5.1 Reflected Signal
If a trace is not properly terminated (terminal matching), the signal pulse from the driving end will reflect at the receiving end, leading to unexpected effects and distortion of the signal profile. Significant distortion can cause various errors and ultimately result in design failure. Additionally, distorted signals become more susceptible to noise, further increasing the risk of design failure. Inadequate consideration of these factors can significantly increase EMI, affecting both the design results and the overall system. The primary causes of reflected signals include overly long traces, improper termination, excessive capacitance or inductance, and impedance mismatches.
5.2 Delay and Timing Errors
Signal delay and timing errors manifest as periods when the signal fails to transition between high and low logic levels. Excessive signal delay can lead to timing errors and disrupt device functionality.
**5.3 Multiple Logic Level Threshold Crossings**
Errors can occur when a signal crosses the logic level threshold multiple times during its transition. This type of error is essentially a form of signal oscillation, where the signal oscillates near the logic level threshold, causing multiple threshold crossings and resulting in logic function disorder. Causes of such reflected signals include long traces, unterminated transmission lines, excessive capacitance or inductance, and impedance mismatches.
**5.4 Overshoot and Undershoot**
Overshoot and undershoot occur due to two primary reasons: long trace lengths or rapid signal changes. While most components are protected by input protection diodes, overshoot levels can sometimes exceed the component’s power supply voltage range, potentially damaging the components.
**5.5 Crosstalk**
Crosstalk happens when a signal on one trace induces a signal on an adjacent trace on the PCB. The degree of crosstalk is influenced by the proximity of the signal line to the ground plane, the line spacing, and the nature of the signals involved. Asynchronous and clock signals are particularly susceptible to crosstalk. To mitigate crosstalk, techniques include shielding heavily interfered signals or rerouting them to minimize interference.
**5.6 Electromagnetic Radiation**
Electromagnetic Interference (EMI) refers to the electromagnetic disturbances that can affect nearby electronic devices. EMI occurs when a digital system, powered on, emits electromagnetic waves that interfere with surrounding electronic equipment. This issue is often caused by high operating frequencies and poor layout design. While EMI simulation tools are available, they are costly and can be challenging to configure, which impacts the accuracy and utility of the results. The most effective approach is to adhere to design rules aimed at controlling EMI, integrating these rules throughout every aspect of the PCB design process.
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