Differential Microstrip Lines Research Articles

Design of T-shaped non-uniform microstrip lines for reducing crosstalk in high-speed PCBs

High-speed and high-density PCB circuit layout design faces serious signal integrity problems. Crosstalk is one of the key problems affecting signal quality. Periodic non-uniform differential microstrip lines have the possibility of reducing crosstalk. This paper proposes a T-shaped non-uniform microstrip line, which adds the T-shaped bump on the inner side of the parallel microstrip lines. The microstrip line adopts the minimum batch production process linewidth to maintain high density, which is suitable for occasions where the line spacing is three times the linewidth. The principle of crosstalk mitigation for T-shaped microstrip lines is explained, and the evolution process of T-shaped microstrip lines is given, compared with parallel microstrip lines and tabbed lines with the same density. The simulation resulting in the time domain and frequency domain shows that the transmission performance of the T-shaped microstrip line is good, and crosstalk can be effectively suppressed in a high-frequency band, even better in some specific frequencies due to the crosstalk fluctuation with frequency.

A Dielectric Sensor Based on Differential Microstrip Lines Coupled With Multiple Magnetic-LC Resonators

A differential microstrip lines-based microwave microstrip sensor loaded with multiple magnetic-LC (MLC) resonators is proposed in this manuscript. The proposed microstrip sensor is evolved from a traditional microstrip sensor, which has a single resonator unit excited by a single microstrip line. Based on the traditional one, the differential microstrip lines are added, which can improve the level of common-mode (CM) suppression. In the MLC resonator structure, a meander slot is embedded into the resonator to enhance the electrical field intensity. Three MLC resonators are etched on the bottom layer of the substrate, which can produce three transmission zeros (TZs) of differential mode (DM) due to the mutual coupling between these resonators and their own RLC equivalent circuit models. The three TZs can be regarded as three modes, the TZ in low-frequency band is the first odd-mode, the middle frequency TZ is even-mode, and the high-frequency TZ is the second odd-mode. As the resonance strength of the second odd-mode is relatively low; thus, it is ignored and the first odd-mode and even-mode are considered. Odd- and even-modes can be utilized to retrieve the permittivity of material under test (MUT). In the measurement, the error removal model is established to guarantee the experimental accuracy of MUT. The proposed microstrip sensor has average sensitivities of about 5.53% and 4.55% for odd- and even-modes, respectively. The proposed sensor has some superiorities to previous designs in terms of design structure, CM suppression, and multimode detection.

A Novel FDTD Approach Considering Frequency Dispersion of FR-4 Substrates for Signal Transmission Analyses at GHz Band

High-speed digital signaling can cause various signal integrity and electromagnetic interference problems. Signal attenuation and distortion can be observed in the gigahertz band owing to the frequency dispersion of substrates. In this article, we propose a frequency-dependent finite-difference time-domain (FDTD) approach for the numerical analyses of flame-retardant type-4 (FR-4) substrates. The proposed FDTD approach provides accurate broadband solutions because it can take into account the frequency dispersion of the substrate. First, the broadband complex permittivity of FR-4 substrates was measured using a balanced-type circular disk resonator and fitted into a Djordjevic model for the FR-4 substrates. Second, we adapted the fast inverse Laplace transform and Prony methods to find the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</i> -domain expressions and incorporate them into the FDTD formulation.The validity of the proposed approach was demonstrated by comparing the numerical and experimental results of the differential insertion loss and phase constant of differential microstrip lines. Both results were in good agreement, indicating the validity and usefulness of the proposed approach for simulations of broadband signal transmission in FR-4 substrates.

