Millimiter-wave stripline bandpass filters

The possibility to use thin (1 mm or less) pseudocombline structures of λ/2 resonators made of sections of symmetric transmission strip lines from a dielectric material with ε _r = 2.2 as millimeter-wave bandpass filters is shown. It is established that the relative bandwidth of such filters can reach FBW = 0.1. Designs of stripline pseudocombline filters with alternating signs of the coupling coefficients, which are characterized by improved selectivity due to the attenuation poles located on both sides of the passband, are proposed; the filters contain stepped-impedance “half-wave” resonators with a new feature. The results of the computer simulation of the frequency characteristics of the filters in the frequency band of 31–122 GHz and the comparison of them with the characteristics of other filters are presented.     

Influence of Geometric Parameters and Permittivity of Stripline Filters on Resonator Coupling Coefficients

It has been proved that electromagnetic coupling coefficients K of resonators in stripline filters with homogeneous dielectric depend only on geometric parameters of the filter designs and are independent of relative permittivity ε_r (if the dielectric is two-layer, coefficients K depend only on geometric parameters of the filter designs and ratio ε_r2/ε_r1). It has been shown that the revealed earlier influence of permittivity ε_r and the operating frequency on K is only a consequence of the influence of the length of stripline resonators on K: the lesser is the length, the larger is coefficient K. It has been found that these propositions enable consideration of frequency characteristics of the same design of the bandpass filter in different frequency bands by changing εr and performing slight changes in outermost resonators. The results of computer simulation of the transfer of frequency characteristics of a stripline bandpass filter from 3 GHz to 6 and 12 GHz with retaining the fractional bandwidth and selectivity are presented.     

Combline filters designed on symmetric stripline

Problems arising during the design of stripline combline filters, which until quite recently were considered as all-stop microwave structures, are considered. It has been shown that the electromagnetic coupling coefficient of quarter-wave stripline resonators increases with the value of material permittivity ε_r, the operating frequency, and the thickness of the filter resonators. For ε_r = 92 and a thickness of stripline resonators of 4 mm, it exceeds 12% at frequencies higher than 2.2 GHz. Two alternative versions of combline filters have been designed: a filter with a coupling strip and a filter without a coupling strip. Experimental data for a nine-resonator combline filter with a thickness of 4 mm, ε_r = 9.7, and the center frequency f ₀ = 2.4 GHz are presented. A method for increasing the selectivity of combline filters with the same number of resonators is proposed. It has been found that amplitude–frequency responses of stripline combline filters are near-symmetric, unlike the responses of microstrip combline filters.     

Interaction Features of Stepped-Impedance Stripline Resonators in Comb Filters

Mixed coefficients of coupling between the closely spaced stepped-impedance resonators in comb filters of stripline design have been investigated. Transmission zeros at frequencies f _zi correspond to mixed coupling coefficients k _i . These zeros can be moved with respect to the filter passband central frequency f₀ by modifying the shape of resonators. It was proved that the reduction of gap between resonators made it possible to locate frequencies f _z and f₀ closer to one another. The existing restrictions on the minimal value of gap between resonators limit the degree of proximity between f _z and f₀. The N-resonator stripline comb filters with mixed coupling can have N?1 transmission zeros. The absence of cross-coupling links in stripline filters simplifies their construction. It has been established that the thickness of central conductors of stripline resonators affects the positive and negative mixed coupling coefficients. The paper presents measurement data of miniature stripline three-resonator comb filter having an enhanced selectivity at the expense of two transmission zeros. The central frequency of filter is f₀ = 1850 MHz, the bandwidth BW = 100 MHz. The filter having dimensions 5.8×4.2×2 mm was implemented by connecting two ceramic substrates having relative dielectric permittivity ε _r = 92 and the metallized patterns deposited on them.     

Varicap-tuned narrowband filters with extended rejection band based on U-shaped microstrip Resonators

A resonance equation for the first unwanted resonant frequency of tunable microwave U-shaped loop-type resonator with variable capacitance has been derived. This frequency does not depend on the variable capacitance value; it is invariable and proportional to λ-type oscillations. The ascertained feature makes it possible to determine the ratio of the first unwanted resonant frequency to the frequency of the main resonance and control the rejection bandwidth of tunable filters with U-shaped resonators. A narrowband (2%) varicap-tuned filter with extended rejection band has been built and its experimental data are presented. The experimental four-resonator filter could be tuned in the frequency range 225–400 MHz. The filter rejection band in terms of the attenuation level of –40 dB was located in the frequency range 420–1290 MHz, i.e., f _max /f _min = 3.07. Varicap-tuned filters with microstrip U-shaped resonators are shown to have a good potential for their practical use.     

