# The Physical Basis of Transmission Lines

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## 7.6 Spatial Extent of the Leading Edge

Every signal has a rise time, RT, usually measured from the 10% to 90% voltage levels. As a signal moves down a transmission line, the leading-edge spreads out on the transmission line and has a spatial extent. If we could freeze time and look at the size of the voltage distribution as it moves out, we would find something like Figure 7-7. Figure 7-7 Spatial extent of the leading edge of the signal as it propagates down a transmission line.

The length of the rise time, Len, on the transmission line depends on the speed of the signal and the rise time:

where:

Len = spatial extent of the rise time, in inches

RT = rise time of the signal, in nsec

v = speed of the signal, in inches/nsec

For example, if the speed is 6 inches/nsec and the rise time is 1 nsec, the spatial extent of the leading edge is 1 nsec × 6 inches/nsec = 6 inches. As the leading edge moves down the circuit board, it is really a 6-inch section of rising voltage moving down the board. A rise time of 0.1 nsec has a spatial extent of 0.6 inch.