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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. |
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. |
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.
PWB Info | Ground/Power Planes | Component Chip Sizes | Capacitor IC By-Pass Info | Resistor Pull-Up equations |
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