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Propagation Delay
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Signal Velocity
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Delay per Inch
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Effective Dielectric Constant
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Signal propagation delay is the time it takes for an electrical signal to travel from one point to another along a PCB trace or transmission line. It's determined by the trace length and the speed at which signals travel through the dielectric material.
Propagation delay is calculated using the formula: Delay = Distance / Signal Velocity. The signal velocity depends on the dielectric constant of the PCB material and trace structure (microstrip or stripline).
Propagation delay is critical for high-speed digital circuits because timing mismatches between signals can cause setup and hold violations, signal integrity issues, and circuit malfunction. Managing delay is essential for synchronous systems.
Signal velocity is primarily affected by the dielectric constant of the PCB material and the trace structure. Lower dielectric constants result in faster signal propagation, while higher values slow down the signal.
Microstrip traces (on surface layers) typically have slightly higher signal velocity than stripline traces (between ground planes) due to the air interface reducing the effective dielectric constant.
Theoretical calculations provide good estimates, but actual delays can vary by ±10-15% due to manufacturing tolerances, trace geometry variations, and temperature effects. For critical timing, measurement verification is recommended.
Air and air-spaced dielectrics have the lowest propagation delay due to their low dielectric constants. PTFE-based materials also offer good high-speed performance with relatively low delay compared to standard FR-4.