Wideband planar filters with an extended stopband

An approach to the construction of wideband bandpass filters of microstrip and stripline designs, which have an extended stopband and can incorporate lumped capacitors, is proposed. This approach is based on resonators with the enhanced frequency diversity of fundamental and parasitic resonance frequencies and coupling circuits formed according to an assigned rule. The potential of the filters achieved due to the proposed technique is established. In particular, a very wide passband whose ratio of cuttoff frequencies is 5: 1, as well as the stopband greater than an octave at a level of 40 dB, is demostrated to be attainable. When the passband is narrower, the stopband can exceed two octaves. The given approach can be applied to filters operating in a broad frequency range, including to that corresponding to centimeter and meter waves. The experimental data are presented.     

Shunt-inductance-coupled waveguide filters with expanded second stopband

A computer-aided design is described for inductive iris and inductive double-strip coupled filters with increased-width resonator sections. The theory includes both the higher-order-mode interaction of all discontinuities and the finite thickness of the irises and inserts. The step-wall discontinuity effect is included in the optimisation process as an additional design parameter. Design examples for midband frequencies of about 11 and 16 GHz are given; the corresponding stop-band attenuation is higher than 50 dB, or 40 dB, up to 18.5 GHz, or 24.5 GHz, respectively.     

Wideband coplanar-waveguide bandpass filters with good stopband rejection

Wideband coplanar-waveguide (CPW) bandpass filters based on the cascade of CPW lowpass and highpass periodic structures are proposed. The upper and lower stopband characteristics of proposed bandpass filter can be easily designed by separately adjusting the lowpass and highpass sections, respectively. Specifically, two wideband CPW bandpass filters with sharp roll-off at passband edges and good stopband rejection are demonstrated.     

Broadband bandpass filters using slotted resonators fed by interdigital coupled lines for improved upper stopband performances

This paper proposes new broadband microstrip bandpass filters based on slotted linear tapered-line resonator (SLTR) and slotted step impedance resonator (SSIR) structures for size reduction and improved stopband performances. A comprehensive treatment of slotted resonators and both ends of the resonator with interdigital coupled lines is described. The design concept is demonstrated by two filter examples including one with an SLTR and another one with an SSIR. These filters have not only compact size but also a wider upper stopband resulting from resonator bandstop characteristics. The simulated and experimental results of stopband performances are better than 15 dB for a frequency range up to 25 GHz.     

Superconducting spiral wide bandpass filters with wide upper stopband

Wide-band superconducting spiral bandpass filters have been successfully designed and tested. This extends the range of parameters that can be implemented using spirals. The shape of the spiral resonators was significantly modified to achieve the higher coupling coefficients required. A wide upper stopband was produced by using quarter-wave resonators for some of the spirals, and by shifting the second harmonic upwards for the others. One of the filters is eighth order with 1560-MHz center frequency and 27% bandwidth, and has an active area of only 9 mm/spl times/6 mm.     

Design of dielectric-filled cavity filters with ultrawide stopband Characteristics

A methodology is presented for the design of small cavity filters, relying entirely on the use of commercial general-purpose design software. Pertinent filter structures comprise resonated segments of ridge waveguide with evanescent-mode inter-resonator coupling sections, supplemented by impedance-transforming port networks. The ridge and evanescent-mode waveguide segments combine to form a metal-clad dielectric core made of either a single dielectric or a composite material. A parameterized equivalent-circuit model is used to describe each segment based on numerical three-dimensional electromagnetic field analyses. For a bandpass filter, the technique yields low passband insertion loss, an ultrawide upper stopband region, and excellent power handling. The approach provides the option to fabricate structures at low per-unit cost with the help of newly available moldable dielectric materials. The efficacy of the technique is demonstrated with an experimental 1-1.45-GHz bandpass filter.     

Parallel-coupled microstrip filters with over-coupled end stages for suppression of spurious responses

In a parallel-coupled microstrip filter, end stages with over-coupling are designed to suppress the unwanted responses at twice the passband frequency (2f/sub 0/). The inherent transmission zero of an over-coupled input/output stage is shown tunable. It is found that increasing the image impedance of the filter sections can further enhance the suppression. The designed bandpass filters thus have a wide upper stopband and improved passband response symmetry. Measured results of fabricated circuits show that the idea works very well.     

