1. High-frequency circuit boards tend to be highly integrated and have high wiring density. The use of high-frequency multi-layer circuit 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 supply layers. Reasonable selection of the number of layers can greatly reduce the size of the printed board. It can make full use of the middle layer to set the mask and better achieve nearby grounding. It can effectively reduce parasitic inductance, effectively shorten the transmission length of signals, and significantly reduce cross-interference between signals, etc. All of these are beneficial to the reliable operation of high-frequency circuit boards. Some data show that when using the same material, a four-layer circuit board has 20dB lower noise than a double-sided circuit board. However, the higher the number of layers, the more complex the manufacturing technology and 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 board wiring are best to use straight lines. If they need to be turned, they can be 45-degree folded lines or arc turns. This requirement is only used to improve the fixation strength of the steel foil in low-frequency circuits, while in high-frequency circuit boards, Meeting this requirement can reduce the external emission and mutual coupling of high-frequency signals. When using Protel for wiring, you can pre-set it in the following two places. One is to reserve 45/90 Line or 90 Arc/Line pipeline wiring in the "Track Mode" sub-menu of the "Options" menu; the other is to make a reservation in the "Auto" menu Select "Add Arcs" in the Routing Passes dialog box opened by the "Setup Autorouter..." item so that the corners are arced when the automatic routing ends.
3. The shorter the leads between the pins of high-frequency circuit board devices, the better. Protel's most effective way to minimize cabling is to make "cabling" reservations for individual key high-speed networks before automatic city lines. First, open the "Edit Net" submenu of the "Netlst" menu, and a? quot; Change Net" dialog box, select "OptimizeMethod (wiring optimization mode)" in this dialog box as "Shortest (minimization)" Rp. Secondly, from the overall consideration, use the Placement Tools in "Auto" when placing components - Compare and adjust Shove' with "Density (density check)" in "Auto" to make the components more compact, and cooperate with the "Length" function in the "Netlist" menu and the "Lengthof selection" function in the "Info" menu. The wiring length of the selected key network that needs to be minimized is measured.
4. The less alternation between lead layers between pins of high-frequency circuit board devices, the better. The so-called "the less inter-layer alternation of leads, the better" means that the fewer vias (Via) used in the component connection process, the better. According to measurements, one via can bring about 0.5 pF distributed capacitance, reducing the number of vias. Can significantly increase speed. Protel software specifically provides this function. In the "Routing Passes" dialog box opened by the "Setup Autorouter..." item in the Auto menu, there is an "Advanced" column. Just set "Smoothing" in it to "On".
5. When wiring high-frequency circuit boards, pay attention to the "cross-interference" caused by close parallel routing of signal lines. If parallel distribution cannot be avoided, a large area of "ground" can be arranged on the opposite side of the parallel signal lines to greatly reduce interference. Parallel traces in the same layer are almost unavoidable, but in two adjacent layers, the trace directions must be perpendicular to each other. This is not difficult to do in Protel but is easy to ignore. In the Routing Lagers dialog box opened by the "Setup Autorouter..." item in the "Auto" menu, the routing direction of each layer is allowed to be predetermined. There are three directions for preselection: "Horizontal, Vertical and No Prefer-ence". Many users are accustomed to choosing "No Preference (no specific orientation)", thinking that this has a higher routing rate. However, in high-frequency circuit board wiring, it is best to alternate horizontal and vertical wiring on adjacent layers.
Parallel wiring within the same layer cannot be avoided, but ground wires can be laid over a large area on the back of the printed board to reduce interference (this is for commonly used double-sided boards. High-frequency multi-layer circuit boards can use the power layer in the middle to achieve this. function), Protel software used to only provide a simple "Fill" function to meet this demand. Now Protel under Windows also provides a more powerful "Place" option in the "Place" option of the "Edit" menu. The function of "Polygon Plane" is: the polygonal grid (strip) copper foil surface. If you take the polygon as a surface of the entire printed board when placing it, and connect this grid (strip) to the GND network of the circuit, Then, this function will be able to realize the "copper laying" operation on a certain side of the entire circuit board. In addition to improving the high-frequency anti-interference ability just mentioned, the "copper laying" circuit board can also improve heat dissipation and printed board strength. etc. has great benefits. In addition, if a tin-plated grid is added to the fixing point on the metal main case of the circuit board, it can not only improve the fixing strength and ensure good contact, but also use the metal main case to form a suitable common 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 connect this item, exit In the dialog box, you will be prompted to give the name of the network you want to connect. If you connect the GND network, it will function as a mask layer. At the same time, you will also be asked whether the "copper" pattern should be made with horizontal strips (horizonta), Vertical strips (vertica) or grid (you can choose both). Choosing grid will have a better masking effect. At the same time, the size of the grid (commonly called "mesh") is determined according to the focus. The mask depends on the interference frequency.
6. Implement ground wire surround measures for particularly important signal lines or local units. This method can also be automatically implemented in the Protel software. It is "Outline Select-ed Items" under "Place" in the "Edit" menu, that is: drawing the outer contour of the selected object. Using this function, you can automatically perform so-called "ground wrapping" processing on selected important signal lines. Of course, using this function to perform local wrapping processing on units such as clocks will also be very beneficial to high-speed systems.
7. Various signal traces cannot form loops, and ground wires cannot form current loops. In addition to the minimization principle mentioned earlier, Protel's automatic wiring routing principles include X-direction, Y-direction-based and daisy-shaped routing pipes. The use of daisy-shaped routing can effectively avoid loops during wiring. Specifically, you can open the "Edit Net" sub-menu of the 'Netlist' menu, a "Change Net" dialog box will appear, and select "Daisy Chain" for "Optimize Method (wiring optimization mode)" in this dialog box.
8. A high-frequency decoupling capacitor should be set near each integrated circuit block. Since the Protel software does not consider the positional relationship between the decoupling capacitor and the decoupled integrated circuit when automatically placing components, allowing the software to place them will make the two too far apart, and the decoupling effect will be greatly reduced. At this time, it must be moved manually. The methods of components ("Edit", "Move" and "component") intervene in the position of the two in advance to bring them closer.
9. Use high-frequency choke links when connecting analog ground wires, digital ground wires, etc. to the public ground wire. When actually assembling high-frequency choke links, high-frequency ferrite beads with wires pierced through the center holes are often used. They are generally not expressed on the circuit schematic diagram, and the resulting network list (netlist) is not If this type of component is included, its existence will be ignored during wiring. To address this reality, you can treat it as an inductor in the schematic diagram, define a separate component package for it in the PCB component library, and manually move it to a suitable position near the meeting point of the common ground before wiring.
