EMC issues of PCB design

writer: G January 05, 2018

(1) key device size:

The physical size of the emitting device that produces the radiation. Radio frequency (RF) current will create an electromagnetic field that escapes from the enclosure through the enclosure. The trace length on the PCB as a transmission path has a direct impact on the RF current.

(2) Impedance matching:

The impedance of the source and receiver, and the transmission impedance between the two.

(3) Interference signal time characteristics:

This issue is a continuous (periodic signal) event, or is it only for a specific operation cycle (eg, a single button operation or power-on interference, periodic disk drive operation, or network burst transmission).

(4) Interference signal strength:

How strong is the source energy level, and how great is its potential for harmful interference.

(5) Interference signal frequency characteristics:

Use the spectrum analyzer to observe the waveform, where the observed problem is in the spectrum, making it easy to find the problem.

In addition, some low-frequency circuit design habits need attention. For example, my usual single-point grounding is very suitable for low-frequency applications, but was later found to be unsuitable for RF applications because of the EMI issues in the RF signal. It is believed that some engineers apply a single point of grounding to all product designs without realizing that more or more complex electromagnetic compatibility (EMC) issues may arise with this method of grounding.

We should also pay attention to the current flow in the circuit components. Knowing the Circuitry We know that current flows from a high voltage to a low voltage and that current always flows through a closed-loop circuit through one or more paths, thus a minimum loop and a very important law. For those measured interference current direction, by modifying the PCB traces so that it does not affect the load or sensitive circuitry. Applications that require a high impedance path from power supply to load must consider all possible paths through which the return current can flow.

There is also a PCB alignment problem. The impedance of the wire or trace contains resistance R and inductance, impedance at high frequencies, and no resistance. When the line frequency is higher than 100kHz, the conductor or trace becomes an inductance. Conductors or traces that work above audio may become RF antennas. In the EMC specifications, it is not permissible for wires or traces to operate below λ / 20 of a particular frequency (the design length of the antenna is equal to λ / 4 or λ / 2 of a particular frequency) Be careful So design, alignment into a high-performance antenna, which makes late debugging becomes more difficult.

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