High speed PCB design introductory questions and answers

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


(Electromagnetic, Interference) there are two kinds of conduction interference and radiation interference. Conducted interference is the coupling of electrical signals across an electrical network to another electrical network via a conductive medium. Radiated interference is the interference of a source by coupling its signal through space (interference) to another electrical network. In the design of high speed and high frequency PCB system, signal lines, integrated circuit pin, various types of connectors are likely to become radiation sources of interference with the antenna characteristics, can emit electromagnetic waves and affect the normal work of other systems or other subsystems within the system. Since the noise reduction technology of electronic system in the middle of 70s, mainly due to the U.S. Federal Communications Commission in 1990 and the EU put forward relevant rules of commercial digital products in 1992, these regulations require all companies to ensure that their products comply with strict emission standards and susceptibility. Products conforming to these Regulations are known as electromagnetic compatibility EMC (Electromagnetic, Compatibility).


2, what is signal integrity (signal integrity)?


Signal integrity refers to the quality of the signal on the signal line. A signal with good signal integrity is the voltage level value that is necessary when needed. Poor signal integrity is not caused by a single factor, but is caused by a variety of factors in board design. The main signal integrity problems include reflection, oscillation, bounce, crosstalk, and so on.


3, what is reflection (reflection)?


Reflection is the echo on the transmission line. A part of the signal power (voltage and current) is transmitted to the line and reached the load, but some of it is reflected. If the source end has the same impedance as the load side, the reflection will not happen. The source and load impedance mismatch will cause the line of reflection, a part of load voltage reflection back to the source of. If the load impedance is less than the source impedance, the reflected voltage is negative; otherwise, if the load impedance is greater than the source impedance, the reflected voltage is positive. Changes in the geometry of the wiring, incorrect line termination, transmission through the connector, and discontinuities in the power supply plane cause such reflections.


4, what is crosstalk (crosstalk)?


Crosstalk is the coupling between two signal lines, mutual inductance and mutual capacitance between the signal lines cause noise on the line. Capacitive coupling induces coupling currents, while inductive coupling induces coupling voltage. The parameters of the PCB board, the signal line spacing, the electrical characteristics of the drive and receiving terminals and the terminal connection of the terminals affect the crosstalk.


5. What is the difference between the time domain (time domain) and the frequency domain (frequency, domain)?


Time domain (time domain) is a time based process of voltage or current variations that can be observed by an oscilloscope. It is usually used to find out the pin to pin the delay offset (delays), (skew), overshoot and undershoot (overshoot) (undershoot) (settling times) and the establishment of time. The frequency domain (frequency domain) is a frequency based process of voltage or current variations that can be observed by a spectrum analyzer. It is usually used for comparison between waveforms and FCC and other EMI control constraints.


6, what is impedance (impedance)?


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


7, what is the establishment time (settling time)?

Setup time is the time required to stabilize an oscillating signal to the specified final value.


8, what is the pin to pin (pin-to-pin) delay (delay)?


The time delay between pins and pins is the time between the changes in the state of the driver's terminal to the state of the receiver. These changes usually occur in a given voltage 50%, the minimum delay occurs when the first output over a given threshold (threshold), maximum delay occurs when the output of the last one across the threshold voltage (threshold) measurements, all of these conditions.


9, what is offset (skew)?


The offset of the signal is the time offset between the same network and the arrival of different receivers. The offset is also used to clock the time difference between the clock and the data at the logical gate.


10, what is the slope (slew rate)?


Slew rate is the slope of the edge (the ratio of the time to voltage of a signal). The technical specifications of I/O (such as PCI) are between two voltages, which is the slope (slew, rate), which can be measured.


11, what is static line (quiescent line)?


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


12, what is a fake clocking (false clock)?


A false clock is a clock that changes the state (sometimes between VIL and VIH) over the threshold (threshold). Usually due to excessive undershoot (undershoot) or (crosstalk) caused by crosstalk.


 

(Source: internal information)

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