High speed digital design fundamentals - Propagation delay in PCBs
Propagation delay
- Propagation delay of signals in conducting wires increases as dielectric constant of medium surrounding the cable increases
- Both trace geometry and type of material used in PCB (Printed circuit board) defines the delay
- The most used FR4 PCBs have the dielectric constant of 4.5
- Delays on outer layer PCBs are always lower than that of inner layers since electric field shares outer air also and dielectric will be between 1 and 4.5
- Popular propagation delay for signal propagating in a PCB is 160 ps/inch
- Circuits with high rising/falling edges are not always lumped.
- Length of rising edge L, in inches is given by ratio of Rise time (ps) to Delay (ps/inch). Circuits smaller than L/6 are lumped circuits
- Treat PCBs with lengthier electrical features as distributed element
