High-speed PCB design basic concepts

writer: G March 19, 2018

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

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

Since the emergence of noise reductiontechnology 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 toensure that their products meet stringent susceptibility standards. And launchcriteria. Products that meet these regulations are called ElectromagneticCompatibility (EMC).  

2. What is signal integrity?

Signal integrity refers to the quality ofthe signal on the signal line. A signal with good signal integrity means havingthe necessary voltage level value when it is needed. Poor signal integrity isnot 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 integrityissues and solutions   

Problems

Possible Solutions

Solutions

Other Solutions

Overshoot

Terminal impedance mismatch

Terminal termination

Use a slow rise time drive source

Bad DC voltage level

Overload on the line

Replace DC load with AC load

Terminate at receiving end, rewire or check ground plane

Excessive crosstalk

Over-coupling between lines

Using a slow rise-time send driver

Use a drive source that provides more drive current

Too much time delay

Transmission line is too long

Replace or check from the new line, check  the serial termination

Use an impedance-matched driver to change the routing strategy

Oscillation

Impedance mismatch

In the sending disconnected damping resistor



3. What is a reflection?

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

Impedance mismatch between the source andload leads to line reflections and the load reflects a portion of the voltageback to the source. If the load impedance is less than the source impedance,the reflected voltage is negative, whereas if the load impedance is greaterthan the source impedance, the reflected voltage is positive. Changes in thegeometry of the wiring, incorrect wire termination, transmission through theconnector, and discontinuity in the power plane can all cause such reflections.

4. What is crosstalk?

Crosstalk is the coupling between twosignal lines, and mutual inductance and mutual capacitance between signal linescause noise on the line. Capacitive coupling induces a coupling current, whileinductive coupling induces a coupling voltage. PCB board parameters, signalline spacing, driving end and receiving end electrical characteristics, andwire termination methods all have some impact on crosstalk.

5. What is overshoot and undershoot?

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

6. What is ringing and rounding?

The phenomenon of oscillation is repeatedovershoot and undershoot. The oscillation of the signal and the surroundingoscillation are caused by excessive inductance and capacitance on the line. Theoscillation 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 becompletely eliminated.

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

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

Since the ground plane (including power andground) is divided, for example, the ground is divided into digital ground,analog ground, shield ground, etc., ground return noise is generated when thedigital signal goes to the analog ground area. The same power plane may also bedivided 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 timedomain and the frequency domain?

The time domain is a time-based voltage orcurrent change that can be observed with an oscilloscope. It is typically usedto find out the delay from pins to pins, skew, overshoot, undershoot, andsettling times.

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

9. What is the impedance?

The impedance is the ratio of the inputvoltage to the input current on the transmission line (Z0 = V/I). When a sourcesends a signal to the line, it will block it from driving until 2 * TD, thesource 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 anoscillating signal to settle to a specified final value.

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

The pin-to-pin delay is the time betweenthe change of the state of the driver side and the change of the state of thereceiver side. These changes usually occur at 50% of a given voltage. Theminimum delay occurs when the output first crosses a given threshold and themaximum delay occurs when the output crosses the voltage threshold. Measuringall these Happening.

12. What is a skew?

The offset of the signal is the time offsetbetween arrivals of different receivers on the same network. The offset is alsoused to 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 (theratio of the change in time of a signal's voltage). The technical specificationof the I/O (such as PCI) is between two voltages. This is the slew rate, whichis measurable.

14. What is a quiescent line?

It does not switch over during the currentclock 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 hasunconsciously changed state (sometimes between VIL or VIH) past the threshold.Usually caused by excessive undershoot or crosstalk.


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