Trace Termination

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[PWB Materials] [Signal Reflection Calculation]
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Signal Reflections

Effects of Trace Voltage Reflections

Ripple frequency is based on the trip delay from receiver to source. Ripple amplitude is based on the difference between load impedance and Trace impedance.
Proper termination of a trace results in no reflections on the line [as shown in the first signal pulse]. If the trace is unterminated or terminated with a resistor value that does not match the trace impedance than a reflection will occur [shown in the next two signal pulses]. The period or frequency of the ripple is based on the trace length. The shorter the trace, the higher the ripple frequency. As the trace gets longer the trip delay increases and lowers the ripple frequency. The amplitude of the ripple [reflection] is based on the difference between the trace and load impedance. The larger the difference between the two impedances, the larger the ripple amplitude.

The ripple frequency produced by an FR4 PWB [~150pS per inch of propagation delay] with a 2 inch trace will be 666MHz, or 1 / [2 inch x 150ps x 5 trace times]. The 5 trace times = the first trace trip + 2 additional round trip delays.

Signal Frequency vs. Ripple Frequency Reflections

The voltage reflection is based only on the rise time of the signal and is caused by the impedance mismatch [ripple amplitude], and the trace length [ripple period]. The frequency of the signal has nothing to do with the reflection, except that it appears worse at high signal frequencies. The reflection hasn't changed with signal frequency. So if the rise time of the signal is fixed at 1nS [for example], and we change the frequency of the signal from 2.5Mz to 10MHz. We see the period of the signal shorten, but the reflection remains unchanged in both frequency and amplitude ~ it just consumes more of the signal.

No matter the clock or data frequency the design uses, the Effective Operating Frequency of a circuit, or trace is: Signal Frequency [GHz] = [0.35] / [Signal Transition Time {nSec}]. For signal Transition time, use the smaller value of Tr [Rise Time] or Tf [Fall Time]. For example: a circuit that uses a signal period of 50MHz with a 1.1nS rise time [or fall time] has an Effective Operating Frequency of 318MHz ~ which is far above the actual operating frequency [Period] of the signal. The FreqKnee = 0.5/Tr, the frequency at which most of the energy resides below.

PWB traces [or cables] should be terminated when the trace length exceeds the following: Length > tr / [ 2 x t pr ]
For a general approximation this page uses: 150ps/inch for FR4 [Board Material], and 130pS/inch for Polyimide [Board Material]. For example, using FR4 [150ps/inch] a trace with a 1.1nS rise time would need to be terminated if it exceeded 3.3 inches. The four main ways to terminate a signal trace are shown below.

Design note: A Printed Wiring Board trace very low resistance, these pages deal with PWB trace impedance. The resistance of a Printed Wiring Board trace has more to do with voltage drop over the signal trace and nothing to do with signal reflections. The terms Printed Wiring Board, or PWB, and Printed Circuit Card, or PCC, and Circuit Card Assembly, CCA are used Interchangeably. Also the information provided works for terminating a cable in addition to a board trace. Unused IC input pins which require a Resistor pull-up are not discussed on this page.

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Modified 3/05/12
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