PCB Technical - Skills of Protel Software in High Frequency Circuit Wiring

Digital devices are developing in the direction of high speed, low power consumption, small size, and high anti-interference. This development trend puts forward many new requirements for the design of printed circuit boards. The application of Protel software in China is quite common. However, it is not Few designers only focus on the "distribution rate" of the Protel software, and the improvements made to the Protel software to adapt to the changes in device characteristics have not been used in the design. This not only makes the waste of software resources more serious, but also makes many new devices. Excellent performance is difficult to play. This article intends to introduce the general requirements of high-frequency circuit wiring at the same time, taking Protel for Windows V1.5 software as an example to introduce some special countermeasures that Protel software can provide when high-frequency circuit wiring.

(1) High-frequency circuits tend to have high integration and high wiring density. The use of multilayer boards is not only necessary for wiring, but also an effective means to reduce interference. Protel for Windows V1.5 can provide 16 copper wire layers and 4 power supplies Layers, reasonable selection of the number of layers can greatly reduce the size of the printed board, make full use of the intermediate layer to set up the shield, can better realize the nearest grounding, can effectively reduce the parasitic inductance, can effectively shorten the transmission length of the signal, and can greatly Reducing cross-interference between signals, etc., all of which are beneficial to the reliable operation of high-frequency circuits. According to data, when the same material is used, the noise of a four-layer board is 20dB lower than that of a double-sided board. However, the higher the number of board layers, The more complex the manufacturing process, the higher the cost.

(2) The less bending of the leads between the pins of high-speed circuit devices, the better. The leads of high-frequency circuit wiring are best to use a full straight line, which needs to be turned. 45-degree fold lines or arcs can be used. This requirement is only in low-frequency circuits. It is used to improve the fixing strength of the steel foil. In high-frequency circuits, meeting this requirement can reduce the external emission and mutual coupling of high-frequency signals. When using Protel wiring, it can be preset in the following two places, one is in In the “Track Mode” submenu of the “Options” menu, make an appointment for wiring in 45/90 Line or 90 Arc/Line mode. Set "Add Arcs" so that the corners will be rounded 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 "wiring" reservations for individual key high-speed networks before the automatic market line. First, open "Netlst" In the "Edit Net" submenu of the menu, a "Change Net" dialog box will appear. Select "OptimizeMethod" in this dialog box as "Shortest (shortest)" Rp. Secondly, consider the overall, Use Placement Tools-Shove in “Auto” and “Density” in “Auto” to compare and adjust the component layout to make the component arrangement compact, and cooperate with the “Length” function and “Length” in the “Netlist” menu. The Length of selection in the Info menu can measure the wiring length of the selected key network that needs to be minimized.

(4) The less the lead layer alternate between the pins of high-frequency circuit devices, the better. The so-called "the less the lead layer alternates, the better" means that the fewer vias (Via) used in the component connection process, the better. It is measured that a via can bring about 0.5 pF of distributed capacitance, reducing the number of vias can significantly increase the speed. Protel software specifically provides this function, it is in the Auto menu Setup Autorouter... "Routing Passes" item turned on In the dialog box, there is an "Advanced" column, just set the "Smoothing" to ON.

(5) High-frequency circuit wiring should pay attention to the "cross interference" introduced by the close parallel routing of the signal line. If parallel distribution cannot be avoided, a large area of "ground" can be arranged on the opposite side of the parallel signal line to greatly reduce the interference. The same Parallel wiring in the layer is almost unavoidable, but in the 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 "Setup" of the "Auto" menu The Routing Lagers dialog box opened by Autorouter... allows you to pre-determine the routing direction of each layer. There are three pre-selected directions: "Horizontal, Vertical and No Prefer-ence". Many users are used to choosing "No Preference( No specific orientation)”, it is considered that the distribution rate is high. However, in the high-frequency circuit wiring, it is best to alternate horizontal and vertical wiring in adjacent layers. Parallel wiring in the same layer cannot be avoided, but it can be printed in the printed circuit. A large area of ground wire is laid on the back of the board to reduce interference (this is for the commonly used double-sided board, the multi-layer board can use the middle power layer to achieve this function), the Protel software only provided a simple "Fill" function in the past. To cope with this demand, now Protel under Windows also provides a more powerful function of placing "Polygon Plane" in the "Place" option of the "Edit" menu, namely: polygonal 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 (bar) is connected to the GND network of the circuit, then this function will be able to realize the "layout of a certain surface of the entire circuit board". "Copper" operation, the "copper" circuit board can not only improve the high-frequency anti-interference ability just mentioned, but also has great benefits for heat dissipation and the strength of the printed board. In addition, the fixed place on the metal chassis of the circuit board If you add a tin-plated grid, you can not only improve the fixing strength and ensure good contact, but also use the metal chassis to form a suitable public line. After opening this function in the software menu, you will see a "Place Polygon Plane dialog box", it will ask Do you want to connect the placed polygonal grid (strip) to the network (connect net), if you connect this option, you will be prompted to give the name of the network you want to connect to when you exit the dialog box, and the given GND network is connected Will be able to play the role of a shielding layer. At the same time, you will be asked whether the "copper" pattern uses horizontal strips (horizonta), vertical strips (vertica) or grid (you can choose both). There will be a better shielding effect, and at the same time, the size of the grid network (usually called "mesh") is determined according to the interference frequency to be shielded.

(6) Implement ground wire surround measures for particularly important signal lines or local units. This measure can also be automatically implemented in the Protel software, which is the "Outline Select-ed Items" under "Place" in the "Edit" menu. Namely: draw the outer contour line of the selected object. With this function, the selected important signal line can be automatically processed by the so-called "wrapped ground". Of course, this function can be used for the part of the clock and other units to wrap the ground. It will also be very beneficial for high-speed systems.

(7) Various signal traces cannot form loops, and ground wires cannot form current loops. In addition to the shortest principle mentioned above, the routing principles of Protel automatic wiring are based on the X direction, the Y direction and the chrysanthemum The daisy wiring method can effectively avoid the formation of loops during wiring. Specifically, you can open the "Edit Net" submenu of the "Netlist" menu, and a "Change Net" dialog box will appear. In the "Optimize Method (wiring optimization mode)", select "Daisy Chain".

(8) A high-frequency decoupling capacitor should be installed near each integrated circuit block. Because the Protel software does not consider the position relationship between the decoupling capacitor and the decoupled integrated circuit when automatically placing the components, let the software place it, If the two are too far apart, the decoupling effect will be greatly reduced. At this time, you must manually move the components ("Edit", "Move" and "component") to intervene in the two positions in advance to make them close.

(9) A high-frequency choke link is used when the analog ground wire, digital ground wire, etc. are connected to the public ground wire. In the actual assembly of the high-frequency choke link, a high-frequency ferrite magnet with a wire through the center hole is often used. Bead, it is generally not expressed on the circuit schematic, the resulting netlist (netlist) does not contain such components, and its existence will be ignored when wiring. In view of this reality, it can be in the schematic diagram Think of it as an inductor, define a component package for it separately in the PCB component library, and manually move it to a suitable position near the common ground junction before wiring.