Photonic bandpass filters with high skirt selectivity and stopband attenuation

A new photonic signal processor topology that simultaneously achieves both a high-Q and a high skirt selectivity and stopband attenuation filter response is presented. It is based on a novel dual-cavity bandpass optical structure in which two pairs of active fiber Bragg grating cavities are used with an optical gain offset to control the poles and stopband attenuation characteristics of the filter. This concept enables a large improvement in the filter stopband attenuation, rejection bandwidth, and skirt selectivity to be realized. Measured results demonstrate both a narrow bandpass bandwidth of 0.4% of center frequency and a skirt selectivity factor of 16.6 for 40 dB rejection, which corresponds to a 6.5-fold improvement in comparison to conventional single cavity high-Q structures. To our knowledge, this is the best skirt selectivity reported for a photonic bandpass filter to date. The new photonic filter structure has been experimentally verified and excellent agreement between measured and predicted responses is shown.     

Single- and dual-band bandstop filters with fully controllable stopband characteristics

This paper presents a novel design method of bandstop filters (BSFs) and dual-band bandstop filter (DBBSF) with completely controlled bandwidth and centre frequency, high rejection levels at mid-stopband and compact sizes. First, bandstop units with simple structures are proposed. Through theoretical analysis, the centre frequency and bandwidth of the bandstop units can be independently controlled by tuning the parameters of the loaded stubs. Then, single-band, dual-band bandstop units and compact J-inverters are adopted to design the filters. Finally, to demonstrate the proposed design method, 2-order, 3-order and dual-band BSFs are fabricated and measured. From the experimental results, the maximum rejection levels at mid-stopband of 2- and 3-order are around 50 dB and 70 dB, respectively. The two stopbands of the DBBSF can be fully controlled. The measured maximum rejection levels at the two mid-stopbands are both around 60 dB. Furthermore, all the simulation and measurement show good agreement.     

Recursive digital filters with predetermined group delay andChebyshev stopband attenuation

Recently, a conformal mapping technique has been described whereby the transfer function of a recursive digital filter with prescribed group delay and Chebyshev stopband can be obtained in such a way that the numerator and denominator polynomials are different in order. It is shown here that such a transfer function can also be determined using the familiar bilinear transformation and the well known method in the s-domain. Numerical examples are included to demonstrate the technique     

Design of filters with prescribed time response and Chebyshev stopband attenuation

This paper presents a new class of low pass filters with Chebyshev stopband attenuation which are intended for pulse applications and characterized by very small values of the dominant polo Q-factor, Their transfer functions are obtained by computational optimization in the time domain of either step response or impulse response under the constraint of double or higher-order multiplicity of the dominant pair of poles. The comparison with Schüssler filters, which are known for their excellent rise-time-overshoot characteristics, reveals that the new filters yield superior frequency and time domain characteristics and yet they have much smaller values of the dominant pole Q-factor. The pole locations and other important frequency and time-domain parameters are tabulated for optimum step response and for optimum impulse response for n = 5, 6, 7, 8 and several values of the maximum tolerable overshoot.     

Compact frequency and bandwidth tunable stopband filters using split ring resonators and varactors coupled transmission line

Two highly compact tunable stopband filters using microstrip transmission lines coupled with split ring resonators (SRRs) and varactor diodes are presented. Frequency or bandwidth tuning capability of each device is demonstrated. The frequency tunable filter, realized by a single stage, shows a wide tuning range of 19.8% with a maximum bandwidth of 5% and an insertion loss of approximately 20 dB at 4 GHz. The bandwidth tunable filter, realized by double stages, shows a 10-dB bandwidth of 19–34% with a biasing voltage of 0–10 V. The implemented frequency tuning and bandwidth tuning devices show a significant area reduction of 60.1% and 53.5%, respectively, in comparison with a similar frequency or bandwidth tunable structure presented by others. Equivalent circuit models are presented. The measured S-parameters are in good agreement with simulated ones.     