Stripe delay filters

There are considered constructions of microsized stripe delay filters, which are realized on a basis of ceramic materials with high dielectric permittivity. Delay time of non-minimal phase filters is 7–12 ns at frequencies of 1900 MHz with relative bandwidth of 3.6–3.85%. Filters dimensions are comparable with ones used in portable communication devices. Dimensions of researched three-resonator filter at frequency of 1900 MHz are 8.4×5×2mmwith material dielectric permittivity ε_r = 92, and 5-resonator filter ones are 9.2×8.6×2 mm. Filters are different from traditional delay filters. Two filters of considered ones contain odd resonator number and the third one contains four resonators and it has two cross couplings. The basis of the filters is amount of step-impedance stripe resonators pairs located close to each others whose electromagnetic coupling behavior is capacitive. There are represented the results of frequency characteristics simulation for different delay filters.     

Stripline bandpass filters with alternating coupling coefficients

This article shows the possibility in principle to design stripline comb filters with alternating signs of the coupling coefficients. The new feature of the stepped-impedance resonators of the quarterwave type was established. They provide the opposite sings of the electromagnetic coupling with adjacent resonators. Such resonators are asymmetric relative to the vertical axes, which is drawn through their middle part. The stripline passband filters with alternating signs of coupling coefficients have attenuation poles that are located to the left and to the right of the passband. Those poles improve the selectivity of such filters. The articles provide the measured characteristics of the stripline comb filter of the forth order with alternating coupling, which has the central frequency of f ₀ = 1835 MHz and the passband width of 90 MHz. The filter is constructed using a dielectric material with relative permittivity of ε_r = 92 and has the dimensions of 7.4×4.2×2 mm.     

Mixed Coupling on Stripline and Microstrip Bandpass Filters

The problems of designing stripline and microstrip bandpass filters with mixed coupling, including the magnetic and electric components of the interaction, are considered. It is shown that the transmission zeros corresponding to mixed coupling coefficients can be shifted along the frequency axis by changing the shape of the stepped-impedance resonators. It is confirmed that N-resonator planar filters can have (N–1) transmission zeros. Designs of microstrip filters with combined coupling, which include mixed coupling and the traditionally used magnetic and electric coupling, are proposed. It is shown that the number of transmission zeros of such filters is smaller than for filters with only mixed coupling, but their designing and tuning are less labor-consuming. The data of the experiment and computer simulation are presented.     

Symmetric stripline duplexer

The compact duplexer using the symmetric stripline and containing the fifth-order comb filters in transmit channel TX (2300–2370 MHz) and receive channel RX (2510–2580 MHz) is constructed, and the characteristics of obtained duplexer are measured. This duplexer is built using dielectric material Al₂O₃ (Alumina, polikor) with high thermal conductivity that makes it possible to use the operating power of 10Wat small dimensions 57-11.8-4 mm. The losses in bandpass of TX and RX filters did not exceed 3 dB during the filter attenuation at adjacent channel frequencies of no less than 60 dB. It is shown that the selection of the width of metallized strip at the base of quarter-wave resonators makes it possible to change the duplexer width for attaining the required value. The circuit of coupling of resonators with loads used in this design made it possible to obtain a sufficiently high level of isolation from each other for RX and TX channels of duplexer. This level does not exceed 60 dB. The finite thickness of internal conductors of stripline amounting to 16 μm is taken into account while building the duplexer that results in good agreement between the simulation and measurement results.     

Effects of Nb Content on the Ferroelectric and Dielectric Properties of Nb/Nd-Co-doped Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> Thin Films

Nd/Nb-co-substituted Bi_3.15Nd_0.85Ti_3?x Nb _x O₁₂ (BNTN _x , x = 0.01, 0.03, 0.05 and 0.07) thin films were grown on Pt/Ti/SiO₂/Si (100) substrates by chemical solution deposition. The effects of Nb content on the micro-structural, dielectric, ferroelectric, leakage current and capacitive properties of the BNTN _x thin films were investigated. A low-concentration substitution with Nb ions in BNTN _x can greatly enhance its remanent polarization (2P _r) and reduce the coercive field (2E _c) compared with those of Bi₄Ti₃O₁₂ (BIT) thin film. The highest 2P _r (71.4 μC/cm²) was observed in the BNTN_0.03 thin film when the 2E _c was 202 kV/cm. Leakage currents of all the films were on the order of 10^?6 to 10^?5 A/cm², and the BNTN_0.03 thin film has a minimum leakage current (2.1 × 10^?6 A/cm²) under the high electric field (267 kV/cm). Besides, the C–V curve of the BNTN_0.03 thin film is the most symmetrical, and the maximum tunability (21.0%) was also observed in this film. The BNTN_0.03 thin film shows the largest dielectric constant and the lowest dielectric loss and its maximum Curie temperature is 410 ± 5°C.     

