PCB Tech - Summarize 13 points of high-frequency circuit layout skills

The less the lead bends between the pins of high-speed circuit devices, the better. It is best to use a full straight line for the wiring of the high-frequency circuit, which needs to be turned. It can be turned by a 45° broken line or a circular arc. Meeting this requirement can reduce the external emission and mutual coupling of high-frequency signals.

The shorter the lead between the pins of the high-frequency circuit device, the better.

High-frequency circuits tend to 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.

Various signal wires cannot form a loop, and the ground wire cannot form a current loop.

A high-frequency decoupling capacitor should be installed near each integrated circuit block.

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" refers to the vias used in the component connection process (Via, the less the better, it is measured that one via can bring about 0.5 pF of distributed capacitance, reducing the number of vias Can significantly increase speed.

For high-frequency circuit wiring, pay attention to the "cross-interference" introduced by the close parallel routing of signal lines. If parallel distribution cannot be avoided, a large area of "ground" 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 in two adjacent layers, the routing directions must be taken perpendicular to each other.

Implement ground wire enclosing measures for particularly important signal lines or local units, that is, draw the outer contour line of the selected object. Using this function, it is possible to automatically perform the so-called "package ground" processing on the selected important signal lines. Of course, using this function for the clock and other units to partially package processing is also very beneficial to high-speed systems.

Before DSP, off-chip program memory and data memory are connected to the power supply, filter capacitors should be added and placed as close as possible to the power supply pins of the chip to filter out power supply noise. In addition, it is recommended to shield around key parts such as DSP and off-chip program memory and data memory to reduce external interference.

Use high frequency choke links when connecting analog ground wires and digital ground wires to public ground wires. In the actual assembly of high-frequency choke links, high-frequency ferrite beads with wires through the center hole are often used. They are generally not expressed in the circuit schematics. The resulting netlist (netlist, not Including this kind of component, it will ignore its existence when wiring. For this reality, it can be regarded as an inductor in the schematic diagram, and a component package can be defined separately for it in the PCB component library, and it can be manually moved to At a suitable location near the junction of the common ground.

The analog circuit and the digital circuit should be arranged separately, and the power and ground should be connected at a single point after independent wiring to avoid mutual interference.

The off-chip program memory and data memory should be placed as close to the DSP chip as possible, and at the same time, the layout should be reasonable to keep the length of the data line and address line basically the same, especially when there are multiple memories in the system, consider the clock line to each memory clock The input distance is equal or a separate programmable clock drive chip can be added. For a DSP system, an external memory with an access speed similar to that of the DSP should be selected, otherwise the high-speed processing capability of the DSP will not be fully utilized. The DSP instruction cycle is nanosecond level, so the most common problem in DSP hardware system is high-frequency interference. Therefore, when making the printed circuit board (PCB, etc.) of the DSP hardware system, special attention should be paid to address lines and data lines. The wiring of the signal line should be correct and reasonable. When wiring, try to make the high-frequency line short and thick, and keep it away from the signal line that is susceptible to interference, such as analog signal line. When the circuit around the DSP is complicated, it is recommended to use the DSP and its clock The circuit, reset circuit, off-chip program memory, and data memory are made into the smallest system to reduce interference.

When following the above principles and proficient in the use of design tools, after manual wiring is completed, high-frequency circuits generally need to be simulated by advanced PCB simulation software in order to improve the reliability and productivity of the system.