Signal Integrity Analysis of Single-Ended and Differential Signaling in PCBs with EBG structure
A. Ciccomancini Scogna #, A. Orlandi+, V. Ricchiuti*
# CST of America Inc, 492 Old Connecticut Path, Suite 505, Framingham, MA, 01701, phone: +1-508-665-4400 Fax: +1-508-665-4401, e-mail: [email protected] + UAq EMC Lab, University of L’Aquila, 67100, Poggio di Roio, L’Aquila, Italy Phone: +39-0862-434432, Fax: +39-0862-434403, e-mail: [email protected] *Technolabs S.p.A., ss. 17, Loc. Pile, 67100, L’Aquila, Italy Phone: +39-0862-344517, Fax: +39-0862-344527, e-mail: [email protected]
Abstract – Object of this paper is the signal integrity analysis of shorting the planes at the physical location of the patches Printed Circuit Boards with Electromagnetic Bandgap within the band-stop frequency range, thus suppressing the structures. In particular the signal quality of single-ended and propagation of electromagnetic noise in between the planes. DIFF lines is discussed both in time and frequency domain (S- Since the last few years, researchers have been focusing parameters, TDR and eye-diagrams). their effort on the more cost effective two dimensional (2D) Two different configurations (two dimensional and three dimensional) of Electromagnetic Bandgap structures are planar EBG: the power (PWR) or the ground (GND) layers analyzed by means of a three dimensional full wave field are patterned and shorting vias connecting the metal patches simulator based on the Finite Integration Technique. Results with an extra metal layer are not required. show a consistent improvement of the signal integrity when On the contrary of the well demonstrated SSN attenuation DIFF signaling is used, while keeping the typical advantage of (within tunable frequency ranges) when the EBGs are noise mitigation due to Electromagnetic Bandgap layers. employed, there is not a systematic and available scientific literature regarding the signal integrity performances of PCB with EBG structures and their impact on the transmission I. INTRODUCTION properties of waveguide structures (like stripline, vias and microstripline) in high speed digital systems. In the present paper the signal integrity analysis of signals propagating on PCB with EBGs is discussed. Electromagnetic band gap (EBG) structures are becoming a In particular the signal quality of single-ended (SE) and popular choice for the suppression of unwanted DIFF (DIFF) lines is analyzed. A recent contribution on this electromagnetic mode transmission and simultaneous topic is [9] where the advantage of DIFF signaling in PCBs switching noise (SSN) in power distribution networks with EBG layers is highlighted. (PDNs) of high speed digital systems [1-8] Nevertheless in [9] only eye diagrams are considered as Other techniques are known in literature for the SSN figure of merit for the investigation of the signal quality. mitigation: 1) decoupling capacitors located over the entire With the present contribution an attempt is made in Printed Circuit Board (PCB), 2) splitting planes to block the presenting more figures of merit computed both in time and propagation of unwanted electromagnetic waves, 3) power frequency domain such as S-parameters, TDR waveforms islands, 4) shorting vias. and the already mentioned eye diagrams. Unfortunately all these techniques are effective only in the Two different configurations of EBG structures, already MHz range and/or are restricted to a very narrowband published for their stop band properties [7,8] are analyzed by frequency ranges. means of CST MICROWAVE STUDIO®, a three The use of EBG structures in PCB environments was first dimensional full wave field simulator based on the Finite introduced by M. Ramahi, but EBG structures have been Integration Technique (FIT) [10,11]. initially employed for antenna applications because of their Results show a consistent improvement of the signal unique behavior. In fact, EBGs can satisfy a Perfect transmission quality when DIFF signals are employed, while Magnetic Conductor (PMC) condition over a certain keeping the typical advantages of SSN mitigation due to frequency band and impose a zero degree reflection phase to EBG layers. normal incident waves, making them suitable for The paper is organized as follows: in Section II the applications such