The length of the terminators is another possible consideration when performing length matching. The actual length of the resistor is irrelevant, however, compared with the timing effect of the actual resistor value.
Figure 4 illustrates a simple schematic with a series terminator. The resistor value was varied from 10 ohms to 75 ohms, and the difference in timing between those two cases was 173 ps. That equates to well over an inch on a pc board, a measure that dwarfs the physical length of the resistor itself.
The bottom line is that timing on a board is about much more than just the length of the traces. Length is only part of the flight time calculation and hence the system timing on the board. Impedance and loading also need to be taken into account when determining board timing.
So when it comes to looking at “lengths” on the pc board, it is important to keep in mind that length only really matters in the context of timing. That understanding, coupled with information gleaned from analysis, will help determine the right trade-offs when considering length constraints on a pc board design.
About the author
Patrick Carrier is technical marketing engineer for high-speed pc board analysis tools at Mentor Graphics Corp.