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1. What are the skills of high-frequency circuit PCB design?
The design of high-frequency PCB is a complicated process, and many factors may be directly related to the working performance of the high-frequency circuit. High-frequency circuit design and wiring are very important to the entire design. The following ten tips for high-frequency circuit PCB design are especially recommended:
One, multilayer PCB board wiring
High-frequency circuits tend to have high integration and high wiring density. The use of multi-layer boards is not only necessary for wiring, but also an effective means to reduce interference. In the PCB Layout stage, a reasonable selection of the printed board size with a certain number of layers can make full use of the intermediate layer to set up the shield, better realize the nearest grounding, and effectively reduce the parasitic inductance and shorten the signal transmission length, while still maintaining a large All of these methods are beneficial to the reliability of high-frequency circuits, such as the amplitude reduction of signal cross-interference. Some data show that when the same material is used, the noise of the four-layer board is 20dB lower than that of the double-sided board. However, there is also a problem. The higher the number of PCB half-layers, the more complex the manufacturing process, and the higher the unit cost. This requires us to select PCB boards with the appropriate number of layers when performing PCB Layout. Reasonable component layout planning, and use correct wiring rules to complete the design.
Second, the less the lead bends between the pins of high-speed electronic devices, the better
It is best to use a full straight line for the wiring of the high-frequency circuit, and it needs to be turned. It can be turned by a 45-degree broken line or a circular arc. This requirement is only used to improve the fixing strength of the copper foil in the low-frequency circuit, while in the high-frequency circuit, it can meet this requirement. One requirement can reduce the external emission and mutual coupling of high-frequency signals.
3. The shorter the lead between the pins of the high-frequency circuit device, the better
The radiation intensity of the signal is proportional to the trace length of the signal line. The longer the high-frequency signal lead, the easier it is to couple to the components close to it. Therefore, for the signal clock, crystal oscillator, DDR data, LVDS lines, USB lines, HDMI lines and other high-frequency signal lines are required to be as short as possible.
Fourth, the less the lead layer alternate between the pins of the high-frequency circuit device, the better
The so-called "the less the inter-layer alternation of the leads, the better" means that the fewer vias (Via) used in the component connection process, the better. According to the side, one via can bring about 0.5pF distributed capacitance, and reducing the number of vias can significantly increase the speed and reduce the possibility of data errors.
2. What needs to be paid attention to between PCB layouts
PCB board routing
The layout of printed wires should be as short as possible, especially in high-frequency circuits; the bends of printed wires should be rounded, and right or sharp corners will affect electrical performance in high-frequency circuits and high wiring density. ; When the two panels are wired, the wires on both sides should be perpendicular, oblique, or bent to avoid parallel to each other to reduce parasitic coupling; printed wires used as the input and output of the circuit should be avoided as far as possible. In order to avoid feedback, it is best to add a ground wire between these wires.
Width of printed wire
The width of the wire should be able to meet the electrical performance requirements and be convenient for production. Its minimum value is determined by the magnitude of the current it bears, but the minimum should not be less than 0.2mm. In high-density, high-precision printed circuits, the wire width and spacing are generally 0.3mm; the wire width should also consider its temperature rise in the case of large currents. The single-panel experiment shows that when the thickness of the copper foil is 50μm and the wire width is 1 The temperature rise is very small when the current is ~1.5mm and the current is 2A. Therefore, it is possible to meet the design requirements without causing a temperature rise by using a wire with a width of 1 ~ 1.5mm.
The common ground wire of the printed wire should be as thick as possible. If possible, use a line larger than 2 to 3 mm. This is particularly important in circuits with microprocessors. Because when the ground wire is too thin, due to the change of the current flowing, the ground potential changes, the level of the microprocessor timing signal is unstable, which will degrade the noise margin; for routing between the IC pins of the DIP package, 10- The principle of 10 and 12-12, that is, when two wires pass between the two pins, the diameter of the pad can be set to 50mil, and the line width and line spacing are both 10mil. When only one wire passes between the two pins, the diameter of the pad can be Set to 64mil, line width and line spacing are both 12mil.
