Identificación de los requerimientos

Guard traces are traces at 0V-potential that surround clocks, periodic

signals, differential pairs, or system critical (high-threat) nets from source to destination. Shunt traces are traces located directly above or below a high-threat transmission line that parallels the trace along its entire route. Both guard and shunt traces have unique applications, implementations, and drawbacks. Depending on functional requirements, one or both techniques may be used. It is up to the design engineer to select which technique is required for suppression of RF energy (flux cancellation/minimization).

Guard and shunt traces have no effect in enhancing the signal integrity of the desired signal. If the purpose of using guard traces is to prevent

crosstalk, or magnetic field coupling between adjacent traces, use of the 3-W rule generally provides an adequate flux boundary, circumventing the usefulness of a guard trace.

Shunt traces are used when the sensitivity of a trace is critical, related to crosstalk corruption, or when a known amount of excessive RF energy will be present within a particular transmission line or trace. Routing a trace at 0V-potential parallel, or adjacent, to the signal trace allows for enhanced flux cancellation in differential mode. The shunt trace acts as an image plane and must be three times the width of the signal trace to fully capture flux that surround the trace. If the trace has an image plane on one side, at 0V-potential, and a shunt trace on the other side, also at 0V-potential, a partial coaxial transmission line structure is developed. If guard traces are provided with a shunt trace, a true coaxial transmission line exists. For many applications, implementing the use of guard traces in a stripline topology is a waste of time. A reason for this statement follows and becomes obvious when Fig. 4.23 is examined.

Figure 4.23: Shunt and guard trace configuration.

The RF field created from electromagnetic distribution within a signal trace is approximately 1-W, or one width distance away from the trace, where W is trace width. Since field distribution is uniform around the trace, magnetic flux will be captured by the guard trace, shunt trace, or image plane,

whichever one is physically closest to the trace.

Figure 4.23 is for illustrative purposes. Two examples will describe how and why guard and shunt traces either work or do not work within a stripline configuration.

1. The width of a trace (W) is 0.010 in. (0.25 mm). The distance spacing between traces is also 0.010 in. (0.25 mm), per manufacturing

requirements. If the distance spacing between signal trace and image (reference) plane (H) is 0.008 in. (0.20 mm), magnetic flux will see the image plane long before the guard trace observes this field. With close dimensional spacing, guard traces become useless. This is the primary explanation for why guard traces do not work well for some stripline configurations. If a guard trace is implemented as stripline to remove radiated RF energy developed within the transmission line, and the image planes located both above and below the signal trace already prevent RF energy from radiating to the environment, why use guard traces?

2. A different stackup assembly is now provided, with the same trace width (W) at 0.010 in. (0.25 mm). Separation distance (W) between the signal traces is also the same, at 0.010 in. (0.25 mm). Our new assembly now has a greater distance spacing between signal trace and the image plane (H) at 0.020 in. (0.5 mm). The distance spacing between guard trace and signal trace is now much less than the distance spacing between signal trace and image plane. For this

particular application, the guard trace works as an enhanced alternate return path for the RF energy developed within the transmission line.

The important item to remember when defining mechanical constraints and constructional details of the PCB prior to placement and routing is the

physical dimension between signal trace and image plane, or signal trace and guard or shunt trace. Whichever distance spacing is "closer" to the signal trace, enhanced suppression of RF energy will be achieved.

The primary function of both guard and shunt traces is to provide an alternate return path for RF currents to return to their source, if a solid image plane is not provided as in multilayer assemblies. If a single- or double-sided PCB is used, provisions for RF return currents will not exist in the same manner owing to lack of solid planar structures. If a guard trace is to be used in a multilayer assembly, this trace will provide a RF return path only if placed physically adjacent to the high-threat trace. The distance from signal trace to guard traces must be as close as can be manufactured.

If a two-layer (double-sided) PCB is used, with typical thickness spacing of 0.062 in. (1.6 mm), the physical distance between the signal trace, if routed on the top layer, and the ground plane, located on the bottom layer, is

physically large. For all practical reasons, the RF return path is too far away to provide any significant amount of flux cancellation. For this application, guard traces are the designer's best friend only when routed as close as physically possible to the signal trace.

Guard and shunt traces are used for specific applications only. Applications are product specific and may not be required in most designs. The

advantages and disadvantages of using both guard and shunt traces include the following:

1. To enforce the 3-W rule. When we increase the distance separation between traces, we minimize the amount of crosstalk that might develop between high-threat traces and other nearby components or traces. A guard trace forces the distance spacing between the source and victim trace to be much greater than if the guard trace was not present between the two. In addition, magnetic flux present within the transmission line containing RF energy will be captured by the guard trace, thus preventing crosstalk from occurring.