Analysis and Modeling of Wideband Common-Mode Absorption With Lossy Complementary Split-Ring Resonator Chain in Resistor-Free Differential Microstrip Lines

Novel resistor-free differential microstrip lines prototype loaded with lossy metamaterial is proposed to achieve wideband common-mode (CM) absorption. The metamaterial is constructed by a chain of lossy complementary split-ring resonators (CSRRs), which are etched on the ground plane. Different from the CM absorption realized by lumped resistors, this work utilizes the inherent dielectric and conductor losses introduced by the differential microstrip lines and CSRRs to absorb CM energy. Furthermore, CSRRs are designed with gradual perturbation to broaden the CM absorption band. The lossy even-mode equivalent circuit models are established and analyzed to reveal the CM absorption mechanism. In order to verify the proposed strategy to design resistor-free differential microstrip lines with CM absorption, a sample was fabricated and measured. The measured <inline-formula> <tex-math notation="LaTeX">$S_{\mathrm {cc21}}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$S_{\mathrm {cc11}}$ </tex-math></inline-formula> are kept below &#x2212;10 dB simultaneously from 4.3 to 5.65 GHz with 27.4&#x0025; fractional bandwidth, where the CM energy is absorbed over 80&#x0025; by the dielectric and conductor losses. Meanwhile, the low insertion loss of <inline-formula> <tex-math notation="LaTeX">$S_{\mathrm {dd21}}$ </tex-math></inline-formula> below 6 GHz as well as the good differential-mode eye diagrams demonstrates that good signal integrity of differential signals is kept.

A Wideband Antenna for mm-Wave Antenna-in-Package Application

Antenna-in-Package has been widely applied for millimeter-wave systems that integrate the MMIC chip and antenna into one package. For organic substrate Antenna-in-Package solutions, probe feeding and aperture-coupled feeding patches are generally used as radiation units. In this work, a patch antenna is proposed using coupled resonation; a wideband, high-gain and compact 3-D antenna package integration is realized. An equivalent circuit model is analysed for the coupled structure. &#x03BB;/4 coupled line is adopted for impedance matching, the whole antenna is designed on the same thick radiation patch substrate, with less organic substrate lamination layers. For millimeter-wave Antenna-in-Package structures, vias are generally used for vertical interconnections, which function well in the lower millimeter-wave frequency range. For operation frequency higher than 100GHz, design difficulties may be encountered for via interconnection, with performance degeneration on impedance matching and transmission loss. This work integrates a H-shaped slot instead of vias on an organic substrate, realizing a high frequency and wideband mm-wave transition. An interconnection structure is also adopted to convert a microstrip line to differential microstrip line in the feeding network. Antenna prototype with 2&#x00D7;4 radiation array is designed for verification purpose at 20GHz frequency range with simulated bandwidth of 23%. The prototype is fabricated with separate PCB boards assembled with screws, measured antenna gain is 12.7dBi-13.5dBi. The proposed structure provides a wideband and low-loss solution for potential higher frequency mm-wave Antenna-in-Package applications.

Scattering Parameters Characterization of Periodically Nonuniform Transmission Lines With a Perturbative Technique

In this article, a novel procedure for the frequency-domain solution of nonuniform transmission lines (NUTLs) is presented. The procedure is based on a recently proposed perturbative technique, which is proven to be computationally more efficient than standard solution approaches, which are based on line subdivision into uniform cascaded sections. With respect to the original perturbation technique, the procedure proposed here offers more flexibility, as it provides a representation of the NUTL under analysis in terms of $S$ and/or $T$ parameters at its ports. Moreover, it retains the same prediction accuracy at the price of a slight increase in computational burden, which can be mitigated anyway through parallel computing. Furthermore, even without ad hoc (parallel) implementations, the proposed procedure outperforms other approaches to solve differential lines with partially or fully repetitive geometries. Namely, it assures accurate prediction of output quantities with reduced simulation time. This is proven by three application examples, namely, two differential trapezoidal tabbed lines (one with interdigital tabs and one with facing tabs), and a differential microstrip line with a varying common-mode (CM) impedance (as such reducing CM noise). Comparison with full-wave simulations allows assessing the prediction accuracy of the proposed procedure. Comparison with the aforementioned transmission-line-based solutions allows appreciating the enhanced computational efficiency.

Compensating Mode Conversion Due to Bend Discontinuities Through Intentional Trace Asymmetry

In this letter, a comparative analysis is carried out between the mechanism of mode conversion in differential microstrip lines due to bend discontinuities on one side and trace asymmetry on the other side. With the help of equivalent modal circuits, a theoretical basis is provided for the idea to compensate the undesired common mode (CM), due to the presence of the bend, by intentionally designing asymmetric traces. As an application example, the proposed CM-reduction strategy is used in conjunction with another recently-presented wideband CM suppression filter for differential microstrip lines. It is shown that the proposed solution enhances the overall CM-reduction performance of the filter by some decibels, while preserving its transmission properties.

