics, which cannot be excited accordingly. Thus, the wideband tuning range of the passband (feven) in the proposed BPF is validated to be meaningful, and the selectivity of the passband is improved due to the suppression of odd-mode spurious near the passband. For demonstration, a wideband tunable BPF is designed, and the simulated and measured results are presented, showing good agreement. The results showcase that the tuning range of passband reaches 84.4%, i.e., from 0.77 to 1.42 GHz with insertion loss from 3.1 to 1 dB.

Novel Compact Microstrip Lowpass Filters with Wide Stopband Using Defected Ground Structure

In this paper, a novel defected ground structure (DGS) is proposed for lowpass filter (LPF) applications. The proposed structure exhibits wideband attenuation without increasing the circuit size, as compared with the conventional DGS. An equivalent circuit for the proposed DGS is described, and its equivalent-circuit parameters are analyzed and extracted. Benefitting from the quasi-elliptic function of the DGS, a demonstration LPF using one DGS unit and a compensated microstrip line is designed, and an attenuation pole near to the passband edge is realized to enhance roll-off of the stopband. To improve rejection level of the stopband, three cascaded DGS units are utilized for the LPF design. The fabricated LPF shows more than 20 dB attenuation from 2.1 GHz to over 8.0 GHz, as well as sharp rolloff. The simulated and measured results are presented, showing good agreement.

Differential Dual-Band Bandpass Filter With Tunable Lower Band Using Embedded DGS Unit for Common-Mode Suppression

This paper presents a varactor- and stub-loaded dumbbell-shaped resonator (VSDSR) for application in differential dual-band bandpass filter, which possesses a tunable lower passband and a fixed upper passband. Under differential-mode (DM) bisection, the VSDSR behaves as a ring resonator with four resonant modes, which can be categorized into two independent groups for constructing two passbands. At the same time, the common-mode (CM) equivalent circuit of the VSDSR generates multiple CM resonant frequencies and some of them inevitably overlap with the DM ones. Accordingly, the traditional CM suppression methods such as adding shunt stub or lumped element are disabled. To suppress the CM signals in both DM passbands, the defected ground structure (DGS) units are embedded underneath the adjacent edges of the two rings directly to form a CM suppression filter without increasing the circuit size. Meanwhile, they have almost no effect on the DM responses. Benefitting from the bandstop and slow-wave effects of the employed DGS units, the CM noises in both DM bands can be rejected effectively. In particular, the level of the CM suppression in the DM lower passband can be improved by adjusting the location of the DGS units. Furthermore, the proposed CM suppression scheme is adaptive in the tunable lower DM passband.

Tri-band bandpass filter with wide stop-band using stub-loaded triple-mode resonator

A novel microstrip tri-band bandpass filter (BPF) based on stub-loaded triple-mode resonator is proposed. The open- and short-circuited stubs are asymmetrically loaded to a uniform impedance open-loop resonator, constructing a triple-mode resonator. Benefiting from the proposed asymmetrical loading scheme, the designed tri-band BPF not only possesses the controllable bandwidths of the three passbands, but also shows very wide stopband. Finally, a tri-band microstrip BPF with good performance is designed at 1.5, 2.57, and 3.83 GHz. Three transmission zeros near the passbands are obtained to improve the selectivity of the tri-band BPF. Meanwhile, the other transmission zeros produced in the stopband enhance the harmonic suppression level. The rejection level from 4.3 to 13.7 GHz is better than 20 dB. The measured results are in good agreement with the simulated data.

Differential Dual-Band Dual-Polarized Dielectric Resonator Antenna

This communication presents a differential dual-band dual-polarized dielectric resonator antenna (DRA) with high isolation and low cross polarization using a cross-shaped DR. In the differential DRA, some of the higher order modes are eliminated and electromagnetic fields of each excited mode have no interaction with its corresponding orthogonal mode. Two groups of modes are utilized to realize dual-band characteristic and each group with two orthogonal modes is used to supply dual-polarized function. Differential feeding provides the DRA with suppression of unwanted modes, much higher isolation and lower cross-polarization in both frequency bands, compared with the traditional single-end feeding. The experimental prototypes of the proposed differential DRA and its single-ended counterpart are designed and implemented. The comparison between them is also made in reflection coefficient, isolation, and radiation properties. Both simulated and measured results show significantly improved performance of the proposed differential DRA.

