The fidelity of a high-speed signal, whether within a device or on a PCB, is affected by the path it takes and its surroundings. In particular, the design of vias used to route the signal between PCB layers and the proximity to other vias have large effects at very high frequencies. Four research papers provide some detailed insights.
In a paper delivered at the EMC SI 2015 Symposium, the relationship between a pair of traces carrying a differential signal and two vias carrying a different differential signal was explored. Through simulation and measurement, the authors showed that a via on either side of a pair of traces exhibited the largest amount of crosstalk from the lines to the vias because each via was close to one of the lines but distant from the other. This means that each via will pick up a greater amount of interference from the line closer to it and less from the more distant line. From symmetry, the crosstalk at the vias will be equal but of opposite phase so when treated as a differential signal will not cancel.
The second case placed both vias next to each other on the same side of the line pair, one via close to the lines and one farther away. Crosstalk will be unequal because the vias are not arranged symmetrically. However, the same signal phase will dominate at each via, so differentially, cancellation will be better than for the first case. The third case placed both vias the same distance away from the lines and on the same side. Differentially, this arrangement will have the most complete cancellation.1
Many times, there are several high-speed signals associated with an IC, so PCB trace routing can become dense near the IC package. A paper presented at last year’s EMC Symposium considered the signal imbalance that ground-return via placement has on nearby differential pairs of vias.
As the authors stated, “Differential signal traces require symmetry to avoid conversion of differential mode to common mode. As part of the return path, this requirement extends to the reference plane. Routing next to the edge of a reference plane can cause mode conversion, for example. This practice can result in radiated emissions and susceptibility issues. Adding more return vias without placing them symmetrically creates an unbalanced return path. One half of the differential signal will have different return-path parasitics than the other; this discontinuity is the source of mode conversion.”2
The present paper extended earlier work that optimized the placement of return vias relative to two pairs of differential vias. Not surprisingly, it was shown that optimizing return via placement for one pair could degrade the performance of the other pair. To better judge the overall success of the process, a fitness function was developed:
By using a multilayer via transition software tool in combination with a fast full-wave solver and genetic algorithms, the effect of adding several return vias to large arrays of differential-signal vias was investigated. Although not providing the absolute lowest amount of mode conversion, a repeating pattern for additional return vias was developed. As well as being simple enough to be incorporated at the PCB layout stage, the approach also can be extended to larger arrays without further analysis. Frequencies up to 20 GHz were considered in simulations.
At higher frequencies, trace and via dimensions become very important. In particular, as stated in a second 2014 paper, “Multiple vias sharing a common anti-pad enable high-density trace routings, and thus [this type of layout] is becoming more and more popular in multilayer printed circuit boards and/or packages, especially for high-speed differential signal interconnects.”3
Two numerical approaches were used to evaluate a parallel-plate structure representing the top and bottom copper layers in a PCB with pairs of vias and shared anti-pads. One method simulated each via hole separately using the 3D finite element method (FEM), creating separate admittance blocks. These were combined with a hybrid 3D/2D FEM analysis of the parallel plate structure to form a combined admittance matrix. This analysis assumed that the transverse electromagnetic mode (TEM) was the only mode propagating in the via holes.
In the other approach, the hybrid 3D/2D FEM was used for the structure and did not assume only TEM propagation. Comparison of the two results was very good when the top and bottom PBC copper layers were simulated with 1-mil thickness. However, the results diverged when the thickness was increased to 10 mils, indicating that TEM propagation was not the only mode present at the anti-pads on the inner surfaces of the top and bottom conductors.
The paper concluded, “… discontinuities at the rim of the [inner] anti-pad excite higher-order modes to satisfy the boundary conditions there. The evanescent higher-order modes, excited by the discontinuities …, the via hole, and the parallel-plate pair, decay exponentially. If the plate thickness is small, the higher-order modes may penetrate into another plate-pair.” Simulations were performed up to 50 GHz in this paper.
A third 2014 paper also dealt with shared anti-pads and higher-order modes of propagation.4 The paper stated that using shared anti-pads for differential vias provided a better impedance match along the via path—better than using two separate anti-pads. And, shared anti-pads also reduce via-to-plate capacitance, which improved the phase velocity. However, the paper determined that “… the generation of the parasitic modes takes place mainly in the launch segment where the current turns from microstrip modes to coaxial modes and excites the higher-order modes….”
