PCB Tech - Practical EMI noise countermeasure technology for PCB

The PCB board often has considerable problems at the beginning of the design, especially in the substrate, layout, wiring...

All other parts need attention, and this article will discuss these aspects.

Noise countermeasures on printed circuit boards

Almost all circuits use printed circuit boards, which means that the noise response of printed circuit boards has become the core of noise countermeasures.

The circuits in the printed circuit board can be divided into three categories:

*Power/Earth circuit

*Main signal circuit

*The main signal circuit of the interface circuit is a part of the actual circuit action. The main signal circuit has the type and purpose of the circuit, and can also be subdivided into several units.

The interface circuit is a circuit that performs the exchange (interface) between the printed circuit board and the external signal. The interface circuit is in the position of noise countermeasures. It has two functions to prevent the noise of the printed circuit board from penetrating into the board and the internal noise that affects the board to radiate to the outside of the board. The main function of the power/ground (ground) circuit is to provide power for the signal circuit and the interface circuit, and the ground wire has the function of the return line of the unbalanced circuit.

Originally, the power supply and the earth must maintain a stable potential, but in reality, both the power supply and the earth have a common impedance (impedance), so it is a very difficult part of noise countermeasures.

From the viewpoint of noise countermeasures, the layout of the PCB substrate needs to be classified according to the type and purpose of the circuit, so that the noise countermeasure configuration (layout) can be arranged on the printed circuit board.

In principle, the high noise hazard circuit and the low noise immunity circuit are best configured on separate circuit boards, but in fact, based on cost and circuit size, it is quite common for the two circuits to be mixed. As mentioned above, the high-noise hazard circuit and the low-noise-resistant circuit must be configured separately as much as possible. In particular, the signal line has a large noise, so as to avoid long-distance winding wiring. The high hazard of wiring is to avoid passing around low-noise circuits as much as possible. If parallel or dense wiring is used, crosstalk will cause greater risks.

The way the wiring is wound depends on the configuration of the components, and the configuration of the components becomes an important theme for realizing the above-mentioned wiring principle.

When the motherboard performs data transactions between substrates, bus traffic is usually available. The digital circuit passes through the interface circuit at the end of the motherboard. In addition to the interface (interface) with other substrates, the digital interface can also perform other interface operations with the outside world.

The analog circuit can be exchanged with external analog signals. The analog circuit unit has an A/D converter to avoid noise interference from the analog circuit to the digital interface. Therefore, the installation of the A/D converter must be far away from the digital interface. The power supply of the analog circuit must be completely separated from the power supply of the digital circuit, but if the power supply voltage of the analog circuit is the same as the power supply voltage of the digital circuit, the noise of the analog circuit except the circuit is very low, and the analog circuit can use part of the digital circuit to supply power. In this case, filter Device must

Eliminate the noise of digital circuits.

As for the earth, the digital and analog units are connected into a point, and then the pattern (pattern) of the digital and analog connections is used to design irregularly so that it has multiple impedances, and then the digital and analog units can be separated by using this impedance.

The wiring bypass capacitor (bypass capacitor) of the printed circuit board is usually installed at the entrance of the board.

In order to strengthen these purposes, some circuits also insert inductors and bypass capacitors used to form LC filters (Figure 3). Once the inductor overlaps with DC, the inductance value is greatly reduced due to the influence of the DC component. In addition, the inductance of the power supply will generate a large direct current, so it is necessary to choose a suitable inductance. Generally, the entrance of the power supply substrate is set on the inductor, and most of the toroidal inductors shown in Fig. 4 are used. The bypass capacitor uses a two-stage structure, and in order for the bypass capacitor to support a wide frequency range, a capacitor capable of supporting low frequencies and a capacitor capable of supporting high frequencies must be used separately.

The capacitor provided at the entrance of the substrate is of low frequency, and although its capacity depends on the value of the current flowing inside the substrate, an aluminum capacitor of about several tens of μF is generally used. Set high-frequency bypass capacitors near the IC, mainly using multiple 0.01μF ceramic capacitors. Ideally, it is best to insert a bypass capacitor near each IC, and a small current IC can be set to 2 to 3 each other.

The second bypass capacitor is also set near the IC. If it is too far away from the IC, the effect of the bypass capacitor may be weakened due to the influence of the inductance in the figure.

Filling Beta mode is very effective. The power and ground (ground) of the multilayer substrate are mostly designed with Beta patterns. The main reason is that the impedance of the Beta pattern is lower than that of the linear pattern. The Beta pattern also has the function of shielding (shielding) the signal line. This means that the multilayer substrate is used in noise countermeasures. very effective.

The first task in designing the signal line is to shorten the length of the signal line, so the configuration skills of the pre-wired components have a decisive influence. Most of the wiring in the substrate is unbalanced. At this time, the circuit must consider signal re-entry lines, including signal lines (that is, ground lines). The circuit composed of the signal line and the ground line must avoid becoming a large-area loop (loop).

In addition, based on considerations such as crosstalk, it is necessary to avoid designing low-noise signal lines and high-victim signal lines adjacent to each other and parallel configuration. When the ground line between the two signals is unavoidable, the line (earth line) cannot be avoided.

The anti-noise ability of the high-impedance part is not as good as the low-impedance, so the wiring at the height of the resistance must be designed to use the shortest distance, otherwise the wiring length of the low-impedance part should be such that a buffer (buffer) can be inserted if necessary. The impedance of the signal line becomes the impedance characteristic of th. When a high-impedance component is inserted between the driver and the receiver, the wiring between the high-impedance component and the receiver becomes high-impedance. At this time, the wiring length and the wiring length of the high-impedance component must be reduced. Wiring length. Low impedance part.

In the past, the substrate was unlikely to have connection problems, mainly because within the general size of the substrate, the frequency of the connection was mostly higher than the frequency of the signal (the pattern was 20cm long, and the frequency was about 250MHz). In addition, the choice of IC depends on the frequency of the signal. The low operating frequency of the IC cannot exceed its own signal frequency. In other words, the IC itself has a filtering effect, even if there is a high-frequency connection, it will not cause trouble.

However, in recent years, the frequency of the signal has been continuously updated, and the internal signal of the substrate is very close to the frequency of the connection, causing more and more serious connection problems. High-frequency noise (noise) will not only propagate in the signal line, but also radiate through the signal line, so you only need to install a filter at the receiving end, and the effect of filtering the connection is very limited. The fundamental countermeasure is to completely eliminate the connection.

When the signal frequency is high, the delaying method of standing the signal can easily make the signal itself dull. Another method is to make the receiving end correct to cancel the connection, but the current at the receiving end will still flow and it will consume power, based on energy saving and other considerations. Take the driver-end terminal design method. If a filter is inserted at the receiving end, the connection can be eliminated at the receiving end, but the connection with the online signal cannot be eliminated.

PCB factories should master the practical EMI noise countermeasure technology of PCB