as coupling reduction between antennas geometric details of the test structures are briefly described; and antenna directivity improvement. in Section III a comparative study between single-ended and When inserting a three dimensional (3D) EBG structure in DIFF signalling is presented for two different types of EBG the parallel-plate waveguide-like structure of the power bus structures: 3D (with shorting vias) and 2D (with patterned of a PCB, a resonant circuit composed of the top plate, a planar GND plane). Section IV offers finally some single patch, the corresponding via and the plane connecting concluding remarks. the vias together is created. This resonant structure provides a low-impedance path to high-frequency currents in the power-planes therefore
978-1-4244-1699-8/08/$25.00 ©2008 IEEE II. EBG STRUCTURES shorting vias, and therefore the stack-up presents an extra layer, as shown in Fig. 2a. The test board used to analyze the performances of single- In particular two different designs are analyzed: 1) triangular ended and DIFF signals is the same proposed in [7]: it is a patches in a hexagonal array and 2) square patches. The details and the dimensions of the patches as well as the 9.15 cm x 4.15 cm board in FR4 dielectric (εr = 4.4 and tan δ = 0.02 at 0.5 GHz) board stack-up are listed in [7, 8]. The EBG layer is characterized by square patches and by double L branches (which increase the inductance) whose geometric details are reported in [7]. Figs. 1a-1c illustrate the SE and DIFF configurations with nominal characteristic impedance of 50 Ω SE and 100 Ω DIFF respectively.
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(c) (b) Fig. 2 – PCB with 3D EBG layer: (a) side view and relevant dimensions, (b) triangle patches and (c) square patches.
III. SIGNAL INTEGRITY ANALYSIS
In this section, the signal integrity of SE and DIFF signals for the proposed EBG structures is investigated both in the time domain and frequency domain by using S-parameters, eye pattern and TDR characterization. Fig. 3 illustrates the comparison of the insertion loss, for the configuration of Fig. 1, between SE and DIFF signals. It is relevant to see how the quality of the S21 consistently improves, as in the classic case of the continuous reference planes, for the DIFF configuration and it is not affected by the stop-band properties of the EBG structure. Considering an allowable attenuation of -3dB, a bandwidth of 0 – 9 GHz can be obtained when a DIFF signaling is used. (c) For the SE configuration, due to a consistent resonance in the relatively low frequency range, only a narrow band Fig. 1 – PCB with 2D planar EBG layer: (a) side view and frequency range (0 – 2 GHz) allows verifying the value of - relevant dimensions in mm, (b) SE case and (c) DIFF case. 3dB constrain. The eye diagram is also calculated for both configurations in order to give an indication of the signal The second kind of configuration is a 3D EBG. The metal quality. patches are connected to the bottom layer by means of A pseudorandom (PRBS) bit sequence has been used as The same kind of analysis has been carried out for the input sequence with the following parameters: Tbit = 0.5 ns, second configuration illustrated in Fig. 2: 3D EBG with tr = tf = 0.05 ns, Vhigh = 1 V, Vlow = 0 V. triangular patches (Fig. 2b) and 3D EBG with square patches (Fig. 2c). In particular Fig. 5 represents the insertion loss for SE and DIFF signaling for the studied 3D EBG structures. In both cases the improvement when the DIFF signaling is used appears relevant, infact the already mentioned figure of merit (-3dB) is applicable up to about 8.5 GHz for the EBG structure with triangular patches and it is valid over the full broadband range (0-10 GHz) for EBG structure with square patches. The slight reduction of performance in the case of EBG with triangular patches is probably related to the large inductive effect of the higher number of shorting vias connecting the metal patches with the bottom metal layer.
Fig. 3 – Insertion loss for SE and DIFF lines for the 2D planar EBG structure in Fig. 1.
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(b) Fig. 5 – Insertion loss for SE and DIFF lines for: (a) the 3D EBG structure with triangle patches and (b) the 3D EBG structure with square patches.
