PCB Tech - High-frequency and high-speed PCB design issues

At present, high-frequency and high-speed PCB design has become the mainstream, and every PCB Layout engineer should be proficient. Next, Banermei will share with you some of the design experience of hardware experts in high-frequency and high-speed PCB circuits, and I hope it will be helpful to everyone.

What problems may be encountered in high-frequency and high-speed PCB design

1. How to avoid high frequency interference?

The basic idea of avoiding high-frequency interference is to minimize the electromagnetic field interference of high-frequency signals, which is the so-called crosstalk (Crosstalk). You can increase the distance between the high-speed signal and the analog signal, or add ground guard/shunt traces next to the analog signal. Also pay attention to the noise interference from the digital ground to the analog ground.

2. How to consider impedance matching when designing high-speed PCB design schematics?

When designing high-speed PCB circuits, impedance matching is one of the design elements. The impedance value has an absolute relationship with the wiring method, such as walking on the surface layer (microstrip) or inner layer (stripline/double stripline), distance from the reference layer (power layer or ground layer), wiring width, PCB material, etc. Both will affect the characteristic impedance value of the trace. That is to say, the impedance value can only be determined after wiring. Generally, simulation software cannot take into account some wiring conditions with discontinuous impedance due to the limitation of the circuit model or the mathematical algorithm used. At this time, only some terminators (termination), such as series resistance, can be reserved on the schematic diagram. Alleviate the effect of discontinuity in trace impedance. The real solution to the problem is to try to avoid impedance discontinuities when wiring.

3. In high-speed PCB design, which aspects should the designer consider EMC and EMI rules?

Generally, EMI/EMC design needs to consider both radiated and conducted aspects at the same time. The former belongs to the higher frequency part (<30MHz) and the latter is the lower frequency part (<30MHz). So you can't just pay attention to the high frequency and ignore the low frequency part. A good EMI/EMC design must take into account the location of the device, PCB stack arrangement, important connection method, device selection, etc. at the beginning of the layout. If there is no better arrangement beforehand, it will be solved afterwards. It will do twice the result with half the effort and increase the cost. For example, the location of the clock generator should not be close to the external connector. High-speed signals should go to the inner layer as much as possible. Pay attention to the characteristic impedance matching and the continuity of the reference layer to reduce reflections. The slew rate of the signal pushed by the device should be as small as possible to reduce the height. Frequency components, when choosing a decoupling/bypass capacitor, pay attention to whether its frequency response meets the requirements to reduce noise on the power plane. In addition, pay attention to the return path of high-frequency signal current to make the loop area as small as possible (that is, loop impedance as small as possible) to reduce radiation. You can also divide the ground to control the range of high-frequency noise. Finally, properly choose the chassis ground between the PCB and the housing.

4. How to choose PCB board?

The choice of PCB board must strike a balance between meeting design requirements and mass production and cost. The design requirements include both electrical and mechanical parts. Usually this material problem is more important when designing very high-speed PCB boards (frequency greater than GHz). For example, the commonly used FR-4 material, the dielectric loss at a frequency of several GHz will have a great influence on the signal attenuation, and may not be suitable. As far as electricity is concerned, pay attention to whether the dielectric constant and dielectric loss are suitable for the designed frequency.

5. How to meet EMC requirements as much as possible without causing too much cost pressure?

The increased cost of PCB board due to EMC is usually due to the increase of the number of ground layers to enhance the shielding effect and the addition of ferrite bead, choke and other high-frequency harmonic suppression devices. In addition, it is usually necessary to match the shielding structure on other institutions to make the entire system pass the EMC requirements. The following only provides a few PCB board design techniques to reduce the electromagnetic radiation effect generated by the circuit.

Try to choose devices with slower signal slew rate to reduce the high frequency components generated by the signal.

Pay attention to the placement of high-frequency components, not too close to the external connector.

Pay attention to the impedance matching of high-speed signals, the wiring layer and its return current path, to reduce high-frequency reflection and radiation.

Place sufficient and appropriate decoupling capacitors on the power supply pins of each device to alleviate the noise on the power plane and ground plane. Pay special attention to whether the frequency response and temperature characteristics of the capacitor meet the design requirements.

The ground near the external connector can be properly separated from the ground, and the ground of the connector can be connected to the chassis ground nearby.

Ground guard/shunt traces can be appropriately used beside some special high-speed signals. But pay attention to the influence of guard/shunt traces on the characteristic impedance of the trace.

The power layer shrinks 20H from the ground layer, and H is the distance between the power layer and the ground layer.

6. What aspects should be paid attention to when designing, routing and layout of high frequency PCB above 2G?

High-frequency PCBs above 2G belong to the design of radio frequency circuits and are not within the scope of discussion of high-speed digital circuit design. The layout and routing of the radio frequency circuit should be considered together with the schematic, because the layout and routing will cause distribution effects. Moreover, some passive devices in the design of radio frequency circuits are realized through parameterized definitions and special-shaped copper foils. Therefore, EDA tools are required to provide parameterized devices and edit special-shaped copper foils. Mentor's boardstation has a special RF design module that can meet these requirements. Moreover, general RF design requires specialized RF circuit analysis tools. The most famous in the industry is agilent's eesoft, which has a good interface with Mentor's tools.

7. Will adding test points affect the quality of high-speed signals?

Whether it will affect the signal quality depends on the method of adding test points and how fast the signal is. Basically, additional test points (do not use the existing via or DIP pin as test points) may be added to the line or pulled a short line from the line. The former is equivalent to adding a small capacitor on the line, the latter is an extra branch. Both of these conditions will affect the high-speed signal more or less, and the extent of the effect is related to the frequency speed of the signal and the edge rate of the signal. The magnitude of the impact can be known through simulation. In principle, the smaller the test point, the better (of course, it must meet the requirements of the test tool) the shorter the branch, the better.