With the rapid development of electronic technology and the widespread application of wireless communication technology in various fields, high frequency, high speed and high density have gradually become one of the significant development trends of modern electronic products. The high frequency and high speed digitalization of signal transmission force PCB to move towards micro-holes and buried/blind holes, fine wires, and uniform thin dielectric layers. High-frequency, high-speed, high-density multi-layer PCB design technology has become an important research field. This time, we mainly want to learn some practical skills in high-frequency PCB design.

　　Many people think that high frequency means higher signal frequency. Although this view is not wrong, high-speed electronic design engineers should have a deeper understanding. In addition to caring about the natural frequency of the signal, we should also consider the influence of high-order harmonics generated when the signal is transmitted. The following formula is generally used to define the signal transmission bandwidth, sometimes also called EMI emission bandwidth: F=1/(Tr*π), F is the frequency (GHz); Tr (nanoseconds) refers to the rise time or fall time of the signal.

　　Usually when F>100MHz, it can be called a high-frequency circuit. Therefore, in digital circuits, whether it is a high-frequency circuit does not depend on the signal frequency, but mainly depends on the rising edge and falling edge. According to this formula, it can be deduced that when the rise time is less than about 3.185ns, it can be considered a high-frequency circuit. In the PCB industry, the transmission bandwidth F>300MHz is called a high-frequency signal, and the signal at this frequency is also called a microwave.

　　High frequency PCB design tips:

　　1. Multilayer board wiring: High-frequency circuits are often highly integrated and have high wiring density. The use of multilayer boards is necessary for wiring and is also an effective means to reduce interference. In the PCB Layout stage, the reasonable selection of the size of a printed board with a certain number of layers can make full use of the middle layer to set up the shield, better achieve the nearest grounding, and effectively reduce parasitic inductance and shorten the transmission length of the signal. At the same time, it can also greatly reduce the cross interference of the signal, etc. All these methods are beneficial to the reliability of high-frequency circuits.

　　2. Avoid loops formed by routing: All kinds of high-frequency signal routing should try not to form loops. If it cannot be avoided, the loop area should be as small as possible.

　　3. The fewer bends in the lead, the better: The leads of high-frequency circuit wiring are best to be straight lines. If a turn is required, a 45-degree bend or arc turn can be used. This requirement is only used to improve the adhesion strength of the copper foil in low-frequency circuits, but in high-frequency circuits, meeting this requirement can reduce the external emission of high-frequency signals and mutual coupling.

　　4. The fewer the alternations between lead layers, the better: The so-called “the fewer the alternations between lead layers, the better” means that the fewer vias (vias) used in the component connection process, the better. According to measurements, a via can bring about 0.5pF of distributed capacitance. Reducing the number of vias can significantly increase speed and reduce the possibility of data errors.

　　5. The shorter the lead, the better: The radiation intensity of the signal is proportional to the length of the signal line. The longer the high-frequency signal lead, the easier it is to couple to components close to it. Therefore, for high-frequency signal lines such as clock signals, crystal oscillators, DDR data, LVDS lines, USB lines, HDMI lines, etc., the lines are required to be as short as possible.

　　6. Isolate the ground wire of high-frequency digital signals from the ground wire of analog signals: When analog ground wires, digital ground wires, etc. are connected to the common ground wire, they should be connected with high-frequency choke beads or directly isolated and connected at a suitable location. The ground potential of the ground wire of high-frequency digital signals is generally inconsistent, and there is often a certain voltage difference between the two. Moreover, the ground wire of high-frequency digital signals often carries a very rich harmonic component of high-frequency signals. When the digital signal ground wire and the analog signal ground wire are directly connected, the harmonics of the high-frequency signal will interfere with the analog signal through ground wire coupling. Therefore, under normal circumstances, the ground wire of high-frequency digital signals and the ground wire of analog signals must be isolated. They can be connected at a suitable location with a single point, or they can be connected with high-frequency choke beads.

　　7. Add high-frequency decoupling capacitors to the power pins of the integrated circuits: Add a high-frequency decoupling capacitor to the power pins of each integrated circuit. Adding high-frequency decoupling capacitors to the power pins can effectively suppress the interference caused by high-frequency harmonics on the power pins.

KKPCB conducts research on special processing technologies such as ordinary double-sided boards, thick copper circuit boards, high-frequency circuit boards, HDI circuit boards, rigid-flexible circuit boards, FPC flexible boards, buried blind hole circuit boards, and IC carrier boards. Provides PCB design, PCB layout, PCB prototyping and PCB assembly services.