EMC Component Diagnostic Tools

EMC Component Diagnostic Tools

EMC Component Diagnostic Tools

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A combination of components such as a Heat-Shrink & Cold-Shrink Sleeves in series with a resistor or an inductor in series with a capacitor works extremely well in helping reduce RF energy within a transmission line or PCB trace. The best filter circuit is one that is placed in series. Another excellent filter element is a ferrite bead. A ferrite exhibit primarily resistive characteristics to signal lines. At low frequencies, inductive performance dominates. Ferrite material exhibits very low impedance at a specific frequency of operation. At higher frequencies, ferrite beads exhibit resistive characteristics, and hence they have high (resistive) impedance. The vector angle formed between the resistive and reactive portion of the ferrite is identified as a “tangent delta” and is important only for phase-sensitive circuits. Due to the RL characteristics of the ferrite, the component is a low-Q device and will NOT resonate, unlike normal inductors that contain parasitic capacitance, forming an LC circuit. An LC circuit will exhibit a reactive resonance, which is exactly what we do not want.

How does one place discrete components in series with a trace or pin on a PCB? Cut open the suspected trace with a sharp knife and scrape away solder mask. Place one end of the filter assembly on one side of the cut. Place the other end of the filter combination on the other end of the cut. Using this technique and a combination of Heat-Shrink & Cold-Shrink Sleeves and a resistor and/or capacitor can significantly reduce the RF energy content within a transmission line, providing for quick identification of a suspected problem area. For differential-mode noise, use of a shunt element such as a capacitor may be required. For this situation, scrape away the solder mask from the transmission line and nearest ground reference and connect the capacitor between these two areas.

For bypassing or shunting effects, place an oscilloscope probe tip on the suspected pin or trace and the other end of the probe on the nearest signal ground connection. This type of probe configuration consists of a series inductor/capacitor circuit as mentioned earlier or a single capacitor. The probe can quickly be used to optimize the location for placement of a bypass component. One can also touch a capacitive probe tip along every pin or pin-to-trace to see what effects occur. Using this technique may make the circuit nonfunctional, which is acceptable, since what is being achieved is location of a fault area containing unwanted RF energy, not system operation

Theresa Lee

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