The improved performance of the DIFF signals is also
visible in the eye diagrams (see Figs. 6a and 6b for (b) triangular patches and Figs. 7a and 7b for square patches) in Fig. 4 – Eye diagrams for the (a) SE and (b) DIFF lines for which the same previous input bit sequence has been used. the 2D planar EBG structure in Fig. 1. In the first case for the SE transmission line, it results MEO = 0.40 V and MEW = 0.41 ns (see Fig.6a) while for the Maximum Eye Opening (MEO) and Maximum Eye Width DIFF signaling, MEO = 0.46 V and MEW = 0.48 ns (see (MEW) are used as metrics of the eye pattern quality. For Fig.6b). With regarding to the square patches configuration the SE transmission line (see Fig. 4a), it results MEO = 0.42 in the SE signaling MEO = 0.38 V and MEW = 0.47 ns, V and MEW = 0.43 ns, for the DIFF signaling (see Fig. 4b), while in the DIFF signaling MEO = 0.44 V and MEW = it results MEO = 0.48 V and MEW = 0.48 ns. 0.45 ns. It is also interesting to observe that the calculated TDR (see Figs. 8a and 8b) reveals oscillations in the SE Each change in characteristic impedance causes the TDR configuration of Fig. 1. trace to bump up or down to a new impedance level. This is due to the periodic gaps among the patches of the EBG structure [12]. In order to minimize this problem, different solutions can be employed: additional reference plane to provide a better return patch to the current (as proposed in [12]), modify the dielectric thickness, change the distance between the EBG layer and the signal trace, partial EBG layer, but typically none of them is very effective. Instead in the corresponding TDR for the DIFF signal type (Fig. 8b) the oscillations are highly attenuated even though a higher impedance variation is observed in the range 0.2 - 0.8 ns.
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(b) Fig. 6 – Eye diagrams for: (a) SE (b) DIFF lines for the 3D EBG structure with triangle patches.
(b) Fig. 8 – TDR waveforms for (a) DIFF and (b) SE lines.
At this point the noise propagation on the PWR/GND plane pairs is investigated by placing a third port (see Fig. 9a). Port 3 is located on the upper corner of the board, close to the edge (x = 8.45 cm , y = 0.7 cm) and it is connected between the continuous PWR plane and the EBG layer in the 2D planar EBG case, and between the continuous PWR plane and the continuous bottom layer in the 3D EBG configurations. (a) The stack-ups with in evidence the port location is illustrated in Figs. 9b-9c.
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(b) Fig. 7– Eye diagrams for the (a) SE and (b) DIFF lines for (b) the EBG structure with square patches. 11a, c, e) with respect to the DIFF signals (Fig.11b, d, f). Port 3 can represent the position of a susceptible device one wishes to place in a “quiet” board’s area.
(c) Fig. 9 – View of the EBG structures used for the noise propagation analysis: (a) top view, (b) stack-up of 2D planar EBG and (c) stack up of 3D EBG.
The results are illustrated in Figs. 10 where the noise propagation from Port 1 to Port 3 (S31) for the SE (Fig. 10a) configuration is compared with the DIFF case (Fig. 10b).
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(b) Fig. 10– Noise propagation from Port 1 to Port 3 (|S31|): (a) SE and (b) DIFF signals.
