Those wiring skills required for high-frequency circuit design

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Those wiring skills required for high-frequency circuit design [Copy link]

    High-frequency circuits are often highly integrated and have high wiring density. The use of multi-layer boards is necessary for wiring and is also an effective means to reduce interference. In the PCB layout stage, the reasonable selection of the size of a certain number of printed circuit boards can make full use of the middle layer to set up shielding, better achieve nearby grounding, and effectively reduce parasitic inductance and shorten the transmission length of the signal. At the same time, it can also greatly reduce the cross interference of the signal, etc. All these methods are beneficial to the reliability of high-frequency circuits. When the same material is used, the noise of a four-layer board is 20dB lower than that of a double-layer 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 that when performing PCB layout, in addition to selecting a PCB board with a suitable number of layers, it is also necessary to carry out reasonable component layout planning and use correct wiring rules to complete the design. The following summarizes some experiences of high-frequency wiring: 1. The fewer the interlayer alternation of the leads between the pins of high-frequency circuit devices, the better. The so-called "the fewer the interlayer alternation of the leads, the better" means that the fewer vias (Via) used in the component connection process, the better. A via can bring about 0.5pF of distributed capacitance. Reducing the number of vias can significantly increase the speed and reduce the possibility of data errors. 2. The shorter the lead between the pins of high-frequency circuit devices, the better. The radiation intensity of the signal is proportional to the length of the signal line. The longer the high-frequency signal lead is, the easier it is to couple to the components close to it. Therefore, for high-frequency signal lines such as signal clocks, crystal oscillators, DDR data, LVDS lines, USB lines, HDMI lines, etc., the shorter the line is, the better. 3. The fewer bends in the leads between the pins of high-speed electronic devices, the better. It is best to use a full straight line for the leads of high-frequency circuit wiring. If a turn is required, a 45-degree fold line or an arc turn can be used. This requirement is only used to improve the adhesion strength of the copper foil in low-frequency circuits, but in high-frequency circuits, meeting this requirement can reduce the external emission of high-frequency signals and mutual coupling. 4. Pay attention to the "crosstalk" introduced by the close parallel routing of signal lines. When wiring high-frequency circuits, pay attention to the "crosstalk" introduced by the close parallel routing of signal lines. Crosstalk refers to the coupling phenomenon between signal lines that are not directly connected. Since high-frequency signals are transmitted along the transmission line in the form of electromagnetic waves, the signal line will act as an antenna, and the energy of the electromagnetic field will be emitted around the transmission line. The unwanted noise signal generated by the mutual coupling of the electromagnetic field between signals is called crosstalk. The parameters of the PCB board layer, the spacing of the signal line, the electrical characteristics of the driver and receiver, and the termination method of the signal line all have a certain impact on crosstalk. Therefore, in order to reduce the crosstalk of high-frequency signals, the following points should be achieved as much as possible during wiring: (1) When the wiring space permits, insert a ground wire or ground plane between two lines with more serious crosstalk to isolate them and reduce crosstalk. (2) When the space around the signal line itself has a time-varying electromagnetic field, if parallel distribution cannot be avoided, a large area of "ground" can be arranged on the opposite side of the parallel signal line to greatly reduce interference. (3) When the wiring space permits, increase the spacing between adjacent signal lines, reduce the parallel length of the signal line, and try to make the clock line perpendicular to the key signal line instead of parallel. (4) If parallel routing within the same layer is almost unavoidable, the routing directions on two adjacent layers must be perpendicular to each other. (5) In digital circuits, clock signals are usually signals with fast edge changes, which have large external crosstalk. Therefore, in the design, the clock line should be surrounded by ground wires and more ground wire holes should be drilled to reduce distributed capacitance, thereby reducing crosstalk; (6) For high-frequency signal clocks, low-voltage differential clock signals should be used as much as possible and grounding should be used. Attention should be paid to the integrity of the grounding holes; (7) Unused input terminals should not be left floating, but should be grounded or connected to the power supply (the power supply is also the ground in the high-frequency signal loop), because the suspended wire may be equivalent to the transmitting antenna, and grounding can suppress the transmission. Practice has proved that this method can sometimes be effective in eliminating crosstalk immediately. 5. When the ground wire of high-frequency digital signals and the ground wire of analog signals are separated, the analog ground wire, digital ground wire, etc. are connected to the common ground wire. High-frequency choke beads should be used to connect or directly isolate and select a suitable place for single-point interconnection. The ground potential of the ground wire of high-frequency digital signals is generally inconsistent. There is often a certain voltage difference between the two. Moreover, the ground wire of high-frequency digital signals often carries a very rich harmonic component of the high-frequency signal. When the digital signal ground wire and the analog signal ground wire are directly connected, the harmonics of the high-frequency signal will interfere with the analog signal through ground wire coupling. Therefore, under normal circumstances, the ground wires of high-frequency digital signals and analog signals should be isolated. This can be done by using a single-point interconnection at a suitable location or by using a high-frequency choke bead interconnection. 6. Add high-frequency decoupling capacitors to the power pins of the integrated circuit block. Each power pin of the integrated circuit block should have a high-frequency decoupling capacitor nearby. Adding high-frequency decoupling capacitors to the power pins can effectively suppress the interference caused by high-frequency harmonics on the power pins. 7. Avoid loops formed by routing. All kinds of high-frequency signal routing should not form loops as much as possible. If it cannot be avoided, the loop area should be as small as possible. 8. Good signal impedance matching must be ensured. During the transmission process, when the impedance does not match, the signal will be reflected in the transmission channel. The reflection will cause the synthesized signal to overshoot, causing the signal to fluctuate near the logic threshold. The fundamental way to eliminate reflection is to make the impedance of the transmission signal well matched. Since the greater the difference between the load impedance and the characteristic impedance of the transmission line, the greater the reflection, the characteristic impedance of the signal transmission line should be equal to the load impedance as much as possible. At the same time, it should be noted that the transmission lines on the PCB should not have sudden changes or corners, and the impedance of each point on the transmission line should be kept continuous as much as possible, otherwise reflections will occur between the sections of the transmission line. This requires that the following wiring rules must be followed when performing high-speed PCB wiring: (1) LVDS wiring rules require LVDS signal differential routing, line width 7mil, line spacing 6mil, the purpose is to control the impedance of HDMI differential signal pairs to 100+-15% ohms; (2) USB wiring rules require USB signal differential routing, line width 10mil, line spacing 6mil, ground line and signal line spacing 6mil; (3) HDMI wiring rules require HDMI signal differential routing, line width 10mil, line spacing 6mil, and the spacing between each two HDMI differential signal pairs exceeds 20mil; (4) DDR wiring rules DDR1 routing requires that signals should not pass through holes as much as possible, signal lines should be of equal width, lines should be equidistant, and routing must meet the 2W principle to reduce crosstalk between signals. For high-speed devices of DDR2 and above, high-frequency data routing is also required to be of equal length to ensure signal impedance matching.