Behavior of Short Pulses on Tightly Coupled Microstrip Lines and Reduction of Crosstalk by Using Overlying Dielectric

Abstract

Forward crosstalk can be a problem in the design of printed circuit boards. We explore two aspects of forward crosstalk in tightly coupled microstrip lines. First, we demonstrate, as far as we are aware, the first direct experimental verification of different velocities of propagation in tightly coupled microstrip lines. (These are the even and odd modes for symmetric lines.) Second, we verify by modeling and by experiments the reduction in forward crosstalk when microstrip lines are covered by the proper overlying thickness of dielectric and the simultaneous reduction in differences in modal velocities. In 1981, Anderson introduced a clever means to reduce forward crosstalk in microstrip lines to zero by burying the lines under a relatively thin dielectric layer of an appropriate thickness. At this critical overlayer thickness, the capacitive and inductive crosstalk components are equal even for inhomogeneous media. Anderson's work is not well known to most packaging engineers because it appeared only in the patent literature and in the Review of Scientific Instruments, an excellent journal, but not one usually read by the packaging community. We extend Anderson's results by demonstrating that at the critical thickness of the overlying dielectric the velocities of the two modes become equal. We verify that Anderson's argument extends to the case where the lines are so tightly coupled that the modal velocities differ by more than 10%. We also generalize his results to the case where the lines are asymmetric. Experimental results on microstrip lines approximately 61 cm in length confirm the modeling predictions.

Enhanced Microstrip Guard Trace for Ringing Noise Suppression Using a Dielectric Superstrate

“…Solving the eigensolution of [L] [C] yields η 1 = 1.22 for the present example. The value of the first voltage peak magnitude at the near end waveform can be obtained using (6) and (12). Table I lists the predicted value and those found by a HSPICE simulation.…”

“…By increasing the capacitive coupling coefficients to equalize the even-and odd-mode velocities, the FEXT can be eliminated. Some approaches have been proposed, e.g., by employing the stub-alternated microstrip line [4] and depositing the extra dielectric layer over the microstrip substrate [5], [6], but fail to reduce the NEXT.…”

“…The overlying dielectric is intentionally inserted to equalize the phase velocities of different modes, which has been found to be efficient in eliminating the FEXT [5], [6]. It was found to be also effective in suppressing the ringing noise as well, with only two shorting vias placed at both ends [15].…”

The impact of coupled conductors in a dielectric material above an ideally conducting plane on the difference of board delay per unit length of even and odd modes

“…For example, coupled lines, immersed into a single dielectric. The peculiarities of their interaction were partially considered in papers (Senthinathan et al, 1987;Muthana and Kroger, 2007). However, an additional study can help discover new regularities in the behavior of these lines' dependencies, in particular, useful modal phenomena, similar to the ones suggested for printed (Gazizov et al, 2012a;b) and cable (Gazizov et al, 2009;Gazizov and Zabolotsky, 2012;Orlov et al, 2013) structures.…”

The Effect of an External Electromagnetic Field on Orthogonal Coupled Microstrip Transmission Lines

“…There are a lot of classical models to analyze simple and parallel coupled multiconductor lines on the same layer or on different layers [5][6][7][8][9][10]. Different parameters of coupled microstrip lines have been studied [20][21][22][23][24][25] and many electronic devices using coupled MTLs have been fabricated with IC technology [26][27][28][29][30][31][32][33][34][35][36][37][38]. Multilayer printed circuit boards are used in several applications to reduce the coupling phenomenon between adjacent layers.…”