Microstrip Bandpass Filter with Left Transmission Zero Controlled by Parasitic Cross Coupling

A third-order microstrip filter is proposed and studied. It is characterized by a left-handed transmission zero f_z, caused by the parasitic cross-coupling between non-adjacent resonators. The filter contains a half-wave middle resonator and two quarter-wave resonators located from different sides, near the open ends of the middle resonator. The coupling between all resonators has magnetic character, and the zero of the filter transfer function f_z is located to the left of the center frequency of the passband f₀. Such filter is described by a modified coupling matrix, where one of the main coupling coefficients is artificially assigned a minus sign. In the proposed filter design, for a given value of the main coupling coefficients, it is possible to provide different values of the cross-coupling coefficient by appropriately selecting the design parameters. This allows adjusting the zero position of the transmission f_z for a given bandwidth of the filter, thereby changing the left slope of the amplitude-frequency characteristic. A sequence of steps is proposed for constructing such a filter. The measured and simulated frequency characteristics of the experimental filter are given.     

Spectra of low-temperature photoluminescence in thin polycrystalline CdTe films

The band of intrinsic (e–h) radiation emission by the subsurface potential barriers of crystal grains and the edge doublet band arising as LO-phonon replicas of the e–h band are observed in the spectra of the low-temperature (4.2 K) photoluminescence of fine-grained (with a grain size of d_cr ≤ 1 µm) CdTe films. Film doping with the In impurity results in quenching of the doublet band, while heat treatment leads to activation of the intrinsic band, a short-wavelength shift of the red boundary (ΔE_r = 16–29 meV) and the halfwidth modulation (Δ_A = 6–17 meV) of which correlate with the height of micropotential barriers and the temperature of recombining hot photocarriers.     

Detection of terahertz radiation by resonant tunneling nanoheterostructures

The phenomenon of terahertz radiation detection by resonant tunneling structures (RTSs) has been studied. The calculations of the changes ΔI₀ in the direct current (DC) component I₀ under the action of an alternating electric field were carried out by the solution of a nonstationary Schrodinger equation with a time-periodic electric field based on the Floquet mode expansion of the wave functions. The dependences of the DC component I₀ in resonant tunneling structures on the frequency ν and AC signal amplitude V_ac have been built. It is shown that the ΔI₀ value in triple-barrier RTSs at resonance frequency hv ≈ E_r2–E_r1 (E_r1 and E_r2 are the energies of the size-quantized levels) can exceed a low-frequency value by more than an order of magnitude. The parameters of the structures have been optmized, in order to use them in the terahertz radiation detectors in the anbsence of an external bias. The possibility of tuning the resonance frequency in the terahertz range by changing the DC bias has been shown.     

A comparative study on the electronic and optical properties of Sb<Subscript>2</Subscript>Se<Subscript>3</Subscript> thin film

The thin film of Sb₂Se₃ was deposited by thermal evaporation method and the film was annealed in N₂ flow in a three zone furnace at a temperature of 290°С for 30 min. The structural properties were characterized by scanning electron microscopy (SEM), transmission electron microscopy (ТЕМ), X-ray diffraction (XRD) and Raman spectroscopy, respectively. It is seen that the as-deposited film is amorphous and the annealed film is polycrystalline in nature. The surface of Sb₂Se₃ film is oxidized with a thickness of 1.15 nm investigated by X-ray photolecetron spectroscopy (XPS) measurement. Spectroscopic ellipsometry (SE) and UV–vis spectroscopy measurements were carried out to study the optical properties of Sb₂Se₃ film. In addition, the first principles calculations were applied to study the electronic and optical properties of Sb₂Se₃. From the theoretical calculation it is seen that Sb₂Se₃ is intrinsically an indirect band gap semiconductor. Importantly, the experimental band gap is in good agreement with the theoretical band gap. Furthermore, the experimental values of n, k, ε₁, and ε₂ are much closer to the theoretical results. However, the obtained large dielectric constants and refractive index values suggest that exciton binding energy in Sb₂Se₃ should be relatively small and an antireflective coating is recommended to enhance the light absorption of Sb₂Se₃ for thin film solar cells application.     

