Reduction of Far-End Crosstalk on Coupled Microstrip PCB Interconnect

Size: px

Start display at page:

Download "Reduction of Far-End Crosstalk on Coupled Microstrip PCB Interconnect"

Antonia Bates
5 years ago
Views:

1 anuscript for the IEEE Instrumentation and easurement Technoogy onference, ay 10-12, 1994, Shizuoka, Japan, paper WEA 2-4 Reduction of Far-End rosstak on ouped icrostrip PB Interconnect Bertaan Eged, Assistant Professor (*) Ferenc ernyei, IEEE (**) István Novák, Associate Professor, SIEEE (*) Péter Bajor, Postgraduate Student (*) (*) Department of icrowave Teecommunications, Technica University of Budapest 1111 Budapest, Godmann tér 3, HUNGARY Phone/FAX: , E-mai:...NOV.HT.BE.HU (**) TERATE, orp. R&D Dept. T, Sect Nakacho, ushasio-shi, TOKYO, 180 JAPAN Phone: , FAX: , E-mai: Abstract - rosstak among interconnects and PB traces is a major imiting of signa quaity in highspeed digita equipment. The paper evauates a possibe way of far-end crosstak reduction with a specia geometry of traces. This paper shows that increasing capacitive couping aong couped surface microstrip traces may be used to reduce far-end crosstak. I. INTRODUTION With umped representation, n couped transmission ines (n+1 conductors) are described by,, R and G matrices of size of n x n. Two ossess, symmetrica, reciproca transmission ines are described by = = = = (1) where and are the mutua inductance, and capacitance, respectivey. This structure may be described by its eigenvaues, even and odd mode characteristics. The wave can be decomposed to an even-mode and odd-mode wave. Each wave has its own characteristic impedance, and propagation constant. The even-mode and odd-mode characteristic impedances and phase constants are: Z Z oe oo = = + + tpde = ( + )( ) (2) tpdo = ( )( + ) (3) We can define the reative capacitive (c c ) and inductive (c m ) couping as c c = c m = (4)

2 anuscript for the IEEE Instrumentation and easurement Technoogy onference, ay 10-12, 1994, Shizuoka, Japan, paper WEA 2-4 Rearranging t pde and t pdo, we can obtain the foowing expressions: tpde = ( + )( ) = ( 1+ cm)( 1 cc) (5) tpdo = ( )( + ) = ( 1 cm)( 1 + cc) (6) In homogeneous medium the capacitive and inductive coupings are equa, c c = c m = c, therefore the two modes propagate with the same speed: t pde = t pdo. If the two modes propagate with the same speed, no farend crosstak wi appear with matched terminations. This is the case of couped pair of panar stripines. In surface microstrip, however, due to the inhomogenity of the medium, t pde and t pdo are different. In surface microstrips, since the dieectric fis ony the voume beow the strips, the capacitive couping is weaker than inductive couping: c c <c m. In the time domain, this resuts in far-end crosstak waveform with an opposite sope with respect to the main edge [1], [2]. To reduce far-end crosstak on microstrip structures, we shoud increase the capacitive couping. We can do it in different ways. In some cases, ike backpanes, there are discrete oading and mutua capacitances on the ines. We can increase the couping with these additiona discrete capacitances [3]. Another way is to buid more ayers with different reative permittivities with the highest permittivity materia being cosest to the ines [4], [5]. This structure increases the couping whie the main capacitances remain neary unchanged. Another method is to use overay ayer above the strips to increase couping [6], [8]. A of the above methods have a disadvantage that they require additiona eements and/or technoogica steps. The method described in this paper wi show a technique, which requires ony the modification of the ayout of the ines. II. OUPED IROSTRIPS A frequenty used trace structure on high-speed ST PBs and mutichip modues is the microstrip configuration. As mentioned earier, this couped structure can be modeed by its and matrices. Our aim is to increase the capacitive couping, so ets ook at the capacitances of a couped pair of microstrip ines; [9], [10]. w mfa m t o mfd of of h of o of Side view Top view Fig. 1.: ouped pair of microstrip ines. The sef and mutua capacitance of the structure: w = ε 0 ε r K c 1 (7) h m t = ε 0 ε r Kc2 (8) s

