PCB Blog - Research on Protel 99 SE High Frequency PCB Board Design

With the advancement of electronic technology, the complexity and scope of application of PCB board (printed circuit boards) have developed rapidly. Designers engaged in high-frequency PCB boards must have the corresponding basic theoretical knowledge, and should also have rich experience in the production of high-frequency PCB boards. That is to say, whether it is the drawing of the schematic diagram or the design of the PCB board, it should be considered from the high-frequency working environment where it is located, so that an ideal PCB board can be designed. This paper mainly studies some problems in the design of high-frequency PCB based on Protel 99 SE from two aspects of manual layout and wiring of high-frequency PCB.

1、Layout design
Although Protel 99 SE has the function of automatic layout, it cannot fully meet the working needs of high-frequency circuits. It is often necessary to rely on the experience of the designer and according to the specific situation, first use the method of manual layout to optimize and adjust the position of some components, and then combine Automatic layout completes the overall design of the PCB board. Whether the layout is reasonable or not directly affects the life, stability, EMC (electromagnetic compatibility) of the product, etc., it must be considered from the overall layout of the circuit board, the accessibility of wiring and the manufacturability of the PCB board, mechanical structure, heat dissipation, EMI ( Electromagnetic interference), reliability, signal integrity and other aspects are considered comprehensively. Generally, components in fixed positions related to mechanical dimensions are placed first, then special and larger components are placed, and small components are placed. At the same time, to take into account the requirements of wiring, the placement of high-frequency components should be as compact as possible, and the wiring of signal lines should be as short as possible, thereby reducing cross-interference of signal lines.

1.1 Placement of positioning inserts in relation to mechanical dimensions
Power sockets, switches, interfaces between PCB boards, indicator lights, etc. are all positioning plug-ins related to mechanical dimensions. Usually, the interface between the power supply and the PCB board is placed at the edge of the PCB board, and there should be a distance of 3 mm to 5 mm from the edge of the PCB board; the indicator light-emitting diode should be placed accurately as needed; switches and some fine-tuning components, Such as adjustable inductance, adjustable resistance, etc. should be placed close to the edge of the PCB board to facilitate adjustment and connection; components that need to be replaced frequently must be placed in a position with fewer components for easy replacement.

1.2 Placement of special components
High-power tubes, transformers, rectifier tubes and other heating devices generate a lot of heat when they work at high frequencies, so ventilation and heat dissipation should be fully considered in the layout, and such components should be placed on the PCB board where the air is easy to circulate. place. High-power rectifier tubes and adjustment tubes should be equipped with radiators, and should be kept away from the transformer. Components that are afraid of heat, such as electrolytic capacitors, should also be kept away from heating devices, otherwise the electrolyte will be dried, resulting in increased resistance and poor performance, which will affect the stability of the circuit. Components that are prone to failure, such as adjusting tubes, electrolytic capacitors, relays, etc., should be placed in consideration of ease of maintenance. For test points that often need to be measured, care should be taken when arranging components to ensure that the test rods can be easily contacted. Due to the 50 Hz leakage magnetic field generated inside the power supply equipment, when it is connected to some parts of the low frequency amplifier, it will interfere with the low frequency amplifier. Therefore, they must be isolated or shielded. All levels of the amplifier can be arranged in a straight line according to the schematic diagram. The advantage of this arrangement is that the ground current of each level is closed and flowed at this level, which does not affect the work of other circuits. The input stage and output stage should be as far away as possible to reduce the parasitic coupling interference between them. Considering the signal transmission relationship between the functional circuits of each unit, the low-frequency circuit and the high-frequency circuit should also be separated, and the analog circuit and the digital circuit should be separated. The integrated circuit should be placed in the center of the PCB board, so as to facilitate the wiring connection between each pin and other devices. Devices such as inductors and transformers have magnetic coupling and should be placed orthogonally to each other to reduce magnetic coupling. In addition, they all have a strong magnetic field, and there should be an appropriately large space or magnetic shielding around them to reduce the impact on other circuits.

