1 Questions

With the large-scale increase in the design complexity and integration of electronic systems, clock speeds and device rise times are getting faster and faster, and high-speed PCB design has become an important part of the design process. In high-speed circuit design, the inductance and capacitance on the circuit board line make the wire equivalent to a transmission line. Incorrect layout of termination components or incorrect wiring of high-speed signals can cause transmission line effect problems, resulting in incorrect data output from the system, abnormal circuit operation or even no operation at all. Based on the transmission line model, to sum up, the transmission line will bring adverse effects such as signal reflection, crosstalk, electromagnetic interference, power supply and ground noise to the circuit design.

In order to design a high-speed PCB circuit board that can work reliably, the design must be fully and carefully considered to solve some unreliable problems that may occur during layout and routing, shorten the product development cycle, and improve market competitiveness.

How to use PROTEL design tools for high-speed PCB design

2 Layout design of high frequency system

In the PCB design of the circuit, the layout is an important link. The result of the layout will directly affect the wiring effect and the reliability of the system, which is the most time-consuming and difficult in the entire printed circuit board design. The complex environment of high-frequency PCB makes the layout design of the high-frequency system difficult to use the learned theoretical knowledge. It requires the person who lays out must have rich experience in high-speed PCB manufacturing, so as to avoid detours in the design process. Improve the reliability and effectiveness of circuit work. In the process of layout, comprehensive consideration should be given to the mechanical structure, heat dissipation, electromagnetic interference, convenience of future wiring, and aesthetics.

First of all, before layout, the entire circuit is divided into functions. The high-frequency circuit is separated from the low-frequency circuit, and the analog circuit and the digital circuit are separated. Each functional circuit is placed as close as possible to the center of the chip. Avoid transmission delay caused by excessively long wires, and improve the decoupling effect of capacitors. In addition, pay attention to the relative positions and directions between the pins and circuit components and other tubes to reduce their mutual influence. All high-frequency components should be far away from the chassis and other metal plates to reduce parasitic coupling.

Second, attention should be paid to the thermal and electromagnetic effects between components during layout. These effects are particularly serious for high-frequency systems, and measures to keep away or isolate, heat and shield should be taken. The high-power rectifier tube and adjustment tube should be equipped with a radiator and kept away from the transformer. Heat-resistant components such as electrolytic capacitors should be kept away from heating components, otherwise the electrolyte will be dried out, resulting in increased resistance and poor performance, which will affect the stability of the circuit. Enough space should be left in the layout to arrange the protective structure and prevent the introduction of various parasitic couplings. To prevent electromagnetic coupling between the coils on the printed circuit board, the two coils should be placed at right angles to reduce the coupling coefficient. The method of vertical plate isolation can also be used. It is best to directly use the lead of the component to be soldered to the circuit. The shorter the lead, the better. Do not use connectors and soldering tabs because there are distributed capacitance and distributed inductance between adjacent soldering tabs. Avoid placing high-noise components around the crystal oscillator, RIN, analog voltage, and reference voltage signal traces.

Finally, while ensuring the inherent quality and reliability, while taking into account the overall beauty, reasonable circuit board planning should be carried out. The components should be parallel or perpendicular to the board surface, and parallel or perpendicular to the main board edge. The distribution of components on the board surface should be as even as possible and the density should be consistent. In this way, it is not only beautiful, but also easy to assemble and weld, and it is easy to mass produce.

3 Wiring of high frequency system

In high-frequency circuits, the distribution parameters of resistance, capacitance, inductance and mutual inductance of the connecting wires cannot be ignored. From the perspective of anti-interference, reasonable wiring is to try to reduce the line resistance, distributed capacitance, and stray inductance in the circuit. , The resulting stray magnetic field is reduced to a minimum, so that the distributed capacitance, leakage magnetic flux, electromagnetic mutual inductance and other interference caused by noise are suppressed.

The application of PROTEL design tools in China has been quite common. However, many designers only focus on the “broadband rate”, and the improvements made by the PROTEL design tools to adapt to the changes in device characteristics have not been used in the design, which not only makes The waste of design tool resources is more serious, which makes it difficult for the excellent performance of many new devices to be brought into play.

The following introduces some special functions that PROTEL99 SE tool can provide.

