Standard Technical Engineering Terms
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Editor note; The alphabetic links above will switch to the engineering dictionary, while the definitions related to Printed Wiring Boards are below.
Trace Reflections
Reflections: Signal reflections on the Printed Circuit Board [PCB] trace are caused by a number of conditions; including mis-terminating a trace and discontinuities on the trace. Avoid discontinuities on PCB traces, as the differences will change the impedance of the trace and cause reflections.
Trace discontinuities are caused any time a trace changes direction, changes shape or interfaces to a component. In fact any time a trace changes direction the shape of the trace has to change. Sharp bends in the trace changes the cross-sectional area of the trace, increasing at the bend, lowering the impedance [Zo]. The bend in the trace, or the actual point of the bend is a change in both trace shape and trace width. The sharper the bend the greater amount of discontinuity. Of course there is no way to avoid bending a trace and still route a PCB. However to reduce the impedance change the trace may be shaved at the corner to reduce the sharp bend.
Sometimes a trace is required to be necked down or reduced in width to fit between two adjacent vias or between other traces. Changing the width also produces a change in the impedance. Normally these changes are also unavoidable, but reducing the length of the change reduces the overall negative effect.
As soon as a trace encounters a surface mount pad or via for component mounting the trace dimensions are changed which then effects the impedance. Because components are required on a PCB, hitting a pad is unavoidable; however the point is not to run a trace through a pad unless the pad acts as the termination point for the signal.
Vias; Are vertical signal runs between board layers and represents an uncontrolled impedance. The impedance is uncontrolled because as the trace moves between layers there is no reference ground, just random near by traces. The via or trace is not routed over a ground plane. In fact as the signal moves between layers the impedance changes for each layer it passes through. A via may also be encountered when a trace arrives at a through-hole component, in which case the signal may not need to traverse the via but instead sees a discontinuity caused by the change in shape of the trace.
Layer changes; Moving from an outside layer {stripline} to an inside layer {microstrip} has the same effect on impedance as passing through a via. Of course routing to different layers is very common and a most for dense boards. The point is to avoid changing board layers when possible, and when a layer change is required the fewer layers the signal moves through the better, keeping the length of impedance mismatch short.
Signal Stubs; Should be Avoided on any high speed signal traveling on a wire or copper trace. Stubs on the signal lines are really traces that require a termination resistor.
Signal Termination; Any high speed trace needs to be terminated at the destination [far-end] by a resistor that represents the impedance of the line. Termination of traces will reduce or eliminate reflections from the 'far-end' IC; see Trace Termination Schemes for the primary reason.
Additional Digital Logic Pitfalls.
