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EMI guidelines that engineers need to know when designing PCBs.

January 26, 2021


Design Guide 1: Minimize the current loop area of power supplies and high-frequency signals


In the design phase, we first need to know two key points:

1. The signal current always returns to the source (that is, the current path always exists in the form of a loop).

2. The signal returns to the path with the least impedance. At megahertz and higher frequencies, the signal current path is relatively easy to identify. This is because at high frequencies, the signal return path is usually the path with the smallest inductive reactance, and here is usually the path with the smallest loop area, and the return signal will return as close as possible to the output signal path. At low frequencies (usually kHz frequencies and below), the path with the least impedance is often the path with the least resistance. Because the resistance value of the path is difficult to determine, it is more difficult to identify the low-frequency return path.

Design Guide 2: Keep the signal return plane intact

A complete signal return plane can effectively reduce the inductance of the high-frequency signal loop. The smaller the inductance, the smaller the noise voltage value generated. This is one of the important reasons why a complete ground plane is required in the middle of the PCB. Of course, in some cases, the signal has to be separated back to the plane due to wiring. However, this situation is less likely to occur on multilayer PCBs. In addition, in the case of a single-layer board, you can wrap the ground around the high-speed signal traces to maintain the integrity of the signal return path.

Design Guide 3: Do not place high-speed circuits near connectors

We often make the following mistakes. In the process of reviewing or evaluating the circuit board design, due to lack of consideration, high-speed circuits are placed near the connector, which causes engineers to do a lot of additional filtering and shielding, thereby increasing the cost and improving the machine. Difficulty of rectification.

Why is the location of the connector so important? At frequencies below three hundred megahertz, the wavelength is approximately one meter or more. The printed circuit board itself and the wiring inside the board are often of small electrical size, so the radiation efficiency is relatively low. However, the cable connected to the connector is generally longer, so the antenna effect will be obvious, and the noise in the board is more likely to radiate out through the cable.

In addition, the high-speed circuit located between the connectors can easily generate a potential difference of several millivolts or more between the connectors. These voltages can drive current to the connected cable, causing the product to exceed the radiation emission requirements.

Design Guide 4: Control signal edge transition time (rising edge and falling edge time)

In many cases, the excessive clock noise is not the fundamental frequency, but the higher harmonics derived from the fundamental frequency. By increasing the transition time of the clock edge, the energy of higher harmonics can be well controlled. Although the excessively long edge transition time will cause signal integrity and heating problems, in many cases the function and EMC effect need to be compromised.

There are three common methods for controlling the rise and fall times of digital signals:

1. Change the chip signal output drive capability

2. The signal line is connected with resistor or ferrite in series

3. Parallel capacitance of signal line

Design Guide 5: Clock Spread Spectrum

As electronic products have more and more functions, the chip clock frequency is also increasing. For high-speed clocks, the risk of controlling the clock edge conversion rate to suppress EMI is increasing. At this time, spread spectrum technology has become a good choice for suppressing electromagnetic interference.

While not changing the rising and falling edges of the clock and maintaining the integrity of the clock signal waveform, the clock jitter is controlled according to certain rules, and the clock energy is dispersed over a wider frequency band to achieve the suppression of clock noise in the frequency domain.

The spread spectrum technology not only modulates the clock source, but other data, address and control signals synchronized with the clock source are modulated at the same time as the clock is spread, and the overall EMI peak value will be reduced accordingly. Therefore, the clock spread is System-level solutions. This is one of the biggest advantages of spread spectrum technology compared to other EMI suppression measures.

In general, engineers should always sound the alarm in their minds during the PCB design process. While considering how to implement circuit functions, they should pay attention to signals that are prone to noise. When they encounter such as clock or PWM, they will generate high-order For harmonic signals, refer to the several EMI guidelines of the appeal to design the PCB, so that it will be easier for products to pass EMC certification.