Electrodynamic analysis and synthesis of selective devices based on thin diaphragms in ridge waveguides

Problems of analysis and synthesis of microwave selective devices are solved in the case when such devices are the bandpass filters based on thin plane transverse metal diaphragms in ridge waveguides. The vector eigenfunctions and cutoff wave numbers of waveguides are calculated with the help of the partial-domain method taking into account the edge singularity of the electromagnetic field. The multimode scattering matrices of a cascade connection of reentrant resonators are calculated with allowance for the interaction of irregularities via higher modes of evanescent H and E waves in a waveguide. The amplitude-frequency and phase characteristics and the dimensions of synthesized bandpass filters are presented for the case of Π and H-waveguides.     

Duplexer designed on the basis of microstrip filters using high dielectric constant substrates

A microstrip duplexer for a modification of a PCS communication system operating at frequencies of 1.84?C1.87 and 1.75?C1.78 GHz is described. The duplexer containing microstrip bandpass filters on high dielectric constant substrates (? _r = 92) are compared with duplexers manufactured on microwave ceramic materials with high values of ? _r . The microstrip bandpass filters use stepped-impedance resonators placed one near another with a gap of 0.1?C0.2 mm. It is shown that the microstrip duplexer has slightly lower insertion loss and occupies smaller volume than a duplexer using coaxial dielectric resonators having a rectangular cross section of 3 × 3 mm and ? _r = 92.     

An asymmetric multilayer pendry lens

The optical-band subwavelength imaging by a multilayer Pendry lens consisting of alternating layers of a metal with the permittivity ε _m < 0 and a dielectric with the permittivity ε _d = ?ε _m is considered. In the earlier papers by Pendry and Ramakrishna, it is shown that, in a nonideal case, i.e., for |ε _d | = |ε _m | ≠ 1, the image can be obtained only with the use of an asymmetric scheme, when the image is formed in a medium with a permittivity equal to ε _d , the source being located in vacuum. However, in this case, the image quality is impaired (the image spectrum is narrowed). This phenomenon is explained in this study.     

Microstrip bandpass filters on substrates with high permittivities

A new class of compact microstrip bandpass filters, the stepped-impedance resonators of which are located close together at a gap of 0.1–0.2 mm, is described. Basic two- and three-element sections of microstrip filters, which are designed around stepped-impedance impedance resonators used to implement tiny selective devices with overall sizes and characteristics comparable to those of microwave ceramic filters, are proposed. It is justified that the mentioned filters can be equipped with closely neighboring metallic shields whose heights are equal to the substrate thickness. As a consequence, these filters turn out to be thinner. The frequency responses of experimental microstrip filters operating at frequencies of 1.75–2 GHz and manufactured on the 1-mm-thick substrates with the permittivity ɛ _r = 100 have been analyzed. The three-pole microstrip filter with a shield (its operating frequency is 1.96 GHz, and sizes are 5 × 5 × 1.5 mm) has been simulated. It is demonstrated that new microstrip filters can be competitive with low-sized ceramic filters in certain applications.     

Small-sized microstpip delay line filters on the substrates with high permittivity

There are considered small-sized microstrip delay line filters on the substrates with permittivity (ɛ _r = 92, 36), whose metallic screen is placed at the height, equal to substrate thickness. Filters contain three step resonators, which are placed close to each other with a gap of 0.1–0.2 mm. The basis of delay line filters construction lies in specificities of electromagnetic interaction of closely placed step resonators with closely placed screen and possibility of modification of coupling between non-adjacent resonators. There are represented data of measurement and simulation. Dimensions of a filter on a substrate with ɛ _r = 92, was 5×5×2 mm, they are comparable with dimensions of small-sized microwave ceramic filters of one-piece construction, used in portable communication devices. It is shown that a set of technical characteristics makes new delay line filters competitive.     

Limiting ratios between symbol rate and minimal carrier frequency in ultra-wideband digital information transmission systems

Limiting ratios between symbol rate and minimal carrier frequency f ₀ in ultra-wideband digital information transmission systems, when using rectangular pulse as the message symbol and transmission efficiencies of radio signal’s energy of 90 and 99% in the frequency band [0–f ₀], are obtained. It is shown that modulation of sine and cosine oscillations by a rectangular pulse, in case of small number of carrier’s periods within the message symbol, gives different efficiency results of radio signal’s energy transmission in the provided frequency band. It is stated that, when using a rectangular pulse as message symbol and a requirement to transmit 99% of the radio signal’s energy in the frequency band[0 ? f ₀], the maximal symbol rate of information transmission should be 12 times less than the carrier frequency. However, if transmission of 90% of the rectangular radio signal’s energy in the frequency band[0 ? 2 f ₀] is acceptable, the symbol rate of information transmission may exceed the minimal carrier frequency f ₀, but no more than 1.45 times.