Stub discontinuity and crosstalk causes

How a stub causes impedance mismatch

A stub is a short branch line connected to the main transmission line (for example a trace to a test point or a second receiver). At the junction, the signal travelling along the main line sees the continuing line and the stub in parallel. The effective impedance at the junction drops (for equal lines, Z₀ ∥ Z₀ = Z₀/2). This is a sudden change in impedance, so part of the wave is reflected (Γ = (Z₂ - Z₁)/(Z₂ + Z₁)). The stub itself is an open-ended line: the wave travels to its end, reflects back with the same polarity and returns after twice the stub delay, adding ringing and distortion to the main signal. The longer the stub, the larger the delay and effect; stub length should be less than about 1/10 of the rise-time length.

 Source ======+=============== Load
              |
              | stub (open end)
              |
 Z drops at the junction -> reflection

Solution: route as a daisy chain (point to point), keep stubs very short, or use terminations.

Causes of crosstalk

1. Mutual (stray) capacitance between adjacent traces: coupled current I = C·dV/dt.
2. Mutual inductance between adjacent loops: V = M·dI/dt.
3. Long parallel runs and small spacing between traces.
4. Fast edges (high dV/dt and dI/dt).
5. Shared or poor return paths (common ground impedance), high-impedance victim lines, and absence of a ground plane.
6. Higher signal amplitude, and routing on the same layer or adjacent layers over a long distance.