(1) The lead between the pins of the high-frequency circuit device should be bent as little as possible. It is best to use a full straight line. When bending is required, 45° bends or arcs can be used, which can reduce the external emission of high-frequency signals and mutual interference. The coupling between. When using PROTEL for routing, you can select 45-Degrees or Rounded in the “Routing Corners” in the “rules” menu of the “Design” menu. You can also use the shift + space keys to quickly switch between the lines.

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

PROTEL 99 The most effective way to meet the shortest wiring is to make a wiring appointment for individual key high-speed networks before automatic wiring. “RouTIng Topology” in “rules” in the “Design” menu

Select shortest.

(3) Alternation of lead layers between pins of high-frequency circuit devices is as small as possible. That is, the fewer vias used in the component connection process, the better.

One via can bring about 0.5pF of distributed capacitance, and reducing the number of vias can significantly increase the speed.

(4) For high-frequency circuit wiring, pay attention to the “cross interference” introduced by the parallel wiring of the signal line, that is, crosstalk. If parallel distribution is unavoidable, a large area of ​​”ground” can be arranged on the opposite side of the parallel signal line

To greatly reduce interference. Parallel wiring in the same layer is almost unavoidable, but in two adjacent layers, the direction of the wiring must be perpendicular to each other. This is not difficult to do in PROTEL but it is easy to overlook. In the “RouTIngLayers” in the “Design” menu “rules”, select Horizontal for Toplayer and VerTIcal for BottomLayer. In addition, “Polygonplane” is provided in “place”

The function of the polygonal grid copper foil surface, if you place the polygon as a surface of the entire printed circuit board, and connect this copper to the GND of the circuit, it can improve the high frequency anti-interference ability , It also has greater benefits for heat dissipation and printing board strength.

(5) Implement ground wire enclosure measures for particularly important signal lines or local units. “Outline selectedobjects” is provided in “Tools”, and this function can be used to automatically “wrap the ground” of the selected important signal lines (such as oscillation circuit LT and X1).

(6) Generally, the power line and grounding line of the circuit are wider than the signal line. You can use the “Classes” in the “Design” menu to classify the network, which is divided into power network and signal network. It is convenient to set the wiring rules. Switch the line width of power line and signal line.

(7) Various types of wiring cannot form a loop, and the ground wire cannot form a current loop. If a loop circuit is generated, it will cause a lot of interference in the system. A daisy chain wiring method can be used for this, which can effectively avoid the formation of loops, branches or stumps during wiring, but it will also bring about the problem of not easy wiring.

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

According to the current, try to increase the width of the power line and reduce the loop resistance. At the same time, make the direction of the power line and the ground line consistent with the direction of data transmission, which helps to enhance the anti-noise ability. When necessary, a high-frequency choke device made of copper wire wound ferrite can be added to the power line and ground line to block the conduction of high-frequency noise.

(9) The wiring width of the same network should be kept the same. Variations in line width will cause uneven line characteristic impedance. When the transmission speed is high, reflection will occur, which should be avoided as much as possible in the design. At the same time, increase the line width of parallel lines. When the line center distance does not exceed 3 times the line width, 70% of the electric field can be maintained without mutual interference, which is called the 3W principle. In this way, the influence of distributed capacitance and distributed inductance caused by parallel lines can be overcome.

4 Design of power cord and ground wire

In order to solve the voltage drop caused by the power supply noise and line impedance introduced by the high-frequency circuit, the reliability of the power supply system in the high-frequency circuit must be fully considered. There are generally two solutions: one is to use power bus technology for wiring; the other is to use a separate power supply layer. In comparison, the latter’s manufacturing process is more complicated and the cost is more expensive. Therefore, the network-type power bus technology can be used for wiring, so that each component belongs to a different loop, and the current on each bus on the network tends to be balanced, reducing the voltage drop caused by the line impedance.

The high-frequency transmission power is relatively large, you can use a large area of ​​copper, and find a low-resistance ground plane nearby for multiple grounding. Because the inductance of the grounding lead is proportional to the frequency and length, the common ground impedance will be increased when the operating frequency is high, which will increase the electromagnetic interference generated by the common ground impedance, so the length of the ground wire is required to be as short as possible. Try to reduce the length of the signal line and increase the area of ​​the ground loop.

