High-speed PCB design basic concepts

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

Electromagnetic Interference, there are two kinds of conducted interference and radiated interference. Conducted interference refers to the coupling (interference) of signals on one electrical network to another electrical network through a conductive medium. Radiatedinterference means that the interference source couples (disturbs) its signal to another electrical network through space. In high-speed PCB and system design, high-frequency signal lines, pins of integrated circuits, various types of connectors, etc., may become radiation interference sources having antenna characteristics, and can emit electromagnetic waves and affect other systems or other subsystems within the system. normal work.

Since the emergence of noise reduction technology for electronic systems in the mid-1970s, the United States FederalCommunications Commission proposed in 1992 and the European Union in 1992regulations concerning commercial digital products that require companies to ensure that their products meet stringent susceptibility standards. And launch criteria. Products that meet these regulations are called electromagnetic compatibility (EMC).  

2. What is signal integrity?

Signal integrity refers to the quality of the signal on the signal line. A signal with good signal integrity means having the necessary voltage level value when it is needed. Poor signal integrity is not caused by a single factor but is caused by a combination of factors inboard design. The main signal integrity issues include reflections, oscillations, ground bounce, crosstalk, and so on. Common signal integrity issues and solutions   

3. What is a reflection?

Reflection is the echo on the transmission line. Some of the signal power (voltage and current) is transmitted to the line and reaches the load, but part of it is reflected. If the source and the load have the same impedance, the reflection will not occur.

Impedance mismatch between the source and load leads to 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, whereas if the load impedance is greater than the source impedance, the reflected voltage is positive. Changes in the geometry of the wiring, incorrect wire termination, transmission through the connector, and discontinuity in the power plane can all cause such reflections.

4. What is crosstalk?

Crosstalk is the coupling between two signal lines, and mutual inductance and mutual capacitance between signal lines cause noise on the line. Capacitive coupling induces a coupling current, while inductive coupling induces a coupling voltage. PCB board parameters, signaling to space, driving end and receiving end electrical characteristics, and wire termination methods all have some impact on crosstalk.

5. What is overshoot and undershoot?

Overshoot is where the first peak or valley exceeds the set voltage—the highest voltage for rising edges and the lowest voltage for falling edges. Undershoot refers to the next valley or peak. Excessive overshoot can cause protection diodes to work and cause premature failure. Excessive undershoot can cause false clock or data errors(misoperations).

6. What is ringing and rounding?

The phenomenon of oscillation is repeated overshoot and undershoot. The oscillation of the signal and the surrounding oscillation are caused by excessive inductance and capacitance on the line. The oscillation is under-damped and the surrounding oscillation is over-damped. Signal integrity problems usually occur in periodic signals such as clocks. Oscillations and ringing are caused by many factors as well as reflections. Oscillations can be reduced by proper termination, but they cannot be completely eliminated.

7. What is the ground plane rebound noise and return noise?

A large current surge in the circuit can cause ground plane bounce noise (referred to as ground bounce). If a large number of chip outputs are turned on at the same time, there will be a large transient current flowing through the power plane of the chip and the board. The inductance and resistance of the chip package and the power plane will cause power supply noise. This will cause voltage fluctuations and changes in the true ground plane (0V). This noise will affect the operation of other components. The increase of the load capacitance, the decrease of the load resistance, the increase of the ground inductance, and the increase of the number of switching devices all result in an increase in ground bounce.

Since the ground plane (including power and ground) is divided, for example, the ground is divided into digital ground, analog ground, shield ground, etc., ground return noise is generated when the digital signal goes to the analog ground area. The same power plane may also be divided into 2.5V, 3.3V, 5V, and so on. Therefore, in the multi-voltage PCB design, the bounce noise and return noise of the ground plane need special care.

8. What is the difference between the time domain and the frequency domain?

The time domain is a time-based voltage or current change that can be observed with an oscilloscope. It is typically used to find out the delay from pins to pins, skew, overshoot, undershoot, and settling times.

The frequency domain is the process of changing the voltage or current based on the frequency and can be observed by a spectrum analyzer. It is commonly used for comparisons between waveforms and other EMI control limitations.

9. What is the impedance?

The impedance is the ratio of the input voltage to the input current on the transmission line (Z0 = V/I). When a source sends a signal to the line, it will block it from driving until 2 * TD, the source does not see its change, where TD is the delay of the line.

10. What is the settling time?

Settling time is the time required for an oscillating signal to settle to a specified final value.

11. What is the pin-to-pin delay?

The pin-to-pin delay is the time between the change of the state of the driver side and the change of the state of the receiver side. These changes usually occur at 50% of a given voltage. The minimum delay occurs when the output first crosses a given threshold and the maximum delay occurs when the output crosses the voltage threshold. Measuring all these Happening.

12. What is askew?

The offset of the signal is the time offset between arrivals of different receivers on the same network. The offset is also used to the time offset the clock and data on the logic gate.

13. What is the slope (slew rate)?

Slew rate is the slope of the edge (the ratio of the change in time of a signal's voltage). The technical specification of the I/O (such as PCI) is between two voltages. This is the slew rate, which is measurable.

14. What is a quiescent line?

It does not switch over during the current clock cycle. Also known as the "stuck-at" line or static line. Crosstalk can cause a static line to switch during a clock cycle.

15. What is a false clocking?

A fake clock means that the clock has unconsciously changed state (sometimes between VIL or VIH) past the threshold.Usually caused by excessive undershoot or crosstalk.