Appropriate high-frequency decoupling capacitors should be configured at key parts of the PCB. For example, an electrolytic capacitor of 10 μF to 100 μF should be connected to the input end of the PCB power supply, and a 0.01 pF should be connected near the power supply pins of the integrated circuit. ceramic capacitors. Some circuits are also equipped with appropriate high frequency or low frequency choke coils to reduce the influence between high and low frequency circuits. This point should be considered when designing and drawing the schematic diagram, otherwise it will affect the performance of the circuit. The spacing between components should be appropriate, and the spacing should take into account whether there is a possibility of breakdown or ignition between them. For amplifiers with push-pull circuits and bridge circuits, attention should be paid to the symmetry of the electrical parameters of the components and the symmetry of the structure, so that the distribution parameters of the symmetrical components are as consistent as possible. After completing the manual layout of the main components, the method of component locking should be adopted so that these components will not move during automatic layout. That is, execute the Edit change command or select Locked in the Properties of the component to lock it and no longer move.

1.3 Placement of common components
For common components, such as resistors, capacitors, etc., it should be considered from several aspects, such as the orderly arrangement of components, the size of the occupied space, the accessibility of wiring and the convenience of welding, etc., and the method of automatic layout can be adopted.

2. Design of wiring
Wiring is the general requirement for realizing high-frequency PCB board design on the basis of reasonable layout. Routing includes automatic routing and manual routing. Usually, regardless of the number of key signal lines, these signal lines are manually routed first. After the wiring is completed, the wiring of these signal lines is carefully checked. After the inspection is passed, they are fixed, and then other wirings are automatically routed. That is, the combination of manual and automatic wiring is used to complete the wiring of the PCB board.

In the wiring process of high-frequency PCB board, special attention should be paid to the following aspects.

2.1 The direction of wiring
The wiring of the circuit adopts a full straight line according to the flow direction of the signal, and can be completed with a 45° broken line or a circular arc curve when turning is required, which can reduce the external emission and mutual coupling of high-frequency signals. The wiring of high-frequency signal lines should be as short as possible. According to the operating frequency of the circuit, the length of the signal line wiring should be reasonably selected, which can reduce the distribution parameters and reduce the loss of the signal. When making double-sided panels, the wiring is perpendicular, oblique, or curved to intersect on two adjacent layers. Avoid being parallel to each other, which can reduce mutual interference and parasitic coupling. High-frequency signal lines and low-frequency signal lines should be separated as much as possible, and shielding measures should be taken if necessary to prevent mutual interference. For the signal input terminal with weak reception, it is easy to be interfered by external signals, and the ground wire can be used as a shield to surround it or shield the high-frequency connector. Parallel wiring should be avoided on the same level, otherwise distribution parameters will be introduced, which will affect the circuit. If it is unavoidable, a grounded copper foil can be introduced between two parallel lines to form an isolation line. In digital circuits, for differential signal lines, they should be routed in pairs, try to make them parallel and close together, and have little difference in length.

2.2 The form of wiring
In the wiring process of the PCB board, the width of the trace is determined by the adhesion strength between the wire and the insulating layer substrate and the strength of the current flowing through the wire. When the thickness of the copper foil is 0.05mm and the width is 1mm to 1.5mm, a current of 2A can be passed. The temperature will not be higher than 3 ℃. Except for some special traces, the width of other traces on the same layer should be as consistent as possible. The spacing of the wiring in the high frequency circuit will affect the size of the distributed capacitance and inductance, thereby affecting the loss of the signal, the stability of the circuit and the interference caused by the signal. In high-speed switching circuits, the spacing of the wires will affect the transmission time of the signal and the quality of the waveform. Therefore, the spacing of the wiring should be greater than or equal to 0.5 mm, and as long as it is allowed, the PCB board wiring should use a relatively wide line. There should be a certain distance between the printed wire and the edge of the PCB board (not less than the thickness of the board), which not only facilitates installation and machining, but also improves the insulation performance. When encountering a line that can only be connected in a large circle in the wiring, use the flying lead, that is, directly connect with a short line to reduce the interference caused by long-distance wiring. Circuits containing magneto-sensitive elements are more sensitive to the surrounding magnetic field, and the corners of the wiring are easy to radiate electromagnetic waves when high-frequency circuits are working. Cabling on the same level is not allowed to have crossovers. For lines that may intersect, the method of "drilling" and "winding" can be used to solve it, that is, let a lead "drill" from the gap under the pins of other devices such as resistors, capacitors, triodes, etc., or from a line that may intersect One end of the lead "wraps" over it. In special cases, if the circuit is very complex, in order to simplify the design, it is also allowed to use wire jumpers to solve the crossover problem. When the operating frequency of the high-frequency circuit is high, the impedance matching of the wiring and the antenna effect also need to be considered.