Differential bandpass filter with high common-mode rejection ratio inside the differential-mode passband using controllable common-mode transmission zero

Controllable common-mode transmission zero is proposed to improve the common-mode rejection ratio (CMRR) inside the differential-mode passband of differential bandpass filters. With the controllability of the common-mode transmission zero, the common-mode transmission zero can be moved into the frequency range of the differential-mode passband, and then the CMRR inside the differential-mode passband can be greatly improved. Two two-order filters using different resonators are designed to demonstrate the existence and controllability of the common-mode transmission zero. A four-order filter with two resistor-loaded stepped-impedance resonators (SIRs) and two resistor-loaded uniform impedance resonators (UIRs) are designed to get good selectivity of differential-mode passband and wide-band common-mode suppression besides the high CMRR inside the differential-mode passband. The design methodology and experimental results of the proposed filters are presented.

Novel equal and unequal power divider using offset parallel-strip lines

The offset parallel-strip line (PSL) allows another design freedom for characteristic impedance except for strip width, i.e. offset distance between the top and bottom strips of the offset PSL. This can reduce the discontinuities effectively resulting from the change of strip width and improve power-handling capability. The design method is applied successfully in our proposed equal power divider with fixed strip width and 5:1 unequal power divider. The theoretical and measured results are presented in this paper.

Differential oscillator using double-sided paralell-strip line with an inserted conductor plane

A novel differential oscillator using double-sided parallel-strip line (DSPSL) with an inserted ground plane is proposed in this letter. Its two sub-oscillators on the opposite sides of the substrate operate in differential mode, leading to the inherent suppression of the second harmonics at the common DSPSL output port, while the fundamental signals are well combined without additional combiner required. To further enhance the suppression of the second harmonic, the inset length of the ground plane into the DSPSL resonator is optimized to make the second harmonics to be suppressed inside each sub-oscillator.

Differential Dual-Band Filters With Flexible Frequency Ratio Using Asymmetrical Shunt Branches for Wideband CM Suppression

In this paper, a group of differential dual-band bandpass filters based on a single quad-mode stub-loaded twin-ring resonator (TRR) are presented, which utilize asymmetrical shunt branches for common-mode (CM) suppression. Under differential-mode (DM) operation, the stub-loaded TRR manifests quad-mode characteristic for dual-band design. The frequency ratio of the two passbands can be flexibly selected according to the form of the loaded stubs. Different from the inline topology, differential multimode structures inevitably suffer from the hardship in CM suppression, especially for dual-band designs. Despite previous work fulfilled moderate CM suppression via embedded defected ground structure units, but the CM noise in the DM lower band can just be shifted away rather than suppressed. Instead, a novel CM-suppression approach is devised attributing to the asymmetrical short-terminated branches along the bisection line of the TRR. The transmission of the CM noise near the DM passbands can be blocked effectively. Furthermore, multiple CM notches can be obtained by the cooperation of the asymmetrical branches and source–load coupling, leading to the favorable CM suppression in a wide frequency range. For verification, a prototype differential dual-band filter based on the open stub-loaded TRR has been realized with frequency ratio which is larger than two. Similarly, a differential dual-band filter using the short-circuited stub-loaded TRR is also presented as an extension for frequency ratio which is smaller than two. The simulation and experiment results of the two demonstrated filters are given, showcasing a good agreement.

Compact LTCC bandpass filters using vertically folded half wavelength resonator

In this paper, a pair of novel compact low temperature co-fired ceramic (LTCC) bandpass filters (BPFs) based on transmission line theory are proposed. The employed half wavelength (λg/2) resonator has been vertically folded to form a multi-layer transmission line resonator for minimizing the circuit size significantly. Since there is a virtual ground in the symmetrical plane of the multi-layer resonator, the characteristic of the resonator keeps almost unchanged. Transmission zeros have been realized near to the passband and in the stopband, which improve the selectivity and harmonic rejection of the BPFs. For demonstration, two BPFs centered at 3.1GHz and 10GHz are designed using the proposed resonator, and the circuit sizes are 5.1×3.9×1.8 mm3 and 4.0×3.9×1.5 mm3.