3 anuscript for the IEEE Instrumentation and easurement Technoogy onference, ay 10-12, 1994, Shizuoka, Japan, paper WEA 2-4 where K c1 and K c2 are modeing the fringing effect on the strips ( of, mfd, mfa ). For crosstak cacuations, we are interested in the mutua capacitance. The mutua capacitance between ines can be cacuated as a function of meta thickness, gap and the correction for fringing capacitances. The main capacitance of the ines can be cacuated as a function of substrate height, ine width and a correction for fringing capacitances. III. OPENSATION ETHOD In the compensated structure we wi increase the ength of the gap without increasing the ength of couped section. Fig. 2.: Increasing mutua capacitance with gap profie function. The new mutua capacitance is: m t = ε 0 ε r Kc2 (9) s The Increasing Factor (IF) of the mutua capacitance is equa to: IF m = = (10) m The main capacitances of the ines remain neary constant because the surface areas of the ines were not changed. The fringing capacitances, on the other hand, wi increase because the contours of the ines become onger. At first we approximate the Increasing Factor (IF) by the ine integra of the couping profie. For the sake of simpe computation, a sinusoida profie was chosen so that the ine integra and increasing factor are: = + [ f ( x) ] T 1+ IF = T 2 1 dx (11) [ f ( x) ] T 2 dx (12)

4 anuscript for the IEEE Instrumentation and easurement Technoogy onference, ay 10-12, 1994, Shizuoka, Japan, paper WEA 2-4 It means that the new mutua capacitance is equa the capacitance of a panar capacitor with ength, height t and gap s. However, the actua capacitance wi be higher, because the eectric fied in the gap wi take the shortest path. We can mode this effect with a correction factor (K p ), which depends on the gap size, gap profie function and its ampitude. IF = K p IF (13) Fig. 3. Eectric fied in the gap. The predicted Increasing Factor as a function of profie ampitude is shown in Figure Increasing Factor [-] A/T [-] Fig. 4. Increasing Factor (IF) computed from ine integra of the gap profie function. IV. DEVIE UNDER TEST For measurement purposes ten DUTs were buit on epoxi fibregass (FR4) materia with copper-etching technoogy with h = 1.6mm, t = mm and ε r = 4.3. The tota ength was = 200 mm, the width before patterning was w = 2.5 mm and the gap was s =.5 mm. The sinusoida function was chosen as a gap profie with a period of T = 2.5mm. On the ten different sampes the reative ampitude of the profie function (A/T) varied in increments of (A = mm, A/T = ), see Figure 5.

5 anuscript for the IEEE Instrumentation and easurement Technoogy onference, ay 10-12, 1994, Shizuoka, Japan, paper WEA 2-4 Fig. 5. Top view of the device under test (DUT) used to measure the and matrices and time- and frequency domain far-end crosstak. V. EASURED AND SIUATED RESUTS First the ow frequency capacitance and inductance matrices were measured with an HP4285A precision R meter. The vaues are shown in Figure 6. A/T [F/mm] [F/mm] [H/mm] [H/mm] E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E-11 Fig. 6. easured ow frequency static capacitances and inductances of the DUTs with different A/T vaues. From these vaues we can cacuate the capacitive and inductive coupings and the rea vaue of Increasing Factor. The capacitances and inductances were measured on a 200 mm ong DUT and the vaues were normaized to one mm ength. We can cacuate the capacitive (c c ) and inductive (c m ) couping factors with Equation (4). If we compare the two couping factors, we wi find the particuar gap profie ampitude, which gives equa capacitive and magnetic coupings, hence this wi be the optimum compensation. Figure 7 shows the computed and measured Increasing Factor vaues for the A/T range covered by the ten DUTs. For the same A/T vaues Figure 8 shows the capacitive couping and inductive couping vaues.

6 anuscript for the IEEE Instrumentation and easurement Technoogy onference, ay 10-12, 1994, Shizuoka, Japan, paper WEA easured and cacuated increasing factor easured (IF ) acuated (IF) A/T Fig. 7. ouping increasing factor cacuated from ine integra of gap profie function (IF), and cacuated from measured capacitances (IF). The rea couping increasing is higher than a cacuated from ine integra. 20 apacitive and inductive couping ratio [%] apacitive couping 10 Inductive couping A/T Fig. 8. acuated capacitive and inductive coupings. The two coupings are equa at A/T = 0.3 we can expect the maximay far-end crosstak canceation with this profie function. With the measured capacitance and inductance matrices we can simuate the far-end crosstak on the ines. The cacuation shows that the A/T = 0.3 reative gap profie ampitude is the best case for far-end crosstak compensation. The simuation was done with icrosim Pspice. The simuation resuts show that the A/T = 0.3 vaue causes a itte overcompensation, whie the A/T = 0.25 vaue is the best case. Figure 9 shows the simuated far-end crosstak for three different gap profies, assuming 0.1 ns input rise time and 1V source magnitude.

