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High Frequency PCB Routing Skills

Posted:02:45 PM July 19, 2018 writer: G

(1) High-frequency circuits tend to have high integration and high wiring density. 

The use of multi-layer boards is both necessary for wiring and an effective means to reduce interference. Protel for Windows V1.5 can provide 16 copper layers and 4 power layers. Reasonable selection of layers can greatly reduce the size of the board, make full use of the middle layer to set the shield, and better achieve near grounding. Ground reduction of parasitic inductance, can effectively shorten the transmission length of the signal, can greatly reduce the cross interference between signals and so on. All of these are beneficial to the reliable operation of high-frequency circuits. When the data shows the same material, the four-layer board is 20dB lower than the noise of the double-layer board, but the higher the number of layers, the more complicated the manufacturing process and the higher the cost.

(2) The less the lead bend between the pins of the high-speed circuit device, the better. 

The lead wire of the high-frequency circuit wiring is preferably a full line, and a turning point of 45 degrees or a circular arc is required for the turning. This requirement is only used to improve the fixing strength of the steel foil in the low frequency circuit, and meeting this requirement in the high frequency circuit can reduce the external emission and mutual coupling of the high frequency signal. When wiring with Protel, you can pre-set it in two places: one is to schedule the 45/90 Line or 90 ArcLine mode in the Track Mode submenu of the Options menu; the second is the RoutingPasses dialog box opened by the Setup Autorouter item in the Auto menu. Add Arcs is selected in it 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. 

The most effective way for Protel to meet the shortest cabling is to make wiring reservations for individual high-speed networks before the automatic market. First open the Edit Net of the Netlst menu. A submenu will display a Change Net dialog box, and the OptimizeMethod in this dialog box will appear. The wiring optimization mode is selected as Shortest, and secondly, the layout of the components is considered by using the Placement Tools Shove in Auto and the Density in Auto to make the components compact and match the Length function in the Netlist menu. The Lengthof selection function in the Info menu measures the routing length of the selected key network that needs to be minimized.

(4) The less alternating between the wiring layers between the pins of the high-frequency circuit device, the better. 

The less alternating between the layers of the lead wire, the better the via hole Via used in the component connection process is better. It is measured that a via hole can bring about a distributed capacitance of about 0.5 pF, and the number of via holes can be significantly increased. Protel software specifically provides this feature. It has an Advanced section in the RoutingPasses dialog that opens in the Setup Autorouter item of the Auto menu. Set Smoothing to ON.

(5) High-frequency circuit wiring should pay attention to the cross interference introduced by the parallel lines of the signal lines. 

If parallel distribution cannot be avoided, a large area can be placed on the opposite side of the parallel signal line to greatly reduce interference. Parallel traces in the same layer are almost inevitable, but the direction of the adjacent two layers must be taken as mutual Vertical, this is not difficult in Protel, but it is easy to ignore. In the Routing Lagers dialog box opened by the SetupAutorouter item in the Auto menu, the routing direction of each layer is allowed to be reserved. There are three horizontal verticals for the preselected direction. And No Prefer ence, many users are accustomed to choose No Preference without specific orientation, which is considered to be high, but in high-frequency circuit wiring, it is best to alternate between horizontal and vertical wiring in adjacent layers, parallel in the same layer. The trace can not be avoided, but the ground wire can be laid on the reverse side of the printing plate to reduce the interference. This is for the commonly used double panel, 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 cope with this demand. Now Protel under Windows also provides a more powerful placement of Polygon Plane in the Place option of the Edit menu, namely polygon grid bar copper. Foil surface. If you place the polygon as a face of the entire plate when you place it, and connect the grid bar to the GND network of the circuit, then this function will enable copper plating on one side of the entire board. In addition to improving the high-frequency anti-jamming capability just mentioned, the copper circuit board also has great advantages for heat dissipation and plate strength. In addition, if a tinned grid strip is added to the fixed portion of the circuit board metal chassis, It can improve the fixing strength, ensure good contact, and make use of 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, which will ask if you want to connect the placed polygon grid bar to the network and connect net. If you open the item, you will be prompted to exit the dialog box. Given the name of the network to be connected, given the network connected to GND will function as a shield, and will also ask if the copper pattern is horizontal horizontal, vertical vertica, or grid. If you choose, the grid will have better shielding effect, and the size of the grid net is used as the purpose to determine the interference frequency according to the key shielding.

(6) Measures for grounding the signal lines or local units that are particularly important. 

This measure can also be implemented automatically in Protel software. It is the Outline Select edItems under the Place of the Edit menu, which draws the outline of the selected object. This function can automatically perform the so-called important signal lines. Packet processing, of course, the use of this function for components such as clocks for packet processing is also very beneficial for high-speed systems.

(7) Various types of signal traces cannot form a loop ground line or form a current loop. 

In addition to the shortest principle described above, Protel's automatic routing guidelines are based on the X-direction, Y-based, and daisy-like daisy routing. Chrysanthemum routing can effectively avoid loops during routing. Specifically, you can open Edit Net in the Netlist menu. A Change Net dialog box appears in the submenu. Select Optimize Method routing optimization mode in this dialog box as Daisy Chain.

(8) A high frequency decoupling capacitor should be placed 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 the component is placed automatically, the software is placed so that the distance between the two is too far apart, and the decoupling effect is greatly reduced. At this time, the component must be manually moved. Edit Move component's approach, intervening in both positions to bring it closer.

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