PCB printed circuit board design high frequency wiring skills

2019-07-24 17:41:17

(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.

(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, which needs to be turned, and can be folded by a 45° fold line or a circular arc. To meet this requirement, the external transmission and mutual coupling of the high-frequency signal can be reduced.

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

(4) The less alternating between the wiring layers between the pins of the high-frequency circuit device, the better. The so-called "the minimum number of interlayer crossings is as good as possible" means that the fewer vias (Via) used in the component connection process, the better, it is estimated that one via hole can bring about a distributed capacitance of about 0.5 pF, reducing the number of vias. Can significantly increase the speed.

(5) The 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 “ground” can be arranged on the reverse side of the parallel signal lines to greatly reduce the interference. Parallel traces in the same layer are almost inevitable, but in the adjacent two layers, the direction of the traces must be taken perpendicular to each other.

(6) A measure of grounding enveloping a particularly important signal line or local unit, that is, drawing the outer contour of the selected object. With this function, the so-called "packet" processing can be automatically performed on the selected important signal lines. Of course, it is also very beneficial for the high-speed system to use this function for the local processing of the components such as the clock.

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

(8) A high frequency decoupling capacitor should be placed near each integrated circuit block.

(9) High-frequency turbulence links should be used when connecting analog ground lines and digital ground lines to public ground lines. In the actual assembly of the high-frequency turbulence link, the high-frequency ferrite bead through which the center hole is threaded is often used, and it is generally not expressed on the circuit schematic, and the resulting netlist is not Including such components, the wiring will ignore its existence. In response to this reality, it can be used as an inductor in the schematic, and a component package is defined separately in the PCB component library, and it is manually moved to a suitable position near the convergence point of the common ground line before wiring.

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

(11) Before the DSP, off-chip program memory and data memory are connected to the power supply, the filter capacitor should be added and placed as close as possible to the chip power supply pin to filter out the power supply noise. In addition, shielding is recommended around the DSP and off-chip program memory and data memory to reduce external interference.

(12) The off-chip program memory and data memory should be placed as close as possible to the DSP chip. At the same time, the layout should be reasonable, so that the length of the data line and the address line are basically the same, especially when there are multiple memories in the system, the clock line should be considered to each memory. The clock input distance is equal or a separate programmable clock driver chip can be added. For the DSP system, the external memory with the same access speed as the DSP should be selected, otherwise the high-speed processing capability of the DSP will not be fully utilized. DSP instruction cycle is nanosecond, so the most common problem in DSP hardware system is high frequency interference. Therefore, when making printed circuit board (PCB) of 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 away from the signal lines that are susceptible to interference, such as analog signal lines. When the circuit around the DSP is more complicated, it is recommended to make the DSP and its clock circuit, reset circuit, off-chip program memory, and data memory into a minimum system to reduce interference.

(13) After following the above principles and skillful use of design tools, after manual wiring is completed, high-frequency circuits generally need to use advanced PCB simulation software for simulation in order to improve the reliability and productivity of the system.

Due to space limitations, this article does not give a detailed introduction to the specific simulation, but the suggestion to everyone is that if there are conditions, we must simulate the system, here are a few basic concepts.

Give everyone a basic explanation.

What is electromagnetic interference (EMI) and electromagnetic compatibility (EMC)?

Electromagnetic interference (Electromagnetic InteRFerence) has both conducted and radiated interference. Conducted interference refers to the coupling (interference) of signals on one electrical network to another electrical network through a conductive medium. Radiated interference refers to an interference source that couples (interferes) its signal to another electrical network through space. In high-speed PCB and system design, high-frequency signal lines, integrated circuit pins, various types of connectors, etc. may become radiation interference sources with antenna characteristics, which can emit electromagnetic waves and affect other systems or other subsystems in the system. normal work.

What is signal integrity?

Signal integrity refers to the quality of a signal on a signal line. Signals with good signal integrity are those that have to be reached when needed. Poor signal integrity is not caused by a single factor, but by a combination of factors in the board design. The main signal integrity issues include reflection, oscillation, ground bounce, crosstalk, and so on.

What is reflection (reflecTIon)?

Reflection is the echo on the transmission line. A portion of the signal power (voltage and current) is transmitted to the line and reaches the load, but a portion is reflected. If the source and load have the same impedance, the reflection will not occur. A mismatch between the source and load impedances causes on-line reflections, and the load reflects a portion of the voltage back to the source. If the load impedance is less than the source impedance, the reflected voltage is negative. Conversely, if the load impedance is greater than the source impedance, the reflected voltage is positive. Variations in routing geometry, incorrect line termination, transmission through the connector, and discontinuity in the power plane can cause such reflections.