PHYSICAL DESIGN AND DEBUGGING
the transmission line from the source. The terminating' impedance combines with' the destination input circuitry to produce a load that closely matches the characteristic impedance of the line (board traces have characteristic impedances in the range of 30 ohms to 200 ohms).
The calculation of characteristic impedance was already discussed. Impedance of the printed circuit board backplane connectors have the impedance in the same range as the traces (Le., 30 to 200 ohms)..
The characteristic impedance of a backplane may change, depending upon the length of the conductors or when using twisted pairs of
11.4.2.1.1 Need for Termination
The transmission line should be terminated when the tpd exceeds
Termination thus eliminates impedance mismatches, increases noise immunity, sup- presses RFIIEMI, and helps to ensure that signals reach their destination with minimum of distortion. There are five methods for terminating traces on the board:
1.Series
2.Parallel
3.Thevenin
4.AC.
5. Active
Terminations can be costly, because they require additional components and power. With passive terminations, extra drivers are needed to deliver more current to the line. With active terminations extra power is needed which increases the power dissipation of the system.
11.4.2.1.2 Series Termination
One way of controlling ringing on longer lines is with the series termination technique also known as damping. This is accomplished by placing a resistor in series with the transmission line at the driving device end. The receiver has no termination. The value of the impedance looking into the driving device (Rdriver + Riine = Zo) should approx- imate the impedance of the. line as closely as possible. In this circuit the ringing dampens out when the reflection coefficient goes to zero. Figure
