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PCB Tech

Some basic concepts of signal integrity in high-speed circuit design

1. Signal Integrity (Signal Integrity): refers to the quality of the signal in the circuit system. If the signal can be transmitted from the source to the receiving end without distortion within the required time, we call the signal complete.

2. Transmission Line: A connection line composed of two conductors with a certain length in a loop. We call it a transmission line or sometimes a delay line.

3. Lumped circuit: In general circuit analysis, all parameters of the circuit, such as impedance, capacitive reactance, and inductive reactance, are concentrated on each point in space. On each component, the signal between each point is Transmitted in an instant, this idealized circuit model is called a lumped circuit.

4. Distributed System: The actual circuit situation is that various parameters are distributed everywhere in the space where the circuit is located. When the signal delay time caused by this dispersion cannot be ignored compared with the change time of the signal itself Hou, the entire signal channel is a complex network with resistance, capacitance, and inductance. This is a typical distributed parameter system.

5. Rise/Fall Time: The time required for the signal to transition from low level to high level, usually measuring the duration of the rising/falling edge between 10% and 90% of the voltage amplitude , Denoted as Tr.

6. Knee Frequency: This is the frequency range (0.5/Tr) that characterizes the concentration of most energy in the digital circuit. It is recorded as Fknee. It is generally believed that the energy above this frequency has no effect on the transmission of digital signals.

7. Characteristic Impedance: Each step of the AC signal propagating on the transmission line encounters a constant instantaneous impedance, which is called the characteristic impedance, also known as the surge impedance, which is recorded as Z0. It can be expressed by the ratio of the input voltage to the input current (V/I) on the transmission line.

8. Propagation delay: refers to the propagation delay of the signal on the transmission line, which is related to the line length and the signal propagation speed, and is recorded as tPD.

9. Micro-Strip: Refers to a transmission line with a reference plane on only one side.

10. Strip-Line: Refers to a transmission line with reference planes on both sides.

11. Skin effect: When the signal frequency increases, the flowing charge will gradually approach the edge of the transmission line, and even no current will flow in the middle. Similar to this, there is the clustering effect, the phenomenon is that the current-intensive area is concentrated on the inner side of the conductor.

12. Reflection: refers to the incomplete absorption of signal energy caused by impedance mismatch, and the degree of emission can be expressed by the reflection coefficient ρ.

13. Over shoot/under shoot: Overshoot means that the first peak or valley value of the received signal exceeds the set voltage-for rising edges, it means that the first peak exceeds the maximum voltage; for falling Edge refers to the first valley exceeding the minimum voltage, and undershoot refers to the second valley or peak.

14. Oscillation: In a clock cycle, overshoot and undershoot appear repeatedly, we call it oscillation. Oscillations can be divided into ringing and circumferential oscillations according to their manifestations. Ringing is underdamped oscillation, while ringing is overdamped oscillation.

Termination: refers to the effect of uniform impedance by adding resistors or capacitors in order to eliminate reflections. Because it is usually used at the source or terminal, it is also called termination.

15. Crosstalk: Crosstalk refers to the undesired voltage noise interference caused by electromagnetic coupling to adjacent transmission lines when the signal propagates on the transmission line. This interference is caused by the mutual inductance and mutual capacitance between the transmission lines.

Return current: Refers to the return current accompanying signal propagation.

16. Self shielding: When the signal propagates on the transmission line, the method of suppressing the electric field by large capacitive coupling and suppressing the magnetic field by small inductive coupling to maintain low reactance is called self shielding.

17. Forward Crosstalk (Forward Crosstalk): Refers to the first interference from an interference source to the receiving end of the sacrifice source, also known as far-end crosstalk.

18. Forward Crosstalk: Refers to the first interference caused by an interference source to the transmitting end of the sacrifice source, also known as near-end crosstalk.

