It has been said that there are only two kinds of electronic engineers in the world: those who have experienced EMI and those who have not. With the increase in PCB board trace density, electromagnetic compatibility (EMC) design has become a critical consideration for electronic engineers. When performing an EMC analysis of a product and its design, there are five key attributes to consider:
1) **Critical Device Size**: This refers to the physical size of the emitting device that generates radiation. Radio frequency (RF) current creates an electromagnetic field that can escape the enclosure through its case. The length of the PCB trace, serving as a transmission path, directly impacts the RF current.
2) **Impedance Matching**: Consider the impedance of the source and receiver, as well as the transmission impedance between them.
3) **Time Characteristics of the Disturbance Signal**: Determine if the issue is continuous (periodic signal) or occurs only during specific operational cycles (e.g., a single key press, power-on disturbance, periodic disk drive operation, or network burst transfer).
4) **Strength of the Interfering Signal**: Assess the energy level of the source and its potential to cause harmful interference.
5) **Frequency Characteristics of the Interference Signal**: Use a spectrum analyzer to observe the waveform and identify where the problem lies in the frequency spectrum. This helps in pinpointing the issue.
Additionally, design practices for low-frequency circuits need special attention. For example, single-point grounding, which is often suitable for low-frequency applications, may not be effective for RF signal applications due to increased EMI problems. Some engineers might apply single-point grounding universally, unaware that this approach could exacerbate or complicate EMC issues.
Attention should also be given to current flow within circuit components. Current flows from regions of high voltage to low voltage and always forms a closed-loop circuit through one or more paths, highlighting the importance of minimizing loop size. To mitigate interference, modify PCB traces so they do not impact the load or sensitive circuits. For applications requiring high impedance paths from source to load, consider all possible return current paths.
Moreover, PCB routing presents challenges. The impedance of a wire or trace includes resistance (R) and inductive reactance, with capacitive reactance becoming negligible at high frequencies. When trace frequency exceeds 100 kHz, the trace behaves inductively and can act as an RF antenna. According to EMC specifications, wires or traces should not be designed to operate below λ/20 of a certain frequency (where λ is the wavelength). A trace acting as a high-efficiency antenna complicates debugging efforts.
Lastly, consider the PCB board layout. A larger PCB board can reduce system interference resistance but increase costs and trace length. Conversely, a smaller board might lead to heat dissipation issues and mutual interference. Proper placement of special components (e.g., clock components) is crucial; clock traces should not be placed on the ground or in proximity to critical signal lines to avoid interference. Arrange related components as close as possible to improve anti-interference performance.
1) **Critical Device Size**: This refers to the physical size of the emitting device that generates radiation. Radio frequency (RF) current creates an electromagnetic field that can escape the enclosure through its case. The length of the PCB trace, serving as a transmission path, directly impacts the RF current.
2) **Impedance Matching**: Consider the impedance of the source and receiver, as well as the transmission impedance between them.
3) **Time Characteristics of the Disturbance Signal**: Determine if the issue is continuous (periodic signal) or occurs only during specific operational cycles (e.g., a single key press, power-on disturbance, periodic disk drive operation, or network burst transfer).
4) **Strength of the Interfering Signal**: Assess the energy level of the source and its potential to cause harmful interference.
5) **Frequency Characteristics of the Interference Signal**: Use a spectrum analyzer to observe the waveform and identify where the problem lies in the frequency spectrum. This helps in pinpointing the issue.
Additionally, design practices for low-frequency circuits need special attention. For example, single-point grounding, which is often suitable for low-frequency applications, may not be effective for RF signal applications due to increased EMI problems. Some engineers might apply single-point grounding universally, unaware that this approach could exacerbate or complicate EMC issues.
Attention should also be given to current flow within circuit components. Current flows from regions of high voltage to low voltage and always forms a closed-loop circuit through one or more paths, highlighting the importance of minimizing loop size. To mitigate interference, modify PCB traces so they do not impact the load or sensitive circuits. For applications requiring high impedance paths from source to load, consider all possible return current paths.
Moreover, PCB routing presents challenges. The impedance of a wire or trace includes resistance (R) and inductive reactance, with capacitive reactance becoming negligible at high frequencies. When trace frequency exceeds 100 kHz, the trace behaves inductively and can act as an RF antenna. According to EMC specifications, wires or traces should not be designed to operate below λ/20 of a certain frequency (where λ is the wavelength). A trace acting as a high-efficiency antenna complicates debugging efforts.
Lastly, consider the PCB board layout. A larger PCB board can reduce system interference resistance but increase costs and trace length. Conversely, a smaller board might lead to heat dissipation issues and mutual interference. Proper placement of special components (e.g., clock components) is crucial; clock traces should not be placed on the ground or in proximity to critical signal lines to avoid interference. Arrange related components as close as possible to improve anti-interference performance.
