Pu-Hua Deng

Wide-stopband microstrip bandpass filters using dissimilar quarter-wavelength stepped-impedance resonators

Wide-stopband and compact microstrip bandpass filters (BPFs) are proposed using various dissimilar quarter-wavelength (/spl lambda//4) stepped-impedance resonators (SIRs) for multiple spurious suppression. The use of /spl lambda//4 SIRs is essential in widening the filter stopband and reducing the circuit size. By properly arranging the individual /spl lambda//4 SIR, which has the same fundamental resonance frequency f/sub 0/, but has different spurious (harmonic) resonance frequencies, and also carefully misaligning the maximum current density nodes, several higher order spurious resonances may be suppressed so that a BPF with wide stopband may be realized. In this study, the basic concept of multiple spurious suppression is demonstrated by thoroughly investigating the spurious characteristics of the fourth-order interdigital BPFs, which consist of two different types of /spl lambda//4 SIRs. To widen the rejection bandwidth, the fourth-order coupled-resonator BPFs based on three and four different types of /spl lambda//4 SIRs are implemented and carefully examined. Specifically, a very wide-stopband microstrip BPF composed of four dissimilar /spl lambda//4 SIRs is realized and its stopband is extended even up to 11.4f/sub 0/ with a rejection level better than 27.5 dB.

Design of Matching Circuits for Microstrip Triplexers Based on Stepped-Impedance Resonators

New matching circuits for microstrip triplexers are proposed based on half-wavelength tapped-connected (or fed) stepped-impedance resonators. The stepped-impedance resonators play important roles for the matching circuits, either to serve as a through pass at the center frequency of a bandpass filter or to provide a short circuit at the center frequency of another bandpass filter. First, three tapped-connected stepped-impedance resonators together with suitable branch transmission lines are utilized to develop the matching circuits for a microstrip triplexer. The design procedure for these matching circuits is much simpler than that for the conventional triplexer structures due to the use of tapped-connected stepped-impedance resonators. Second, to reduce the number of stepped-impedance resonators and to improve the spurious resonances associated with the proposed matching circuits, modified matching circuits for the triplexer are also proposed. Agreement between measured and simulated results is observed and supports the usefulness of the design procedure

Unequal Wilkinson Power Dividers With Favorable Selectivity and High-Isolation Using Coupled-Line Filter Transformers

In the conventional unequal-split Wilkinson power divider, poor selectivity for each transmission path is usually a problem. To surmount this obstacle, the parallel coupled-line bandpass filter structure is utilized as an impedance transformer as well as a band selector for each transmission path in the proposed unequal-split Wilkinson power dividers. However, the bandpass filters in the proposed dividers require careful design because they may not be functional under certain conditions. For example, the odd-order coupled-line filters are not appropriate for impedance transformers in the proposed unequal-split dividers and high-isolation requirement. Using the even-order coupled-line filter transformers, this study proposes two types of unequal-split Wilkinson power dividers. The first type of the proposed dividers arranges two filter transformers near two output ports, respectively, and is capable of achieving a highly remarkable isolation between the two output ports and a good band selection in each transmission path. Specifically, not only the operating band but also the lower and higher stopbands can achieve highly favorable isolation for this type of divider. By arranging the load impedance of each port properly, the second type of the proposed dividers, which has only one filter transformer to be shared by each transmission path near the input port, is also proposed to provide effective isolation between two output ports and favorable selectivity in each transmission path.

New Wilkinson Power Dividers Based on Compact Stepped-Impedance Transmission Lines and Shunt Open Stubs

This study presents new Wilkinson power dividers using compact stepped-impedance structures and capacitive loads to achieve the required power splitting. This approach can produce additional transmission zeros and efiectively suppress the desired stopbands because shunt open stubs realize capacitive loads. This study proposes two equal-split dividers and two unequal-split dividers. For the flrst equal-split case, one shunt open stub forms the needed capacitor in each transmission path, creating one additional transmission zero in each path. To obtain one more transmission zero in each transmission path, the second Wilkinson power divider uses two shunt open stubs in each path to achieve the same capacitor value as the flrst divider. This study also tests unequal-split dividers with one and two transmission zeros in each path to conflrm that compact stepped-impedance transmission lines and shunt-to-ground capacitors can be utilized in unequal power division.