Important considerations can be addressed: the noise excited by the SE configuration is stronger than the one excited by the DIFF configuration and a stop-band of -40dB is visible in the wideband frequency range 1 – 4 GHz and 4.2 - 8.5 GHz. A further confirmation of the previous observations also comes from the plot of the surface current distribution (d) on the EBG layers at 6 GHz, a frequency value within the stop band of all three EBGs. The current variation is represented by a color contrast in these figures: dark color is blue meaning low value, light color is red meaning hot spots). Isolation is of course desirable between the input Port 1 and the output Port 3. Figs 11 show the spatial distribution at 6 GHz of the surface currents on the EBG layers for the 2D planar EBG, the 3D triangular and square EBG excited by the SE and DIFF signals respectively. It is clear that the value of the magnitude of the surface currents is higher for the case of SE transmission (see top right corner of Figs. (e) IV. CONCLUSIONS
In this paper the signal integrity analysis of SE and DIFF signals propagating on PCB with EBGs is presented. Three different configurations (with 2D and 3D EBG structures) and multiple results (in time and frequency) are used to validate the results. It is demonstrated how DIFF signaling allows a consistent improvement of the signal transmission quality (insertion loss and eye-diagram) for the all studied configurations. The noise propagation on the power bus layer is also (f) investigated and results confirm a better isolation (-10dB or more) with DIFF signals. Fig. 11 – Surface current distribution at 6 GHz for 2D planar EBG (a) DIFF and (b) SE signals; 3D EBG triangular (c) DIFF and (d) SE signals; 3D EBG square (e) DIFF and (f) ACKNOWLEDGMENT SE signals. This work was supported by the Italian Ministry of University (MIUR) under a Program for the Development of As comparison the 2D EBG layers of Fig.1 (GND layer) is Research of National Interest (PRIN grant # 2006095890). replaced with a continuous metal plane and Fig. 12 illustrates the surface current distribution on the GND plane; it is straightforward to see the higher value of the current REFERENCES even for the DIFF case. It should be also noted that the scale for the surface current [1] J.Choi, V.Govind, M. Swaminathan, “A novel electromagnetic band distribution in Fig. 12a and 12b is increase to 0-5 [A/m] in gap (EBG) structure for mixed signal system applications”, Proc. of IEEE Radio and Wireless, Atlanta, Georgia, September 2004. order to better visualize the surface current pattern around the ports. [2] R. Abhari, G.V. Eleftheriades, “Metallo - dielectric electromagnetic band gap structures for suppression and isolation of parallel-plate noise in high speed circuits”, IEEE Trans. On Microwave Theory and Tech, vol. 51, no. 6, pp. 1629–1639, June 2003. [3] S. D. Rogers, “Electromagnetic-Bandgap layers for broad-band suppression of TEM modes in power planes”, IEEE Trans. MTT, vol. 53, no. 8, pp. 2495–2505, August 2005. [4] S. Shahparnia, O. M. Ramahi, “A simple and effective model for electromagnetic bandgap structures embedded in printed circuit boards”, IEEE Microwave and Wireless Comp. Letters, vol. 15, no. 10, pp. 621–623, October 2005. [5] J. Qin and O. M. Ramahi, “Ultra-Wideband Mitigation of Simultaneous Switching Noise Using Novel Planar Electromagnetic Bandgap Structures”, IEEE Microwave and Wireless Components Letters, Vol. 16, No. 9, pp. 487-489, September 2006. [6] G. Chen, K. L. Melde, “Cavity resonance suppression in power delivery systems using electromagnetic band gap structures”, IEEE Trans. on Advanced Packaging, vol.29, No. 1, pag. 21-30, February 2006. (a) [7] A.Ciccomancini Scogna, M.Schauer, “A Novel Electromagnetic Bandgap Structure for SSN Suppression in PWR/GND plane pairs”, Proceedings of IEEE International Symo. ECTC, Arizona, USA, May 2007. [8] A. Ciccomancini Scogna, “Noise suppression in high speed digital circuits by means of a novel EBG structure with triangle patches and hexagonal arrays”, Proceedings of IEEE International Symp. on EMC, Hawaii, USA., July 2007. [9] Jie Qin, Omar M. Ramahi, and Victor Granatstein, “Novel Planar Electromagnetic Bandgap Structures for Mitigation of Switching Noise and EMI Reduction in High-Speed Circuits”, IEEE Transaction on Electromagnetic Compatibility, vol. 49, no. 3, pag. 661-669 August 2007. [10] T. Weiland, “A Discretization Method for the Solution of Maxwell’s Equation for Six Component Fields”, Electronics and communication, (AEÜ), Vol.31, (1977). TM (b) [11] CST Studio Suite 2008 , www.cst.com [12] J. Choi, V. Govind, M. Swaminathan et Al., “Noise Suppression and Fig. 12 – Surface current distribution at 6GHz for case with Isolation in Mixed-Signal System using Alternating Impedance Electromagnetic Bandgap (Al-EBG) Structure”, accepted for continuous reference plane, a): DIFF signal and b): SE publication on IEEE Transaction on Electromagnetic Compatibility. signals.