Common-Mode Conversion Noise Mitigation With Embedded Coupled Lines in Differential Serpentine Delay Microstrip Lines

In this article, a novel method to suppress differential-to-common mode conversion noise in differential serpentine delay microstrip lines (DSDMLs) is proposed. To suppress the remnant mode conversion noise, embedded coupled lines are adopted in the parallel traces section of DSDMLs. Embedded coupled lines can minimize the timing delay between even- and odd-modes in the parallel traces section. In specific dimensions, embedded coupled lines propagate even- and odd-mode signals with the same velocity without changing the differential-mode impedance, so DSDMLs with embedded coupled lines can suppress differential-to-common mode conversion noise without degrading differential-mode signal quality. In simulations, the proposed DSDMLs exhibit about 93.0% suppression of the peak common-mode conversion voltage compared with conventional DSDMLs, regardless of the length of the parallel traces section of DSDMLs. The performance of our proposed DSDMLs is verified by conducting measurements with millimeter-scale test boards in the frequency and time domains. The test board exhibited 78.7% suppression of common-mode conversion noise without degrading the differential insertion loss compared with conventional DSDMLs. Simulations and measurement results exhibited good agreement.

E-Band RF MEMS Differential Reflection-Type Phase Shifter

A systematic approach in the design methodology, leading to the optimization of a differential reflection-type phase shifter (RTPS), is presented. The key parameters of the quadrature coupler and the reflective loads are analyzed for optimum RTPS performance, showing that: 1) equality between the coupling and through transmission coefficients provides the best return and insertion loss; 2) a minimized coupler reflection coefficient with negative phase optimizes the return loss and the phase performance; and 3) a wide tunable reflective load with minimized resistance assures lower insertion loss and larger phase range. The even–odd mode analysis leads to the coupler miniaturization. The differential microstrip line dimensions are determined by the minimum feature size of our fabrication process. By choosing the separation between the coupled lines, the even- and odd-mode characteristic impedances satisfy the derived design equations for equalized quadrature outputs and provide the matching capacitance. By exploiting the virtual ground of the differential structure, a novel RF MEMS reconfigurable short circuit is introduced for the reflective loads aiming at 180° phase shift range and low insertion loss. Over the 70–86-GHz range, the measurements show good agreement with the simulations. The measured RTPS achieves a 195.6° phase shift range with a phase error less than 8.6° and insertion loss error less than 0.7 dB. For all phase states, the reflection coefficients are below −18 dB, and at 74 GHz, the insertion loss is 4.4 ± 0.5 dB.

$F$ -Band Differential Microstrip Patch Antenna Array and Waveguide to Differential Microstrip Line Transition for FMCW Radar Sensor

This paper describes a differential microstrip patch antenna array and a rectangular waveguide to coupled differential microstrip line transition operating at 122 GHz. The antenna array is realized in series-fed topology/architecture and is very suitable for MIMO radar applications. This on-board antenna provides a high antenna gain and radiation efficiency at millimeter-wave frequencies. The rectangular waveguide transition with low insertion loss offers the facilities to characterize the on-board antenna and can also be utilized in a radar system in combination with a horn antenna. The measured bandwidth of the antenna array is 7 GHz with a maximum gain of 12.98 dBi at 122 GHz. The waveguide transition has a bandwidth of 20 GHz at 10-dB return loss. Radar measurements were performed using radar sensors that were equipped with the developed antenna array as well as the waveguide transition in combination with horn antenna for comparison purposes. The radar measurement results with on-board antenna array show a good performance for detecting the range of the target.

