Having covered the impact of attenuation and reflections on link bandwidth, we can now examine the role crosstalk has in potentially diminishing throughput. Crosstalk impacts signals by coupling undesired energy into victim lines reducing noise margins and possibly inducing false switching on quiet lines.
Crosstalk between results from mutual capacitance and inductance as illustrated in Figure 1.
Figure 1: Mutual capacitance and inductance between traces.
There are several steps that can be taken to reduce crosstalk.
1. Increase separation between traces. Typical 50-ohm striplines routed with equal width traces and spaces can result in > 10 percent single active crosstalk at Tr = 200 ps.
2. Decrease board dielectric constant. This will reduce capacitive coupling between lines.
3. Reduce the length at which traces run parallel. Less opportunity for crosstalk.
4. Increase the input pulse risetime. Far end crosstalk varies with risetime as does near end crosstalk to the point of saturation.
5. Match termination impedance to line to minimize reflections. In a poorly terminated system near end noise can be reflected and become a far end problem as well
6. Increase separation between groups of signals with differing voltage swings. A 3.3V LVTTL signal will couple 330mV into a neighboring 1.5V HSTL signal at 10% crosstalk dramatically reducing margins.
7. Control current loops. Inductance is a phenomenon of current loops. When signals share common return paths as in the ribbon cable in figure 2, overlapping current loops increase mutual inductance. Additional grounds will reduce this.
8. Proper connector selection. Many low crosstalk connectors are available with low inductance ground paths. Pay careful attention to the pin field routing as there is little impedance control in this region and it can be susceptible to crosstalk.
Figure 2: Overlapping current loops in ribbon cable increase crosstalk..
When currents share a common ground path, the inductance of that path can generate noise on a fairly remote signal, absent of any direct near end or far end crosstalk. This effect is referred to as common mode impedance noise or ground bounce. Currents on the return path can cause the reference level to vary, generating a "signal" on an otherwise quiet line. The waveforms in Figure 3 show noise generated in a package from a single 2.5V, 100 ps. edge. Pad 2 is the most proximate signal, and shows evidence of near end crosstalk. Pads 5 and 9 show 190 mV of common mode impedance noise with no evidence of direct crosstalk. To remedy this, enough ground and power paths through the package must be supplied to adequately reduce the inductance.
Figure 3: Common mode impedance noise in die package.
Reclaiming signal bandwidth begins early in the design process. While passive and active equalization techniques can be applied to manage attenuation resulting from conductor and dielectric losses encountered within the net, the lion's share of the signal integrity effort will be applied to minimizing the impact of reflections, crosstalk and signal skew. With enough money and silicon, many of these impacts can be managed with equalization circuitry as well, however the solution may not be cost effective.
Effort spent on the selection of well designed connectors and packages can payoff with immediate bandwidth improvement. The area that is somewhat out of the hands of the vendors of these components is the via region that escapes the components. This requires careful signal routing and pin out selection by the system designer. Meticulous management of signals and their return paths will produce benefits in terms of the reduction of crosstalk and ground bounce between the nets.
Sound engineering practices coupled with extensive simulation on the front end of the design process will produce superior bandwidth performance in the most cost effective manner. Changes in the SPICE deck are generally much easier to implement than board and package respins.
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Dally, William J. and Poulton, John W., Digital Systems Engineering, Cambridge University Press, Cambridge, UK, 1998, pp 267 " 280.
Elco, Richard A., PhD, "Transmission Line and Interconnect Seminar Notes", Interactive Products Corporation Training Seminar, Camp Hill, Pa., January 2003.
Lemke, T. A., "Microstrip Traces vs. Stripline Traces in Backplanes", Broadside Technology Application Note, November 2002.