Because the shared anti-pads are relatively large, the associated cutoff frequency for these unwanted modes may be low enough that they can propagate down the vias. Therefore, the suggestion was to reduce the size of the anti-pad area on the top and bottom reference-plane layers by reverting to separate and smaller anti-pads. Continuing the reference plane between the separate anti-pads eliminates the TE10 and TE01 modes.
Smaller anti-pads correspond to a higher cutoff frequency, which minimizes propagation of higher-order TE20 and TE11 modes. Simulations using 0.4-mm radius anti-pads instead of the initial 0.6-mm value showed about a 15-GHz improvement in transmission bandwidth. The paper summarized, “The improvement from the proposed design is a result of the combination of a better impedance match and the suppression of higher-order modes.”
References
- Xiao, K., and Ye, X., “A Study of Trace-to-Via Coupling Effect in High-Speed Differential Interconnects,”EMC SI 2015 Proceedings, pp. 131-134.
- Cracraft, M. et al, “Unintended Effects of Asymmetric Return Vias and Via Array Design for Reduced Mode Conversion,“ IEEE International Symposium on Electromagnetic Compatibility Proceedings, 2014, pp. 250-256.
- Zhang, Y. –J, et al, “Modeling of Multiple Vias with a Shared Anti-pad in an Irregular Plate Pair using Domain Decomposition Approach,” IEEE International Symposium on Electromagnetic Compatibility Proceedings, 2014, pp. 265-270.
- Duan, X., et al, “Optimization of Microstrip-to-Via Transition for High-Speed Differential Signaling on Printed Circuit Boards by Suppression of the Parasitic Modes in Shared Antipads,” IEEE International Symposium on Electromagnetic Compatibility Proceedings, 2014, pp. 234-239.
EMC news and product briefs
While the technical program at events like the EMC Symposium provides a forum for exposing basic research, organizations are looking at market research, expanding the services their labs offer, and offering products that can help measure and control electromagnetic interference.
For example, recent analysis from Frost & Sullivan, “EMC Test Equipment and Services Market in North America,” finds that the market earned revenues of $453.4 million in 2014 and estimates this to reach $607.6 million in 2021.
“The rising popularity of electric vehicles and smart grid systems will unlock substantial opportunities for EMC test equipment and service vendors in North America,” said Frost & Sullivan measurement and instrumentation research analyst Janani Balasundar. “The highest demand for EMC testing will stem from the dynamic wireless communication industry in the region.”
On the laboratory front, Retlif Testing Laboratories, a global provider of EMI/EMC and environmental (ESS) testing, has finalized the accreditation process for its new dedicated Composites Testing Laboratory.
Located within the Composite Prototyping Center (CPC) in Plainview, NY, the new laboratory is now fully accredited to IS0-STD-17025, the international standard that represents the basis for accreditation from an accrediting body through formal recognition of demonstration of that competence. Administered by the Laboratory Accreditation Bureau (L-A-B), the accreditation enables a broader acceptance of test data generated, as the requirement for accreditation is a requisite in industries where composites are widely used, including aerospace, military, rail, and automotive industry sectors.
Leonard Poveromo, CPC executive director, commented, “We are extremely pleased that Retlif is fully approved as the Composite Prototyping Center structural testing source for composites. This furthers our goals of helping organizations cross boundaries, expand manufacturing scope, and effectively compete in fast-expanding, dynamic markets. Retlif engineers and technicians will have full access to utilize all equipment within the center.”
And as for applicable instruments, Keysight Technologies announced in August the availability of real-time spectrum analysis (RTSA) as an option for its standards-compliant MXE EMI receiver. Adding RTSA to an MXE enables test labs to observe and diagnose transient and wideband emissions during electromagnetic compatibility compliance and precompliance testing.
With RTSA, the company said, engineers can more easily see and understand high-speed transient signals that are difficult to capture with traditional spectrum or signal analyzers. The company said the RTSA capability is especially useful in applications such as radar, automotive, and wireless communications that often experience fast-moving, short-duration emissions.