PCBA Tech - How to use Protel circuit design software to design high-speed PCB

The significance of circuit design software lies in circuit design. Without circuit design software, circuit design will become very troublesome. For circuit design software, small series has been introduced in many previous articles. In order to continue to enhance our understanding of circuit design software, this paper will explain how to design high-speed PCB based on Protel circuit design software. If you are interested in circuit design software, please read on.

First, the raising of questions

In high-speed circuit design, the inductance and capacitance of the circuit board line make the wire equivalent to a transmission line. Incorrect layout of terminal components or wrong wiring of high-speed signals can cause transmission line effect problems, which can make the system output incorrect data, circuit work abnormal or even completely not work. Based on the transmission line model, it can be concluded that the transmission line will bring bad effects on circuit design, such as signal reflection, crosstalk, electromagnetic interference, power supply and grounding noise.

In order to design a reliable high-speed PCB board, it is necessary to give full consideration to the design, solve some unreliable problems in the layout and wiring, shorten the product development cycle, and improve the market competitiveness.

In the process of realizing the design of high-speed circuit printed circuit board with PROTEL design software, this paper discusses some related principles of layout and wiring, and provides some practical and verified high-speed circuit layout and wiring technology, which improves the reliability and effectiveness of high-speed circuit board design. The results show that the design can shorten the product development cycle and enhance the market competitiveness.

Second, high frequency system layout design

In PCB design of circuit, layout is an important link, the quality of layout results will directly affect the effect of wiring and system reliability, which is time-consuming in the whole PRINTED circuit board design. The complex environment of HF PCB makes it difficult to carry out the layout design of HF system with the theoretical knowledge learned, which requires that the board layout person must have rich experience in high-speed PCB manufacturing, so as to reduce detachments in the design process and improve the reliability and effectiveness of circuit work. In the process of layout, ought to from mechanical structure, come loose, electromagnetic interference, the convenient sex of wiring in the future, beautiful sex wait for a respect to consider comprehensively.

First of all, before the layout, the whole circuit should be functionally divided, the high frequency circuit and the low frequency circuit should be separated, the analog circuit and the digital circuit should be separated, and each functional circuit should be arranged as close as possible to the chip as the center, and the shorter the connection should be, in order to avoid transmission delay caused by too long wires and improve the decoupling effect of capacitors. In addition, pay attention to the relative position and direction of pins to circuit components and other pipes to reduce mutual influence. All high-frequency components should be kept away from housing and other metal plates to minimize parasitic coupling.

Secondly, attention should be paid to the thermal and electromagnetic effects between components in the layout. These effects are particularly serious for high-frequency systems, and measures should be taken to keep away or isolate, heat dissipation and shielding. High power rectifier and adjustment tubes shall be fitted with radiators and shall be kept away from transformers. Electrolytic capacitors and other heat-resistant elements should be kept away from heating elements, otherwise the electrolyte will be baked dry, resulting in increased resistance, poor performance, affecting the stability of the circuit. The layout should leave enough space for protective structures and prevent the introduction of various parasitic couplings. To prevent electromagnetic coupling between the coils on the PCB, the two coils should be placed at right angles to reduce the coupling coefficient. Vertical plate isolation can also be used. Weld directly on the circuit with the lead of its component, as short as possible, do not use connectors and welds, because there is distributed capacitance and distributed inductance between adjacent welds. Avoid placing high-noise components around crystal oscillator, RIN, analog voltage and reference voltage signal wiring.

In order to ensure the inherent quality and reliability at the same time, take into account the overall aesthetic, reasonable circuit board planning, components should be parallel or vertical board surface, and parallel or vertical with the main board edge. Components should be evenly distributed on the board with the same density. In this way, not only beautiful and easy to install welding, easy to mass production.

Wiring of high frequency system

In the high frequency circuit, the distribution parameters of resistance, capacitance, inductance and mutual inductance of the wire connected can not be ignored. Considering from the Angle of anti-interference, reasonable wiring is to try to reduce the line resistance, distributed capacitance and stray inductance in the circuit, and reduce the stray magnetic field generated thereby to the degree of. Thus, the distributed capacitance, flux leakage, electromagnetic mutual inductance and other interference caused by noise can be suppressed.

The following describes some special functions provided by the protel99 SE tool.

(1) the less bending of leads between pins of high-frequency circuit devices, the better, the full line is used. When bending, 45° broken line or arc bending can be used, so as to reduce the external emission of high-frequency signals and the coupling between each other. When RouTIng with PROTEL, choose 45-Degrees or Rounded in "RouTIng Corners" in "Rules" of "Design". You can also use the Shift + Space key to quickly switch lines.

(2) The shorter the lead between pins of high-frequency circuit devices, the better.

The effective means of PROTEL 99 to satisfy the short-wiring is to make wiring reservation for some key high-speed networks before automatic wiring. RouTIng Topology under "Rules" in the "Design" menu

Choose Shortest.

(3) The less the lead layer alternating between pins of high-frequency circuit devices, the better. That is, the less holes used in the connection process, the better.

A single through-hole can deliver about 0.5pF of distributed capacitance, and reducing the number of through-holes can significantly improve speed.

(4) For high frequency circuit wiring, attention should be paid to the "cross interference", that is, crosstalk introduced by parallel route of signal line in the entering distance. If parallel distribution cannot be avoided, a large area of "ground" can be arranged on the opposite side of parallel signal lines.

To drastically reduce interference. Parallel cabling in the same layer is almost unavoidable, but in the two adjacent layers, the direction of cabling must be perpendicular to each other, which is easy to do in PROTEL but easy to ignore. In "Design" menu "Rules" in "RouTIngLayers" Toplayer select Horizontal and BottomLayer select VerTIcal. In addition, "Polygonplane" is provided in "Place".

(5) The implementation of ground wire encirclement measures for particularly important signal lines or local units. Outline Selectedobjects is provided in Tools to automatically "wrap" selected critical signal lines.

(6) Generally, power cables and ground cables should be set wider than signal cables. You can use "Classes" in the "Design" menu to classify networks into power networks and signal networks. In combination with wiring rules, you can easily switch the width of power cables and signal cables.

(7) All kinds of wiring can not form a loop, the ground can not form a current loop. If a loop circuit is generated, it will cause a lot of interference in the system. In this regard, Daisy chain can be used for wiring, which can effectively avoid the formation of loops, branches or stumps during wiring, but it will also bring problems that are not easy to wire.

(8) According to the data and design of various chips, estimate the current passed by the power line and determine the required wire width. According to the empirical formula, W (line width) ≥L (mm/A) *I (A).

According to the current size, as far as possible to increase the width of the power line, reduce the loop resistance. At the same time, the direction of power cables and ground cables should be consistent with the direction of data transmission, which helps enhance the anti-noise capability. When necessary, the power line and ground line can be added with copper wire wound ferrite high-frequency choke device, used to block the transmission of high-frequency noise.

(9) The wiring width of the same network should be kept consistent. The change of line width will lead to uneven line characteristic impedance. When the transmission speed is high, there will be reflection, which should be avoided as far as possible in the design. At the same time, increase the line width of parallel lines. When the distance between the center of lines is not more than 3 times the line width, 70% of the electric fields can be kept from interfering with each other, which is called the 3W principle. In this way, the influence of distributed capacitance and inductance brought by parallel lines can be overcome.