Compact microstrip bandpass filters with good selectivity and stopband rejection

Compact microstrip bandpass filters (second- and fourth-order) are proposed based on the folded quarter-wavelength (/spl lambda//4) resonators, which are mainly coupled through the shunt inductors connected to the ground. By introducing a cross-coupling capacitance directly between the input and output ports of the second-order filter, a pair of transmission zeros may be created to improve the selectivity. Moreover, by an extension of the proposed second-order filter with the incorporation of an additional cross-coupling capacitance, a fourth-order filter is also proposed in which two pairs of transmission zeros may be created to improve both the selectivity and stopband rejection. The proposed fourth-order filter also has the merits of small circuit area and no spurious response up to 3f/sub 0/, where f/sub 0/ is the passband center frequency. To provide effective design tools, simple equivalent-circuit models are also established.

A Switched Reconfigurable High-Isolation Dual-Band Bandpass Filter

This letter presents an independently switched, reconfigurable, dual-band filter of high isolation between two closely-positioned adjacent bands. The reconfigurable fourth-order band pass filter (BPF) prototype consists of two individual filters and a double-diplexing configuration, which enables independent synthesis of BPF on each band. This letter reports the diode-loaded resonator approach to the dual-band BPF design, achieving independent switching control of band selection without affecting the original bandpass characteristics. Measured results are in close agreement with the simulations and validate the design methodology presented in the letter, showing two mid-band frequencies fA and fB at 1.5 and 2 GHz, of fractional bandwidths 11.4% and 10.8%, respectively. The measured worst-case passband insertion loss is less than 3.1 dB, while achieving band isolation higher than 37 dB.

Novel Broadside-Coupled Bandpass Filters Using Both Microstrip and Coplanar-Waveguide Resonators

Novel quasi-elliptic coupled-resonator bandpass filters with wider fractional bandwidth are proposed. By using the broadside-coupled mechanism to couple the half-wavelength microstrip resonators and the quarter-wavelength coplanar-waveguide (CPW) resonators together with introducing two CPW shorted stubs, the required mixed and magnetic couplings associated with the resonators may be enhanced so that a wider bandwidth cross-coupled filter may be realized. Specifically, a fourth-order quasi-elliptic broadside-coupled bandpass filter with a center frequency at f0=1.48 GHz, a minimum insertion loss of 0.68 dB, and a wider 3-dB fractional bandwidth of 34.6% is implemented, and its stopband is extended up to 6 GHz (4f0) with a rejection better than 20 dB

Design of Microstrip Lowpass-Bandpass Diplexer

The matching circuit design of a lowpass-bandpass diplexer may be a challenge in conventional designs because a wideband (from zero frequency to approximately the cutoff frequency of the lowpass filter) open condition is required to achieve in bandpass channel circuit. This paper proposes a new lowpass-bandpass diplexer that involves using a simple matching design that can easily meet the required matching condition in each channel circuit. The measured results are in good agreement with the simulated responses. For the lowpass channel, the measured in-band insertion loss and cutoff frequency are less than 0.25 dB and approximately 1.5 GHz, respectively. For the bandpass channel, the measured minimal insertion loss, center frequency, and 3 dB fractional bandwidth are approximately 2.42 dB, 2.4 GHz, and 7.6%, respectively. The measured isolation between the two operating bands is better than 35 dB.

Coplanar-Waveguide-Fed Microstrip Bandpass Filters With Capacitively Broadside-Coupled Structures for Multiple Spurious Suppression

Coplanar-waveguide (CPW)-fed microstrip bandpass filters are proposed with capacitive couplings suitably introduced at the input/output (I/O) ports, as well as between the resonators for spurious suppression. By adopting these capacitive couplings, several open stubs are established so that adjustable multiple transmission zeros may independently be created to suppress several unwanted spurious passbands, thereby extending the stopband and improving the rejection level. In this study, the capacitive couplings required at the I/O ports, as well as across the resonators, are realized by the broadside-coupled transition structures between the top microstrip layer and the bottom CPW layer so that the I/O ports may properly be matched and the spurious responses may effectively be suppressed. Specifically, a fifth-order bandpass filter, centered at f0=1.33 GHz with a stopband extended up to 8.67 GHz (6.52 f0) and a rejection level better than 30 dB, is implemented and carefully examined

Design of Microstrip Cross-Coupled Bandpass Filter With Multiple Independent Designable Transmission Zeros Using Branch-Line Resonators

This letter proposes a novel branch-line resonator to design a fourth-order cross-coupled bandpass filter (BPF). Theoretically, the proposed branch-line resonator plays two crucial roles. First, the arbitrary number transmission zeros can be designed in the stopband of a fourth-order cross-coupled BPF. Second, the required external quality factor can also be designed without an impedance transformer. This study examines a novel fourth-order cross-coupled BPF with a single pair of transmission zeros produced by a cross-coupled mechanism near the passband and five independent designable transmission zeros created by the proposed resonator in the stopband.