A Compact and Vialess Vertical Microstrip-to-Microstrip Differential Transition

Abstract In this paper, a new compact, vialess and vertical microstrip-to-microstrip differential transition is presented based on half-wavelength slotline structure. The equivalent-circuit model is constructed, and theoretical analyses based on equivalent circuit model reveal that good differential mode (DM) signal transmission between the top- and bottom-layer differential microstrip lines can be achieved over a wide frequency range determined while the common mode (CM) signal is rejected. Moreover, the systematic design method is presented to extract the values of electrical parameters from prescribed performance in terms of its operating bandwidth and ripple levels. In addition, owing to the employment of half-wavelength slot, the proposed differential transition has a compact size. Finally, the proposed differential transition and design theories are well vindicated by an exemplary circuit centered at 2.45 GHz with the measured relative frequency bandwidth (FBW) of 114 %.

Perturbative Analysis of Differential-to-Common Mode Conversion in Asymmetric Nonuniform Interconnects

In this paper, a perturbative technique and the related computational algorithms are presented for the prediction of differential mode (DM) to common mode (CM) conversion arising from asymmetry and nonuniformity in differential interconnects. It is shown that discontinuities evolving from nonuniformities and geometrical imbalance along the (usually desired uniform) lines give rise to equivalent distributed sources that couple the (ideally decoupled) DM and CM circuits. The prediction of modal quantities is then achieved numerically, as an iterative perturbative refinement of the frequency response of an ideal differential line. Different levels of approximation are introduced and discussed. Specifically, the assumption of weak imbalance allows analyzing the propagation of the two modes separately. Moreover, the assumption of weak nonuniformity allows avoiding CM refinements. The proposed technique applies to both ideally uniform and inherently nonuniform interconnects, and leads to a significant simulation speed-up compared to traditional approaches that analyze nonuniform lines by their subdivision into uniform sections. The methodology is illustrated based on two differential microstrip lines, one tapered and one with sinusoidally varying trace edges. Validation against full-wave simulations and measurements is provided.

Differential Bandpass Filters Based on Dumbbell-Shaped Defected Ground Resonators

This letter presents a dumbbell-shaped defected ground resonator and its application in the design of differential filters. The operation principle of the dumbbell-shaped resonator (DSR) coupled to differential microstrip lines is studied through a circuit model analysis. The proposed circuit model is validated through the comparison with the electromagnetic simulation results. It is shown that the bandpass configuration of microstripline- coupled DSR can be used to design higher order bandpass filters. The design procedure is explained by developing a thirdorder filter prototype. The designed filter shows more than 57 dB common mode rejection within the differential passband.

A right-angle wideband transition between differential microstrip line and rectangular waveguide

A right-angle wideband transition between differential microstrip line and rectangular waveguide

Differential Lines for Common-Mode Suppression Based in Hybrid Microstrip/CPW Technology

A symmetrical pair of differential microstrip lines implemented in hybrid microstrip/coplanar waveguide (CPW) technology is proposed. Transmission-line models are used to analyze differential- and common-mode responses, allowing efficient design with minimal optimization effort. The structure behaves as a conventional transmission line pair under differential-mode excitation, whereas asymmetrical coupled transmission line theory has to be applied to characterize common-mode operation. The common mode is strongly suppressed thanks to the introduction of a controllable transmission zero. A two-stage version of the structure is used to increase the common-mode rejection bandwidth. All the electrical parameters of the transmission lines have been obtained using an in-house fast quasi-TEM code. The good agreement between transmission-line models, full-wave simulations and measurements confirms the benefits of the structure and the design procedure.

Wideband tunable common‐mode suppression filter based on varactor‐loaded slot‐ring resonator for high‐speed differential signals

In this study, a varactor-loaded slot-ring resonator (SRR) is investigated and applied in the design of tunable common-mode (CM) suppression filter for high-speed differential signals. Two tunable capacitors (realised by two kinds of varactors) with different values are symmetrically added to the SRR. Under the stepped-width structure of the SRR, the frequency-tuning principle is studied. It can be found that the fundamental resonant frequency f fund can be shifted up and down by properly controlling the respective values of the two loaded capacitors, i.e. bi-directional tuning. As a result, the tuning range of f fund can be greatly enlarged, approximately double that of the traditional tunable resonator. Meanwhile, the employed stepped-width SRR is useful not only for miniaturising the circuit size, but also further increasing the tuning range of f fund. By etching the proposed SRRs on the ground plane underneath the differential microstrip line, two wideband tunable CM-suppression filters using one and three SRR units, respectively, are designed, and their simulated and measured results showing good agreement are presented. It is found that the CM stopband frequency can be tuned from 0.78 to 1.52 GHz, while the differential signals still keep good signal integrity in eye-pattern observation.