2.3 Wiring requirements for power cables and ground cables
According to the size of different working current, try to increase the width of the power line. The high-frequency PCB board should use a large area ground wire as much as possible and lay it on the edge of the PCB board, which can reduce the interference of external signals to the circuit; The voltage is closer to the ground voltage. The grounding method should be selected according to the specific situation. It is different from the low-frequency circuit. The grounding wire of the high-frequency circuit should be grounded at the nearest ground or at multiple points. The grounding wire should be short and thick to minimize the ground impedance. The allowable current requirements can reach 3 times the working current standard. The grounding wire of the speaker should be connected to the grounding point of the output stage of the power amplifier on the PCB board, and must not be grounded arbitrarily. During the wiring process, some reasonable wiring should be locked in time to avoid repeated wiring. That is, execute the EditselectNet command and select Locked in the pre-wiring properties to lock it and no longer move.

3. Design of pads and copper plating

3.1 Pads and Apertures
In the case of ensuring that the wiring spacing does not violate the designed electrical spacing, the design of the pad should be larger to ensure sufficient ring width. Generally, the inner hole of the pad is slightly larger than the lead diameter of the component, and the design is too large, and it is easy to form virtual welding during welding. The outer diameter D of the pad is generally not less than (d+1.2) mm, where d is the inner diameter of the pad. For some PCB boards with relatively high density, the value of the pad can be (d+1.0) mm. The shape of the pad is usually set to be circular, but the pad of the DIP packaged integrated circuit adopts a racetrack shape, which can increase the area of the pad in a limited space, which is beneficial to the soldering of the integrated circuit. The connection between the wiring and the pad should be a smooth transition, that is, when the width of the wiring entering the circular pad is smaller than the diameter of the circular pad, the teardrop design should be used. It should be noted that the size of the inner diameter d of the pad is different, and it should be considered according to the size of the actual component lead diameter, such as component holes, mounting holes and slot holes. The hole spacing of the pads should also be considered according to the installation method of the actual components. For example, components such as resistors, diodes, and tubular capacitors have two installation methods: "vertical" and "horizontal". The hole spacing of these two methods is different. In addition, the design of pad hole spacing should also consider the gap requirements between components, especially the gap between special components needs to be guaranteed by the hole distance between pads. In the high-frequency PCB board, the number of vias should be minimized, which can not only reduce the distributed capacitance, but also increase the mechanical strength of the PCB board. In a word, in the design of high-frequency PCB board, the design of the pad and its shape, aperture and hole spacing should not only consider its particularity, but also meet the requirements of the production process. Standardized design can not only reduce product cost, but also improve production efficiency while ensuring product quality.

3.2 Copper plating
The main purpose of copper coating is to improve the anti-interference ability of the circuit, and at the same time, it is very beneficial to the heat dissipation of the PCB board and the strength of the PCB board. However, large-area strip copper foil cannot be used, because when the PCB board is used for too long, a large amount of heat will be generated, and the strip copper foil is prone to expansion and falling off. Copper foil, and connect the grid with the grounding network of the circuit, so that the grid will have a better shielding effect. The size of the grid is determined by the frequency of interference to be shielded. After completing the design of routing, pads and vias, a DRC (Design Rule Check) should be performed. The differences between the designed graph and the defined rules are listed in detail in the inspection results, and the network that does not meet the requirements can be found. However, the DRC should be parameterized before wiring before running the DRC, that is, executing the ToolsDesign Rule Check command.

4、Conclusion
The design of high-frequency circuit PCB board is a complex process, involving many factors, which may be directly related to the working performance of high-frequency circuit. Therefore, designers need to continuously research and explore in actual work, accumulate experience, and combine new EDA (Electronic Design Automation) technology to design high-frequency circuit PCB board with excellent performance.