2. To prevent common-mode RF coupling from a high-threat signal trace to other circuit traces (minimize crosstalk). This application is one layout technique for preventing crosstalk, except instead of coupling magnetic flux between traces, common-mode currents are involved. If we prevent coupling, signal integrity is enhanced.

3. To provide a low-impedance alternative RF return path and to minimize RF common-mode currents that may develop within the transmission line. This is observed more with shunt traces than with guard traces. Chapter 2 discusses the need for RF currents to return to their source through the least amount of impedance. If a direct path is not provided, air becomes the path. Free space has an impedance of 377 ohms. A return path using a guard trace will provide much lower impedance to the RF current than no return path.

4. To create an impedance-controlled, coaxial-based transmission line

for specific nets. This coaxial type of configuration was shown in Fig.

4.23. Shunt traces perform best in multilayer boards (six or more layers). Shunt traces sandwich a high-threat signal trace between two separate reference sources (image plane and shunt trace). The

advantage of using a shunt trace results from the skin effect of the currents flowing on the copper trace. There is no significant current flow inside the center of a trace. Placing a shunt trace directly above or below the signal trace provides for additional magnetic flux

coupling of the trace, observed in Fig. 4.23.

5. To enhance performance of low-technology stackup assignment related to signal integrity. Guard traces are commonly found on single- and double-layer boards (e.g., those without power or ground planes). RF field capture occurs between a signal trace and reference plane through both capacitive and inductive coupling (including

mutual inductance). This coupling removes common-mode currents and minimizes the signal-to-return loop area. Image planes provide tight RF coupling for return currents. Without a solid image plane on a single- or two-layer board, a guard trace provides this RF current return path.

If a shunt trace must be provided in a multilayer board, it must be placed immediately adjacent to the high-threat signal trace. Both ends of the shunt trace are then connected to the ground planes or 0V-reference. The shunt trace should not have voids in them—particularly those caused by vias.

This is applicable "only" when stripline layers are provided between two planar structures. The width of the shunt trace must be at least three times the width of the signal trace. Additional via connections to the ground

planes remove possible standing waves of RF currents projected onto the shunt trace [1]. Implementation of this scheme is rarely feasible and is impossible in many cases since the signal trace is routed on the horizontal axis, while the shunt trace is implemented on the vertical layer. This type of routing makes it nearly impossible to autoroute the PCB.

When using guard traces, the trace must be grounded at both source and destination. This ground connection must be as close as possible to the component. If the routing lengths of both the signal and guard trace are significant, multiple connections to the ground planes by vias, along the edges of the guard trace, are also required. These additional vias break up the resonant effects that occur from this potential "dipole" antenna.

When shunt or guard traces are connected to a 0V-reference, or ground, an interesting phenomenon is observed. An LC resonance can be

developed (L from the trace and C from the distributed capacitance between trace and plane). Depending on the physical distance spacing between ground connections, sharp resonant impedances may be present.

If any harmonic of the clock signal is at this exact resonant frequency, suppression of RF currents is made more difficult because the

transmission line will create significant levels of RF energy. Should this occur, additional ground connections must be installed between

0V-reference and the shunt trace. What we are doing is changing the physical characteristics of the transmission line. The distance spacing of the ground vias should be altered to shift this resonant frequency away from where clock harmonics are anticipated to be observed.

When guard traces are used, the spacing between the guard and signal trace must be minimized to the smallest manufacturable distance. This distance must be maintained throughout the length of the route. Although the capacitive contribution of this spacing is minimal, suppression of RF energy could be significant.

When a guard trace is forced away from a signal trace due to vias or through-hole component leads in the routing path, this trace must be

returned to normal as soon as the detour is cleared. Never locate anything between a signal trace and its guard trace. When two or more periodic signal or clock traces are routed side by side, they may "share" a common guard trace between them for only a short distance (see Fig. 4.24). All effort must be made to prevent routing two traces within the same guard trace if possible. Exceptions do exist, such as differential or paired signals.

Differential pair traces usually do not require use of either guard or shunt traces.

Figure 4.24: Guard trace implementation.

It must be restated here that "guard" traces are primarily effective on one- or two-layer circuit boards. On multilayer stackup assemblies, the flux boundary provided by the 3-W rule will accomplish much of the benefit provided by guard traces, taking up significantly less real estate!

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Table of Contents 4.4: CAPACITIVE LOADING OF SIGNAL TRACES 4.13.7: Differential or Paired Signaling

REFERENCES

Chapter 4 - Clock Circuits, Trace Routing, and Terminations

Printed Circuit Board Design Techniques for EMC Compliance: A Handbook for Designers, Second Edition by Mark I. Montrose

IEEE Press © 2000 Recommend this title?