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PCB Technology

PCB Technology - Strategy for designing 9 high-frequency circuit boards

PCB Technology

PCB Technology - Strategy for designing 9 high-frequency circuit boards

Strategy for designing 9 high-frequency circuit boards
2023-04-24
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Author:Circuit Board

1. High-frequency circuit boards often have high integration and high wiring density. Using high-frequency multilayer circuit boards is necessary for wiring and is an effective means of reducing interference. Protel for Windows V1.5 can provide 16 copper layers and 4 power layers. Reasonably selecting the number of layers can significantly reduce the board size, fully utilize the inner layers to set shields, achieve better local grounding, effectively reduce parasitic capacitance, shorten signal transmission length, and significantly reduce signal cross-talk interference, all of which are beneficial for the reliable operation of high-frequency circuit boards. Data shows that for the same data, a four-layer circuit board has 20dB lower noise than a double-sided board. However, the higher the number of layers, the more complex the manufacturing technology and the higher the cost.


2. It is preferable to minimize the wire bending between the pins of high-speed circuit components. The wiring of high-frequency circuit boards is best done using straight lines. If turns are necessary, 45-degree angled lines or curved turns can be used. In low-frequency circuits, this requirement is only used to increase the adhesion strength of the foil, but in high-frequency circuit boards, meeting this requirement can reduce the emission of high-frequency signals and mutual coupling. When using Protel for wiring, you can pre-set in two places: first, in the "Options" menu, under the "Track Mode" submenu, reserve routing with 45/90 Line or 90 Arc/Line; second, in the "Auto" menu, under "Setup Autorouter...", select "Add Arcs" in the "Routing Passes" dialog box to make the corners arc-shaped automatically when routing is finished.


3. The shorter the wiring between the pins of high-frequency circuit board components, the better. Protel meets the most effective means of minimizing wiring by pre-booking individual high-speed networks for "wiring" before automatic wiring. First, open the "Netlst" menu and select the "Edit Net" submenu to bring up the "Change Net" dialog box, and choose "Shortest (minimize)" as the "Optimize Method" in this dialog box. Next, from an overall perspective, use the "Auto" Placement Tools - shove and "Auto" Density Check to adjust the component layout for a compact arrangement. Then, use the "Netlist" menu's "Length" function and the "Info" menu's "Length selection" function to measure the wiring length of the selected key networks that need to be minimized.


4, High-frequency circuit board device pins between the lead layer alternation;the less, the better. The so-called "less alternation between the layers of the lead, the better" refers to the components used in the process of connecting the hole (Via),The less the better.According to the test, a hole can bring about 0.5 pF of the distribution capacitance; reduce the number of holes can significantly improve the speed. Protel software specifically provides this function; it is in the Auto menu Setup Autorouter ..." item opened by the Routing Passes "dialogue box, there is a" Advanced "column; set the "Smoothing" to on.

 

5. High-frequency circuit board layout should pay attention to the signal lines close to the parallel alignment of the introduction of "cross-interference." if you can not avoid parallel distribution, parallel signal lines can be laid on the reverse side of a large area of "ground" to significantly reduce interference. Parallel alignments within the same layer are almost unavoidable, but in two neighboring layers, the alignments must be perpendicular to each other, which is not difficult to do in Protel but easy to ignore. The Routing Lagers dialogue box opened in the "Setup Autorouter..." section of the "Auto" menu allows the routing direction of each layer to be predefined, and there are three preselected directions: quot; Horizontal, Vertical, and No Prefer-ence", Many users are accustomed to selecting "no preference (no specific orientation)", thinking that this is a high throughput rate, however, in the high frequency However, in high-frequency circuit board wiring, it is best to alternate between horizontal and vertical wiring on adjacent layers. 

 

Parallel routing within the same layer is unavoidable, but it is possible to reduce interference by laying a large area of earth wire on the reverse side of the printed circuit board (this is for the commonly used double-panel, high-frequency multi-layer circuit boards that can make use of the intermediate power supply layer to realise this function), and in the past, Protel software only provided a simple "Fill" function to cope with this demand. Now Protel for Windows provides a more powerful "Polygon Plane" function in addition to the "Place" option in the "Edit" menu, i.e., the ability to place a "Polygon Plane". The function of "Polygon Plane", i.e., polygonal grid (strip) copper foil surface, is that, if you take the polygon as a surface of the whole printed circuit board when placing it, and connect this grid (strip) with the GND network of the circuit, then this function will be able to realize the operation of "copper-laying" for a certain side of the whole circuit board. "Copper" of the circuit board in addition to improve the high-frequency anti-interference capability mentioned earlier, but also on the heat dissipation, printing plate strength and so on have a great deal of good, in addition, in the circuit board metal chassis of the fixed place if the tinned grid, not only to improve the fixed strength to protect the contact is good, but also the use of metal chassis can be constructed into a suitable common line. 


High-frequency Circuit Board

High-frequency Circuit Board

In the software menu open this function can be seen after a "Place Polygon Plane dialogue box, it will ask you whether you want to place the polygonal grid (bar) and the network connection (connect net). If connected to the item, exit the dialogue box will prompt you to give the name of the network you want to connect to the given connection the GND network will be able to play the role of the mask layer. If it is connected, you will be prompted to give the name of the network you want to connect to when you exit the dialogue box. You will also be asked if you want to use horizonta, vertica, or raster for the "copper tiling" pattern (choose both). Raster will have a better masking effect, and the size of the raster mesh (customarily called "eye") is determined by the frequency of the interference to be masked.

 

6. The particularly important signal lines or local units of the implementation of ground encirclement measures. The measures in the Protel software can also be achieved automatically, it is the "Edit" menu "Place" under the "Outline Select-ed Items" under "Place" in the "Edit" menu, i.e. Drawing the outer contour lines of the selected object. With the use of this function, you can automatically select the important signal line for the so-called "packet" processing, of course, this function is used in the clock, and other units of local packet processing of high-speed systems will also be very beneficial.

 

7. All kinds of signal alignment can not form a loop, ground can not form a current loop. Protel automatic routing principles, in addition to the shortest principle mentioned earlier, there are based on the X-direction, based on the Y-direction and daisy-shaped (daisy) routing pipeline, daisy-shaped routing can effectively avoid the formation of loops when routing. Specifically, you can open the "Edit Net" sub-menu of the 'Netlist' menu and a "Change Net" dialogue box will appear. Optimize Method" in this dialogue box, select "Daisy Chain" to "Optimise Method".

 

8, Each integrated circuit block should be set near a high-frequency decoupling capacitor. As the Protel software in the automatic placement of components does not take into account decoupling capacitors and decoupled from the position of the integrated circuit between the relationship, let the software placed, so that the two are too far apart, the decoupling effect is greatly reduced, it is necessary to manually move the components ("Edit", "Move", "Component"). " "component") of the method of prior intervention in the location of the two, so that it is close.

 

9, Analogue ground, digital ground, etc. should be connected to the common ground with a high-frequency choke. In the actual assembly of high-frequency choke link is often used in the centre of the hole wearing a wire of high-frequency ferrite beads, in the circuit schematic on the general expression of it, the formation of the network meter (netlist) does not contain such components, wiring will be ignored in this and its existence. In order to deal with this reality, it can be treated 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 common ground convergence point before wiring.