A Compact and Broadband Differential Microstrip Line to Rectangular Waveguide Transition Using Dipole Antenna

In this paper, a compact full Ka-band differential microstrip line (DML) to rectangular waveguide transition is proposed. The dipole antenna with semi-elliptic arms is introduced to transform the differential mode of DML to the TE10 mode of the rectangular waveguide directly. The two arms of the dipole antenna are connected together by a shorting strip to reduce the size of the dipole. Compared with the DML-to-waveguide transition using the fin-line topology, the size of the proposed transition has been reduced by 86 %. To verify this transition, a back-to-back structure is fabricated and tested. It provides a return loss of better than 15.2 dB and an insertion loss of 0.73 to 1.07 dB within a wide frequency range from 26.5 to 40 GHz. The measurement results show good agreement with the simulation results. Furthermore, a tolerance analysis is also performed via the simulation to prove that this transition is robust in the fabrication and mechanical assembly.

Common-Mode Filters With Interdigital Fingers for Harmonics Suppression and Lossy Materials for Broadband Suppression

Common-mode (CM) noise on differential microstrip lines can be suppressed by PCB-embedded filters. These filters use resonating or broadband structures such that only the field from the CM signal couples to them. Based on a quarter-wavelength resonator, a new PCB filter is designed having an electrical size of only 0.15 λ × 0.065 λ, where λ is the effective wavelength in the CM filter. Its interdigital structure is also capable of suppressing higher order harmonics of the CM signal and can be used for USB 3.0 to mitigate electromagnetic interference. Further, a novel broadband suppression structure is described that uses magnetically lossy material to suppress the CM signal from 4.6 to 20 GHz without strongly affecting the intended differential signal.

A Wideband Ultra-Thin Differential Loop-Fed Patch Antenna for Head Implants

An ultra-thin differential patch antenna is proposed for head implants. Through the proximity coupling between the rectangular patch and the loop structure fed by a differential microstrip line, an antiphase mode is excited, which doubles the bandwidth of an aperture-coupled rectangular patch antenna. Moreover, the directional radiation allows maximum energy to be directed to the receiver and lowers the specific absorption rate (SAR). An antenna prototype with an overall size of $24 \times 10 \times 0.95\,\text{mm}^{3}$ exhibits the ${-} 10\,\text{dB}$ reflection bandwidth of $4.1 - 4.5\;\text{GHz}$ , directivity more than 5 dBi, and gain higher than ${-} 8\,\text{dBi}$ at the boresight. The maximum transmit power of 28 mW is permitted for the SAR limit of 1.6 W/kg across the impedance bandwidth.

Differential Microstrip Lines With Common-Mode Suppression Based on Electromagnetic Band-Gaps (EBGs)

A technique for the suppression of the common mode in differential (balanced) microstrip lines, based on electromagnetic band-gaps (EBGs), is presented in this letter. It is demonstrated that by periodically modulating the common-mode characteristic impedance of the line and simultaneously forcing the differential-mode impedance to be uniform (and equal to the reference impedance of the differential ports), the common mode can be efficiently suppressed over a certain frequency band, while the line is transparent for the differential-mode. The main advantage of EBGs, as compared to other approaches for common-mode suppression in differential microstrip lines, is the fact that the ground plane is kept unaltered. Moreover, the design of the differential line is straightforward since the required level of common-mode suppression and bandwidth are given by simple approximate analytical expressions. As a design example, we report a four-stage common-mode suppressed differential line with 68% fractional bandwidth for the common-mode stopband centered at 2.4 GHz, and maximum common-mode rejection ratio (CMRR) of 19 dB at that frequency. Furthermore, we have designed and fabricated a six-stage double-tuned common-mode suppressed differential line in order to enhance the stopband bandwidth for the common mode around 2.4 GHz.