Convert line length to delay time
Pattern length to propagation delay conversion Reverse calculation of delay time
Inner layer (stripline): TD = L × √εr / c [ps]
Outer layer (microstrip): TD = L × √εeff / c [ps]
εeff = (εr+1)/2 + (εr-1)/2 × (1+10h/w)^-0.5
Outer layer (microstrip): TD = L × √εeff / c [ps]
εeff = (εr+1)/2 + (εr-1)/2 × (1+10h/w)^-0.5
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Calculates propagation delay from trace length. This is the basis of length matching and skew control on high-speed interfaces.
DDR-SDRAM example:
• DDR4-3200: one bit period ≈ 313 ps
• Skew budget ≈ ±20 ps, which is about ±1.4 mm of trace length
Working out the allowable length error from the delay budget before setting up length matching constraints in your PCB CAD tool makes the layout go much more smoothly.