19. Shielding efficiency (SE): It is a parameter for evaluating the applicability of shielding, in decibels.

Absorption loss: Absorption loss refers to the amount of energy loss when electromagnetic waves pass through the shield.

20. Reflection loss: Reflection loss refers to the amount of energy loss caused by the internal reflection of the shield, which varies with the ratio of wave resistance and shielding impedance.

21. Correction factor: a parameter indicating the decrease in shielding efficiency. Since the absorption efficiency of the shield is not high, the internal re-reflection will increase the energy passing through the other side of the shielding layer, so the correction factor is a negative number and is only used for thin Analysis of the situation where there are multiple reflections in the shield.


22. Differential mode EMI: The EMI generated by the coupling between the current on the transmission line from the driving end to the receiving end and its return is called differential mode EMI.

23. Common mode EMI: When two or more transmission lines output from the driving end to the receiving end with the same phase and direction, common mode radiation will be generated, that is, common mode EMI.

24. Emission bandwidth: the highest frequency emission bandwidth. When the digital integrated circuit switches from logic high to low, the square wave signal frequency generated at the output is not the only component that causes EMI. The square wave contains sinusoidal harmonic components with a wider frequency range. These sinusoidal harmonic components are the EMI frequency components that engineers care about, and the highest EMI frequency is also called the EMI emission bandwidth.

25. Electromagnetic environment: the sum of all electromagnetic phenomena existing in a given place.

26. Electricity: Electromagnetic phenomena that can cause the performance of devices, equipment or systems to decrease or cause damage to animate or inanimate substances.

27. Electromagnetic interference: Electromagnetic interference causes degradation of equipment, transmission channels and system performance.

28. Electromagnetic compatibility: The ability of a device or system to work normally in an electromagnetic environment and not constitute unbearable electromagnetic disturbance to anything in the environment.

29. Interference within the system: Electromagnetic interference caused by electromagnetic disturbance within the system appears in the system.

30. Inter-system interference: electromagnetic interference caused by other systems to a system.

31. Electrostatic discharge: The charge transfer caused by objects with different electrostatic potentials approaching or touching each other.

Setup Time (Setup Time): The setup time is the time that the receiving device requires data to stably exist on the input terminal ahead of the clock edge.

32. Hold Time: In order to successfully latch a signal to the receiving end, the device must require the data signal to continue to be maintained for a period of time after being triggered by the clock edge to ensure the correct operation of the data. This minimum time is what we call the hold time.

33. Flight Time (Flight Time): Refers to the delay between the signal transmission from the driving end to the receiving end and reaching a certain level, which is related to the transmission delay and rise time.

34. Tco: refers to the time difference between the input clock edge trigger of the device and the output signal is effective. This is the sum of all the delays of the signal inside the device, generally including logic delay and buffer delay.

Buffer delay: Refers to the time required for the signal to pass through the buffer to reach a valid voltage output

35. Clock jitter (Jitter): Clock jitter refers to the random error of the clock trigger edge. It can usually be measured by the difference between two or more clock cycles. This error is generated internally by the clock generator and later The wiring does not matter.

36. Clock skew (Skew): Refers to the delay difference between multiple sub-clock signals generated by the same clock.

Fake clock: Fake clock refers to the unconscious change of state (sometimes between VIL or VIH) when the clock crosses the threshold. It is usually caused by excessive undershoot or crosstalk.

37. Power Integrity: Refers to the quality of power and ground in the circuit system.

38. Simultaneous Switch Noise (Simultaneous Switch Noise): refers to when the device is in the switching state, the current (di/dt) that changes instantaneously, when passing through the inductance existing on the return path, forms an AC voltage drop, which causes noise, referred to as SSN . Also called Δi noise.

39. Ground Bounce: Refers to the phenomenon that the chip ground and the system ground are inconsistent due to the fluctuation of the ground plane caused by the package inductance. Similarly, if the power difference between the chip and the system is caused by the package inductance, it is called Power Bounce.