PCB Technical - Printed board high-frequency PCB wiring skills

High-frequency PCBs tend to have high integration and high wiring density. The use of multi-layer boards is not only necessary for wiring, but also an effective means to reduce interference. It can provide 16 copper wire layers and 4 power layers. A reasonable selection of the number of layers can greatly reduce the size of the PCB printed board, make full use of the intermediate layer to set up the shield, better realize the nearest grounding, and effectively reduce the parasitic inductance. Can effectively shorten the transmission length of the signal, can greatly reduce the cross interference between the signals, and so on. All of these are beneficial to the reliable operation of high-frequency circuits. When there are data showing that the same material, the noise of the four-layer board is 20dB lower than that of the double-sided board, but the higher the number of PCB layers, the more complicated the manufacturing process and the higher the cost.

(2) The less the lead bends between the pins of high-speed PCB devices, the better. It is best to use a full straight line for the wiring of the high-frequency circuit, and a 45-degree broken line or a circular arc can be used for turning. This requirement is only used to improve the fixing strength of the steel foil in a low-frequency PCB, while meeting this requirement in a high-frequency PCB can reduce the external emission and mutual coupling of high-frequency signals. There are two pre-settings when using Protel for wiring: one is to schedule 45/90 Line or 90 ArcLine wiring in the Track Mode submenu of the Options menu; the other is the RoutingPasses dialog box opened in the Setup Autorouter item of the Auto menu Select Add Arcs to round the corners at the end of the automatic routing.

(3) The shorter the lead between the pins of the high-frequency circuit device, the better. Protel’s most effective way to meet the shortest wiring is to make a wiring appointment for individual key high-speed networks before the automatic market line. First, open the Edit Net of the Netlst menu. A Change Net dialog box will appear in the submenu. Change the OptimizeMethod in this dialog box. The layout optimization mode can be selected as Shortest Shortest. Secondly, when considering the overall component layout, use Placement Tools Shove in Auto and Density in Auto to compare and adjust the components to make the arrangement compact, and cooperate with the Length function and the Length function in the Netlist menu. The Lengthof selection function in the Info menu measures the wiring length of the selected key network that needs to be minimized.

(4) The less the lead layer alternate between the pins of the high frequency circuit device, the better. The so-called inter-layer alternation of the leads, the better means that the fewer vias used in the component connection process, the better. It is measured that one via can bring about 0.5 pF of distributed capacitance, and reducing the number of vias can significantly increase the speed. The Protel software specifically provides this function. It has an Advanced column in the RoutingPasses dialog box opened by the Setup Autorouter item of the Auto menu, and you can set Smoothing to ON.

(5) When wiring high-frequency circuits, pay attention to the cross-interference introduced by the parallel routing of signal lines at close distances. If parallel distribution is unavoidable, a large area of ground can be arranged on the opposite side of the parallel signal lines to greatly reduce interference. Parallel wiring in the same layer is almost unavoidable, but the directions of the two adjacent layers must be taken as mutual Vertical, this is not difficult to do in Protel, but it is easy to overlook. The Routing Lagers dialog box opened by the SetupAutorouter item of the Auto menu allows the routing direction of each layer to be pre-determined. There are three pre-selected directions for Horizontal Vertical And No Prefer ence, many users are accustomed to choosing No Preference without specific orientation. They think that the distribution rate is high. However, in high-frequency circuit wiring, it is best to alternate horizontal and vertical wiring in adjacent layers, and parallel in the same layer. The wiring cannot be avoided, but a large area of ground can be laid on the reverse side of the printed board to reduce interference. This is for the commonly used double-sided board, and the multi-layer board can use the middle power layer to achieve this function. In the past, Protel software only provided a simple Fill function to meet this demand. Now Protel under Windows also provides a more powerful function of placing Polygon Plane in the Place option of the Edit menu, that is, polygon grid strip copper. Foil surface. If the polygon is taken as a surface of the entire printed board when it is placed, and this grid strip is connected to the GND network of the circuit, then this function will be able to realize the copper paving operation on one side of the entire circuit board. In addition to improving the high-frequency anti-interference ability mentioned earlier, the copper circuit board also has great benefits for heat dissipation and printed board strength. In addition, if tinned grids are added to the fixed place on the metal chassis of the circuit board, not only It can improve the fixing strength, ensure good contact, and can use the metal chassis to form a suitable public line. After turning on this function in the software menu, you will see a Place Polygon Plane dialog box, it will ask you whether you want to connect the placed polygon grid strip to the network connect net, if you turn on this option, you will be prompted when you exit the dialog box. Give the name of the network you want to connect. Given that the GND network is connected, it will act as a shielding layer. At the same time, you will be asked whether the copper pattern is horizontal stripe horizonta, vertical stripe vertica, or grid both. Just choose, the grid will have a better shielding effect and the size of the grid will be used as the purpose. The determination depends on the interference frequency to be shielded.

(6) Implement ground wire surround measures for particularly important signal wires or local units. This measure can also be automatically implemented in the Protel software. It is the Outline SelectedItems under Place in the Edit menu, that is, the outline of the selected object is drawn. This function can be used to automatically perform the so-called Packet processing, of course, using this function for clock and other units to perform packet processing will also be very beneficial to high-speed systems.

(7) Various PCB signal traces cannot form a loop ground wire or a current loop. In addition to the shortest principle mentioned above, the routing principles of Protel automatic routing include X-direction, Y-direction and daisy-like routing methods. The use of daisy-like routing can effectively avoid the formation of loops during PCB routing. Specifically, you can open the Edit Net of the Netlist menu, and a Change Net dialog box will appear in the submenu. Select the Optimize Method wiring optimization mode in this dialog box as Daisy Chain. www.yijunchina.com

(8) A high-frequency PCB decoupling capacitor should be set near each integrated PCB circuit block. Since the Protel software does not consider the positional relationship between the decoupling capacitor and the integrated circuit to be decoupled when automatically placing the components, if the software is placed to make the two too far apart, the decoupling effect will be greatly reduced. At this time, the components must be moved manually Edit Move component method, intervene in advance to make the two positions closer.