PCB Blog - Wiring Skills of High Frequency Circuit of Printed Circuit Board

Protel software has been widely used in China. However, many designers only focus on the deployment rate of Protel software, and the improvements made to Protel software to adapt to the changes in device characteristics have not been used in the design. This not only causes a serious waste of software resources, but also makes it difficult to play the excellent performance of many new devices. This paper is intended to introduce the general requirements for high-frequency circuit wiring, Take Protel for Windows V1.5 software as an example to introduce some special countermeasures that Protel software can provide when wiring high-frequency circuit wiring.

(1) High frequency circuits often 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 supply layers. A reasonable selection of layers can greatly reduce the size of printed boards, make full use of the intermediate layer to set shielding, better realize nearby grounding, effectively reduce parasitic inductance, effectively shorten the transmission length of signals, and greatly reduce cross interference between signals. All these are beneficial to the reliable operation of high-frequency circuits. Some data show that the noise of the four layer board is 20dB lower than that of the double-sided board when the same material is used. However, the higher the number of layers, the more complex the manufacturing process, and the higher the cost.

(2) The lead between pins of high-speed circuit devices shall be bent as little as possible. The lead wire of high frequency circuit wiring should preferably be full straight line, and 45 degree broken line or arc turning can be used if necessary. This requirement is only used to improve the fixation strength of steel foil in low-frequency circuit, while meeting this requirement in high-frequency circuit can reduce the external transmission and mutual coupling of high-frequency signals. When using Protel for wiring, the following two settings can be made in advance: one is to reserve wiring in 45/90 Line or 90 ArcLine mode in the Track Mode submenu of the OpTIons menu; The other is to select Add Arcs in the RouTIngPasses dialog box opened by the Setup Autorouter item in the Auto menu, so that the corner can be arced at the end of automatic routing.

(3) The shorter the lead between pins of High frequency circuits components, the better. The most effective way for Protel to meet the shortest routing is to make routing reservation for some key high-speed networks before the automatic marketing. First, open Edit Net in the Netlst menu, and a Change Net dialog box will appear in the submenu. Select Shortest as the routing optimization mode of OpTimizeMethod in this dialog box, Secondly, when considering component layout as a whole, use Placement Tools Show in Auto and Density in Auto to compare and adjust to compact the component arrangement, and coordinate with the Length function in Netlist menu and the Length selection function in Info menu to measure the routing length of the selected key networks that need to be minimized.

(4) The lead layer alternation between pins of high-frequency circuit devices shall be as little as possible. The so-called less inter layer alternation of the lead wire is the better, which means that the less vias Via is used in the component connection process, the better. According to the measurement, a vias can bring about about 0.5 pF of distributed capacitance. Reducing the number of vias can significantly improve the speed. Protel software provides this function specifically. It has an Advanced column in the RoutingPasses dialog box opened by the Setup Autorouter item in the Auto menu. Set Smoothing in the Advanced column to On.

(5) For High frequency circuits wiring, attention shall be paid to the cross interference introduced by the parallel wiring of signal lines in a short distance. If parallel distribution cannot be avoided, a large area can be arranged on the opposite side of the parallel signal lines to greatly reduce interference. Parallel routing in the same layer is almost unavoidable, but the direction of routing in two adjacent layers must be perpendicular to each other. This is not difficult to do in Protel, but it is easy to ignore. In the Routing Lagers dialog box opened by the SetupAutorouter item in the Auto menu, it is allowed to preset the routing direction of each layer, There are three kinds of horizontal vertical and no preference directions for preselection. Many users are used to choosing no preference without specific orientation. They think that this is a high routing rate. 

However, in High frequency circuits wiring, it is better to take horizontal and vertical wiring alternately in adjacent layers. Parallel wiring in the same layer cannot be avoided, but ground wires can be laid in large areas on the reverse side of the printed board to reduce interference. This is for commonly used double-sided boards, The multilayer board can use the middle power layer 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 function to place Polygon Plane in the Place option of the Edit menu, that is, polygonal grid strip copper foil surface. If the polygon is taken as a surface of the entire printed board when it is placed, and the grid bar is connected to the GND network of the circuit, this function will be able to realize the copper laying operation on one side of the entire printed board. In addition to improving the high-frequency circuit wiring ability just mentioned, the high-frequency circuit wiring after copper coating is also very beneficial to heat dissipation, printed board strength, etc. In addition, if a tinned grid bar is added to the fixed position on the metal chassis of the circuit board, It can not only improve the fixed strength and ensure good contact, but also make use of metal chassis to form a suitable public line. After opening this function in the software menu, you can see a Place Polygon Plane dialog box, which will ask whether you want to connect the placed polygon grid bar with the network. If you connect this item, when you exit the dialog box, you will be prompted to give the name of the network you want to connect. 

If you connect the GND network, High frequency circuits will play the role of a shielding layer. At the same time, you will be asked whether the pattern of laying copper is horizontal bar, vertical bar, vertical bar It is OK to choose both grids. The grid will have a better shielding effect and the size of the grid is used as the purpose to determine according to the interference frequency of the key shielding.