Bandstop monolithic crystal filters having allpass response outside stopband

A dual-resonator monolithic crystal filter (MCF) is basically a narrowband bandpass filter. It is well known that a narrowband bandstop response can be obtained by connecting an inductor in parallel to an MCF element, however, it yields a lowpass response outside the stopband with a cutoff frequency about square root (2) times the stopband frequency. Ideally, outside the stopband, it should have an allpass characteristic. The author shows a simple method of designing a bandstop MCF filter having an allpass response outside the stopband by adding a series-connection inductor. The series inductor together with two capacitors provide the required highpass structure that complements the lowpass response to yield an allpass response.<>     

Equiripple stopband characteristic for circularly symmetric 2-D recursive filters

A circularly symmetric 2-D recursive digital filter whose magnitude-squared response approximates a Gaussian function in the two frequency variables has a separable transfer function. Such a circularly symmetric filter has a monotonic frequency response. One may, therefore, want to sharpen the response in the transition band. Due to the separability feature, it is shown how to obtain an equiripple characteristic in the stopband without affecting the circular symmetry in the passband. This reduces the transition bandwidth.     

A novel cross-shape DGS applied to design ultra-wide stopband low-pass filters

This letter presents a novel low-pass filter with an ultra-wide stopband. The proposed filter is comprised of a new cross-shape defected ground structure (CSDGS). By using this structure, the filter not only supports conventional DGS performances with a sharp rejection, but also exhibits an ultra-wide stopband. For the deigned low-pass filter, an insertion loss of less than 2dB from dc to 3.5GHz and the rejection is better than 20dB from 4.3 to 15.8GHz. Predicted performances show widened and deepened stopband beyond the low passband. Furthermore, it is confirmed by measurement.     

Design of microstrip wide stopband quad-band bandpass filters for multi-service communication systems

This paper presents two planar high performance quad-channel bandpass filters, which are designed based on a novel circular multi-mode resonator. In this paper and for the first time, the proposed resonator is utilized to achieve quad passbands. It consists of diverged feeding lines that are coupled to etched circular cells. The first filter has quite close channels at 2.62, 2.88, 4.34 and 4.67 GHz, which make it appropriate for frequency division duplex (FDD) scheme. Meanwhile, the second filter is designed for WCDMA and WiMAX applications. Both filters are able to attenuate the harmonics up to 19 GHz with a maximum harmonic level of −20 dB. The insertion losses and return losses of both filters at all channels are better than 1.2 dB and 17.5 dB, respectively. The harmonic attenuation method is presented employing a LC equivalent circuit of the proposed resonator. In order to verify the designing methodology, the proposed filters are fabricated and measured where there are good agreements between the simulation and measurement results.     

Pseudo-elliptic microstrip line filters with zero-shifting properties

The paper presents a new design of pseudo-elliptic microstrip line filters. These filters employ a coupling scheme which allows shifting of transmission zeros from one side of the passband to the other by only changing the resonant frequencies of the resonators. To demonstrate the performance of these filters, a fourth-order bandpass filter with one transmission zero in the upper stopband is first designed. A second filter with a transmission zero in the lower stopband is then designed by simply adjusting the lengths of the resonators followed by minor optimization to account for the fact that the resonant frequencies can not be changed without slightly affecting the coupling coefficients in the used layout. Measured and simulated results are presented.     

Experimental determination of finline post discontinuity parameters: application to stopband and lowpass filters

The shunt reactance of a finline post discontinuity is shown to be of resonant type. By using an experimental transmission procedure, we have determined accurately the parameters R, L and C of its equivalent circuit. These parameters, in the first approximation, can be considered as frequency-independent. Thus the use of the equivalent circuit in CAD is very easy. Applications to stopband and lowpass filters are described.     

Decoupled solutions for design of filters having TEM resonatorswith attenuation poles in upper stopband

The equivalent circuits together with decoupled equations are provided as a solution to the problem of designing the filters having TEM resonators with an attenuation pole in the upper stopband. The proposed method simplifies the design of inserting attenuation poles and gives us much flexibility. The presented design example verifies the exactness of the solutions and shows the usefulness of the approach     

Generalized dual-plane multicoupled line filters

The authors describe the general realization of elliptic function narrow-bandpass filters constructed from two adjacent planes of microstrip quasi-TEM line resonators. This dual-plane configuration allows nonadjacent coupling to be realized through small slots between the planes. Alternate sign coupling is realized by the appropriate positioning of the slot along the resonator length. A model developed from H. Bethe's small-hole coupling theory (1944) is used to predict these couplings. Adjacent resonator couplings are modeled using a coupled-line analysis modified to characterize the coupling of an inhomogeneous medium. The filter synthesis technique previously developed for waveguide cavity filters can be directly applied in designing a dual-plane filter