7 anuscript for the IEEE Instrumentation and easurement Technoogy onference, ay 10-12, 1994, Shizuoka, Japan, paper WEA 2-4 Far-end crosstak magnitude 400mV Overcompensated A/T = V -400mV No compensation A/T = 0 Optimum compensation A/T = 0.3 0s 1.0ns 2.0ns 3.0ns 4.0ns Time Fig. 9. Simuated time-domain far-end crosstak with three different gap profie ampitude. The DUTs were measured with an HP 8750 vector network anayzer with time-domain option. Figure 10 shows the measured time domain far-end crosstak. H2 S21 Re 100mU/ REF 0 U Overcompensated A/T = 0.45 Optimum compensation A/T = 0.25 No compensation A/T = 0 START 0s STOP 4ns Fig. 10. easured time-domain far-end crosstak. With the same network anayzer the far-end crosstak was aso measured in the frequency domain. Figure 11 shows the far-end crosstak in the uncompensated case and on the optimay compensated ines. The magnitude response of an overcompensated DUT was neary equa to magnitude response of the uncompensated DUT.

8 anuscript for the IEEE Instrumentation and easurement Technoogy onference, ay 10-12, 1994, Shizuoka, Japan, paper WEA 2-4 H1 S21 og AG 5 db/ REF 0 db No compensation A/T = 0 Optimum compensation A/T = 0.25 START.20GHz STOP 20 GHz Fig. 11. easured frequency-domain far-end crosstak. VI. ONUSIONS On couped microstrip structures the capacitive couping is weaker then magnetic couping. This effect generates arge far-end crosstak. This crosstak can be compensated with a modified trace geometry. The crosstak reduction depends on the gap profie function parameters. With the actua dimensions used for the series of DUTs, it was possibe to span a fu range from undercompensation, optimum compensation, and aso overcompensation. AKNOWEDGEENT The authors express their thanks to r. ajos Kimes and Peter Rakita for their carefu construction of the DUTs. The project has been party funded by the Nationa ommittee for Technica Deveopment (OFB) in connection with the OST 229 project, the Nationa Found for Scientific Research (OTKA) under project T and by the Found for the Hungarian Science. REFERENES [1] A. R. Djordjevic, T. K. Sarkar, R. F. Harrington, "Time-Domain Response of uticonductor Transmission ines", Proceedings of IEEE, Vo. 75, No. 6., June 1987 pp [2] I. Novak, B. Eged,. Hatvani, "easurement by Vector-Network-Anayzer and Simuation of rosstak Reduction on Printed ircuit Boards with Additiona enter Traces", IEEE Instrumentation and easurement and Technoogy onference, ay 18-20, 1993, Irvine A, onference Procedings pp [3] I. Novak, B. Eged,. Hatvani, "easurement and omputer Simuation of Discrete Discontinuities Aong ouped PB Traces", IEEE Instrumentation and easurement and Technoogy onference, ay 18-20, 1993, Irvine A, onference Procedings pp d. [4] J. P. K. Gib,. A. Baanis, "Asymetric, uti-onductor ow-ouping Structures for High Speed, High-Density Digita Interconnects", IEEE Trans. on icrowave Theory and Techniques, December 1991, pp [5] J. P. K. Gib,. A. Baanis, "ouping Reduction in High-Speed, High-Density Digita Interconnects with Substrate ompensation", IEEE 1992, X/92 pp [6] R. R. Weirather,"A Sma I ouper with Good Directivity", IEEE Trans. on icrowave Theory and Techniques, January 1974, pp [8] B. Sheeg, B. F. Spiemann, " Broadband (7...18GHz) 10dB overay coupers for I appication" Eectronic etters, No. 8. pp [9]. S. Waker, "apacitance, Inductance and rosstak Anaysis", ARTEH HOUSE, 1990, ISBN [10] T.. Edwards, "Foundations for icrostrip ircuit Design", John Wiey & Sons td., 1981, ISBN

CSE464 Coupling Calculations via Odd/Even Modes Spring, David M. Zar (Based on notes by Fred U. Rosenberger)

CSE464 Coupling Calculations via Odd/Even Modes Spring, David M. Zar (Based on notes by Fred U. Rosenberger) SE464 ouping acuations ia Odd/Een Modes Spring, 009 aid M. Zar (Based on notes by Fred U. Rosenberger) dzar@wust.edu - aid M. Zar - 3/4/009 ouped Lines Here are two couped ines I I ( )/ I ( I )/ I ( Odd,