Electronic Design - Wiring skills for high-frequency circuits

(1) Wiring skills for high-frequency circuits

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.

The less the lead bends between the pins of high-frequency circuit devices, the better. The lead wire of high-frequency circuit wiring is best to adopt a full straight line, which needs to be turned. It can be turned by 45° broken line or circular arc. This requirement is only used to improve the fixing strength of copper foil in low-frequency circuits, while in high-frequency circuits, this is satisfied. One requirement can reduce the external emission and mutual coupling of high-frequency signals.

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

The fewer lead layers of high-frequency circuit device pins alternate, the better. That is, the fewer vias (Via) used in the component connection process, the better. It is measured that one via can bring about 0.5pF distributed capacitance, and reducing the number of vias can significantly increase the speed.

For high-frequency circuit wiring, pay attention to the crosstalk introduced by the signal lines in close parallel routing. If parallel distribution cannot be avoided, a large area can be arranged on the opposite side of the parallel signal lines to greatly reduce interference. It is almost unavoidable to run horizontally in the same layer, but the directions of the two adjacent layers must be perpendicular to each other.

At least one high-frequency decoupling capacitor should be set near each integrated circuit block (IC), and the decoupling capacitor should be as close as possible to the Vcc of the device.

When the analog ground wire (AGND), digital ground wire (DGND), etc. are connected to the public ground wire, high-frequency choke should be used. In the actual assembly of the high-frequency choke link, the high-frequency ferrite bead with a wire in the center is often used. It can be used as an inductor in the schematic diagram, and a component package and wiring are defined separately for it in the PCB component library. Move it manually to a suitable position close to the common ground line.

(2) Design method of electromagnetic compatibility (EMC) in PCB

The choice of PCB base material and the setting of the number of PCB layers, the selection of electronic components and the electromagnetic characteristics of electronic components, the layout of components, the length and width of the interconnection lines between components, etc. all restrict the electromagnetic compatibility of the PCB. The integrated circuit chip (IC) on the PCB is the main energy source of electromagnetic interference (EMI).

1. Wiring rules in the electromagnetic compatibility (EMC) design of high-frequency digital circuit PCB

High-frequency digital signal cables should be short, generally smaller than 2inch (5cm), and the shorter the better.

The main signal lines are best concentrated in the center of the PCB board.

The clock generating circuit should be near the center of the PCB board, and the clock fan-out should be wired in daisy chain or parallel.

The power line should be as far away as possible from the high-frequency digital signal line or separated by the ground line. The distribution of the power supply must be low inductance (multi-channel design). The power layer in the multi-layer PCB is adjacent to the ground layer, which is equivalent to a capacitor, which plays a filtering role. The power line and ground line on the same layer should be as close as possible. The copper foil around the power layer should be retracted 20 times the distance between the two plane layers than the ground layer to ensure that the system has better EMC performance. The ground plane should not be divided. If the high-speed signal line is to be divided across the power plane, several low-impedance bridge capacitors should be placed close to the signal line.

The wires used for the input and output terminals should try to avoid being adjacent and parallel. It is best to add ground wires between wires to avoid feedback coupling.

When the thickness of the copper foil is 50um and the width is 1-1.5mm, the temperature of the wire will be less than 3 degree Celsius through a current of 2A. As far as possible, the wires of the PCB board should be as wide as possible. For integrated circuits, especially the signal wires of digital circuits, usually 4mil-12mil wire width is used. The power and ground wires are better to use a wire width greater than 40mil. The minimum spacing of the wires is mainly determined by the insulation resistance and breakdown voltage between the wires in the worst case, usually a wire spacing of more than 4mil is selected. In order to reduce the crosstalk between the wires, the distance between the wires can be increased if necessary, and the ground wire can be inserted as the isolation between the wires.

In all layers of the PCB, digital signals can only be routed in the digital part of the circuit board, and analog signals can only be routed in the analog part of the circuit board. The ground of the low-frequency circuit should be grounded in parallel at a single point as much as possible. When the actual wiring is difficult, it can be partially connected in series and then grounded in parallel. To realize the division of analog and digital power supplies, the wiring cannot cross the gap between the divided power supplies. The signal line that must cross the gap between the divided power supplies should be located on the wiring layer close to the large area ground.

There are two main electromagnetic compatibility problems caused by power supply and ground in PCB, one is power noise, and the other is ground noise. According to the size of the PCB board current, try to enlarge the width of the power line and reduce the loop resistance. At the same time, make the direction of the power line and ground line consistent with the direction of data transmission, which helps to enhance the anti-noise ability. At present, the noise of the power supply and ground plane can only be set to the default value through the measurement of prototype products or the capacity of decoupling capacitor by experienced engineers based on their experience.

2. Layout rules in the electromagnetic compatibility (EMC) design of high-frequency digital circuit PCB

The layout of the circuit must reduce the current loop, and shorten the connection between high-frequency components as much as possible. The distance between the susceptible components should not be too close, and the input and output components should be as far away as possible.

Arrange the position of each functional circuit unit according to the circuit flow, so that the layout is convenient for signal circulation, and the signal is kept in the same direction as possible.

Take the core component of each functional circuit as the center and lay out around it. The components should be evenly, neatly and compactly arranged on the PCB, and the lead connections between the components should be shortened as much as possible.

The PCB is partitioned into independent and reasonable analog circuit areas and digital circuit areas, and the A/D converters are placed across partitions.

One of the conventional methods of PCB electromagnetic compatibility design is to configure appropriate decoupling capacitors on each key part of the PCB.

(Three), signal integrity (SI) analysis

Signal Integrity (Signal Integrity) referred to as SI, refers to the quality of the signal on the signal line, and the ability of the signal to respond with the correct timing and voltage in the circuit.

The high switching speed of integrated circuit chips (IC) or logic devices, incorrect layout of termination components or incorrect wiring of high-speed signals can cause such as reflection, crosstalk, overshoot, and undershoot. Signal integrity problems such as undershoot) and ringing (ringing) may cause the system to output incorrect data, and the circuit may not work properly or even not work at all.

PCB signal integrity and design

In PCB design, PCB designers need to integrate the layout and wiring of components and which SI problem solving methods should be used in each case to better solve the signal integrity problem of the PCB board. In some cases, the choice of IC can determine the number and severity of SI problems. Switching time or edge rate refers to the rate of IC state transition. The faster the IC edge rate, the higher the possibility of SI problems. It is very important to correctly terminate the device.

The commonly used method to reduce signal integrity problems in PCB design is to add termination components on the transmission line. In the process of termination, it is necessary to weigh the requirements of the number of components, signal switching speed and circuit power consumption. For example, the addition of termination components means that PCB designers have less space for wiring, and it will be more difficult to add termination components in the later stages of the layout process, because appropriate space must be reserved for new components and wiring. Therefore, at the beginning of the PCB layout, it is necessary to figure out whether termination components need to be placed.