Tom
10-23-2002, 09:54 AM
IPC Advanced Study Guide Page Reference: Pages 238 & 239, Section 4.9
For signals routed side-by-side, forward crosstalk requires extremely long parallel runs to couple enough energy to cause circuit malfunction. Even with very tight conductor-to-conductor spacing, parallel runs of 5 to 6 times ‘signal rise distance’ are required before crosstalk becomes a problem. For a 1.0ns rising edge signal, forward crosstalk won’t reach it’s maximum level until approximately 760 mm [30 inches] or more of parallelism. Backward crosstalk reaches it’s maximum with parallel runs as short as ˝ rise distance. This is referred to as crosstalk critical length.
For a 1.0ns signal, backward crosstalk critical length is reached with only 75 mm [3.0"] of parallel routing between the source and receptor (victim) lines. Beyond crosstalk critical length, the amplitude of backward coupling does not increase. The percentage of backward crosstalk energy coupled when parallelism is equal to or greater than critical length is a function of conductor separation verses height above the nearest power or ground plane.
Circuits do not have to be perfect. Some crosstalk is acceptable. To determine what conductor spacing is required to achieve acceptable levels of crosstalk, the PCB designer or the engineer designing the logic circuit should determine what percentage of the circuit’s noise budget can be attributed to crosstalk. In most digital circuits (within ‘like’ logic families), backward coupling of 10% or less is usually acceptable.
To maintain coupling of 10% or under, with conductors sandwiched between planes (Stripline), keep spacing between conductors at least as great as the distance to the nearest power or ground plane. If conductor to plane distance is 0.18 mm [0.007”] the conductor to conductor spacing must be at least 0.18 mm [0.007”].
For signals routed side-by-side, forward crosstalk requires extremely long parallel runs to couple enough energy to cause circuit malfunction. Even with very tight conductor-to-conductor spacing, parallel runs of 5 to 6 times ‘signal rise distance’ are required before crosstalk becomes a problem. For a 1.0ns rising edge signal, forward crosstalk won’t reach it’s maximum level until approximately 760 mm [30 inches] or more of parallelism. Backward crosstalk reaches it’s maximum with parallel runs as short as ˝ rise distance. This is referred to as crosstalk critical length.
For a 1.0ns signal, backward crosstalk critical length is reached with only 75 mm [3.0"] of parallel routing between the source and receptor (victim) lines. Beyond crosstalk critical length, the amplitude of backward coupling does not increase. The percentage of backward crosstalk energy coupled when parallelism is equal to or greater than critical length is a function of conductor separation verses height above the nearest power or ground plane.
Circuits do not have to be perfect. Some crosstalk is acceptable. To determine what conductor spacing is required to achieve acceptable levels of crosstalk, the PCB designer or the engineer designing the logic circuit should determine what percentage of the circuit’s noise budget can be attributed to crosstalk. In most digital circuits (within ‘like’ logic families), backward coupling of 10% or less is usually acceptable.
To maintain coupling of 10% or under, with conductors sandwiched between planes (Stripline), keep spacing between conductors at least as great as the distance to the nearest power or ground plane. If conductor to plane distance is 0.18 mm [0.007”] the conductor to conductor spacing must be at least 0.18 mm [0.007”].