Set one or several high-frequency decoupling capacitors on the power and ground of the chip to provide a nearby high-frequency channel for the transient current of the integrated chip, so that the current does not pass through the power supply line with a large loop area, thereby greatly reducing The noise radiated to the outside. Choose monolithic ceramic capacitors with good high-frequency signals as decoupling capacitors. Use large-capacity tantalum capacitors or polyester capacitors instead of electrolytic capacitors as energy storage capacitors for circuit charging. Because the distributed inductance of the electrolytic capacitor is large, it is invalid for high frequency. When using electrolytic capacitors, use them in pairs with decoupling capacitors with good high-frequency characteristics.

5 Other high-speed circuit design techniques

Impedance matching refers to a working state in which the load impedance and the internal impedance of the excitation source are adapted to each other to obtain the maximum power output. For high-speed PCB wiring, in order to prevent signal reflection, the impedance of the circuit is required to be 50 Ω. This is an approximate figure. Generally, it is stipulated that the baseband of coaxial cable is 50 Ω, the frequency band is 75 Ω, and the twisted wire is 100 Ω. It is just an integer, for the convenience of matching. According to the specific circuit analysis, the parallel AC termination is adopted, and the resistor and capacitor network are used as the termination impedance. The termination resistance R must be less than or equal to the transmission line impedance Z0, and the capacitance C must be greater than 100 pF. It is recommended to use 0.1UF multilayer ceramic capacitors. The capacitor has the function of blocking low frequency and passing high frequency, so the resistance R is not the DC load of the driving source, so this termination method does not have any DC power consumption.

Crosstalk refers to the undesirable voltage noise interference caused by electromagnetic coupling to adjacent transmission lines when the signal propagates on the transmission line. Coupling is divided into capacitive coupling and inductive coupling. Excessive crosstalk may cause false triggering of the circuit and cause the system to fail to work normally. According to some characteristics of crosstalk, several main methods to reduce crosstalk can be summarized:

(1) Increase the line spacing, reduce the parallel length, and use the jog method for wiring if necessary.

(2) When high-speed signal lines meet the conditions, adding termination matching can reduce or eliminate reflections, thereby reducing crosstalk.

(3) For microstrip transmission lines and strip transmission lines, restricting the trace height to within the range above the ground plane can significantly reduce crosstalk.

(4) When the wiring space permits, insert a ground wire between the two wires with more serious crosstalk, which can play a role in isolation and reduce crosstalk.

Due to the lack of high-speed analysis and simulation guidance in traditional PCB design, the signal quality cannot be guaranteed, and most of the problems cannot be discovered until the plate-making test. This greatly reduces the design efficiency and increases the cost, which is obviously disadvantageous in the fierce market competition. Therefore, for high-speed PCB design, people in the industry have proposed a new design idea, which has become a “top-down” design method. After a variety of policy analysis and optimization, most of the possible problems have been avoided and a lot of savings have been made. Time to ensure that the project budget is met, high-quality printed boards are produced, and tedious and costly test errors are avoided.

The use of differential lines to transmit digital signals is an effective measure to control factors that destroy signal integrity in high-speed digital circuits. The differential line on the printed circuit board is equivalent to a differential microwave integrated transmission line pair working in quasi-TEM mode. Among them, the differential line on the top or bottom of the PCB is equivalent to the coupled microstrip line and is located on the inner layer of the multilayer PCB The differential line is equivalent to a broadside coupled strip line. The digital signal is transmitted on the differential line in an odd-mode transmission mode, that is, the phase difference between the positive and negative signals is 180°, and the noise is coupled on a pair of differential lines in a common mode. The voltage or current of the circuit is subtracted, so that the signal can be obtained to eliminate common mode noise. The low-voltage amplitude or current drive output of the differential line pair fulfills the requirements of high-speed integration and low power consumption.

6 concluding remarks

With the continuous development of electronic technology, it is imperative to understand the theory of signal integrity to guide and verify the design of high-speed PCBs. Some experience summarized in this article can help high-speed circuit PCB designers shorten the development cycle, avoid unnecessary detours, and save manpower and material resources. Designers must continue to research and explore in actual work, continue to accumulate experience, and combine new technologies to design high-speed PCB circuit boards with excellent performance.