Howard Johnson, PhD - January 5, 2012
Some ideas are just too good to keep to yourself. Figure 1 illustrates the blueprint for a differential connector that radically reduces crosstalk between nearest-neighbor pairs. This cross-sectional view of a nine-pair connector shows the arrangement of 18 flat pins as they pass through the body of a connector. Each pair of pins comprises a broadside configuration, meaning that the broad sides of the two pins face each other.
Many differential connectors use broadside coupling. The Molex I-Trac, for example, uses broadside-coupled differential pairs with no additional solid reference layers. If a user requires additional grounds, the grounds are assigned to ordinary data-pin locations.
Other differential connectors use edgecoupled pins. For example, the Ernie ERmet ZD uses edge-coupled pairs with solid-plane layers between rows of pins. The FCI Airmax VS uses edge-coupled pairs with no solid reference layers.
In the Molex, Ernie, and FCI designs, the pairs appear in a regular array, with all of the pairs arranged in the same orientation. The unique aspect of the connector in Figure 1 is not the use of broadside coupling but the alternating 90° rotation of every other pair. The orientation of Pair H, for example, is rotated 90° with respect to the orientation of Pair G. In this quad-connector layout, each pair is rotated into a position that stands in quadrature to its nearest neighbors.
The quadrature alignment dramatically reduces crosstalk. To see why, examine the curved lines in Figure 1. They represent magnetic lines of force that the central pair creates when they are operated in a differential mode. In a cross-sectional diagram such as this one, the crosstalk a victim pair receives varies in proportion to the number of magnetic lines of force that pass between the two elements of that victim pair, crossing perpendicular to the axis connecting the two elements. A line passing parallel to the axis connecting the two elements causes no crosstalk (Reference 1).
Consider victim Pair H, which uses a side-by-side layout. Field Line Y passes straight from one element to the other, parallel to their axes, without passing between the two of them. That orientation eliminates unwanted crosstalk from Field Line Y. If Pair H had used an over-and-under orientation, as the central pair does, Line Y would have penetrated the space between the elements of H in a perpendicular direction, causing crosstalk. That arrangement is the usual one in a connector and one of the main drawbacks of a uniformly oriented pin array.
The benefits of exact quadrature layout also accrue to pairs B, D, and F, which each pick up zero differential crosstalk from the central pair. The arrangement in Figure 1 suppresses crosstalk from the central pair to each of its next four nearest neighbors, A, C, G, and I, as an ordinary connector would.
A practical quad connector must solve the problems of how to twist the pins to obtain the correct alignment and how to maintain a consistent quadrature alignment moving all the way down through the PCB attachment into the underlying PCB.
If these problems prove tractable, a practical low-crosstalk connector would permit the design of serial links with larger link-loss budgets than is now possible, enabling the use of multilevel signaling, which greatly boosts the effective digital bandwidth.
My last article, “Quadrature-via layout,” (Reference 1), discusses how, in the presence of a magnetic field, you can always orient a differential pair so that it picks up no differential crosstalk. The proof is simple. While stimulating any aggressor, pick one victim pair. Looking at a cross-sectional view of the differential-pair layout, grab the victim pair and slowly pinwheel that pair around its center of gravity. The pinwheeling motion does not change the shape of the victim signal, but it does changes its amplitude.
Suppose that, in your starting position the victim signal has amplitude +A. If you rotate the pair through a full 180°, effectively interchanging the positive and negative elements of that pair, the crosstalk signal inverts, producing amplitude –A. Mathematical continuity asserts that, if one orientation produces amplitude +A and another orientation produces –A, then there must exist some intermediate orientation at which the received amplitude passes precisely through zero.
Howard Johnson, PhD, of Signal Consulting, frequently conducts technical workshops for digital engineers at Oxford University and other sites worldwide. Visit his Web site at www.sigcon.com, or e-mail him at firstname.lastname@example.org.
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