Defining Signal Integrity: the Characteristics of High Speed Digital Signals

High Frequency Design From April 2009 High Frequency Electronics Copyright © 2009 Summit Technical Media, LLC SIGNAL INTEGRITY Defining Signal Integrity: The Characteristics of High Speed Digital Signals By Gary Breed Editorial Director ignal integrity, as it overlapping signals or violations of digital cir- Signal Integrity is the applies to high cuit setup and propagation delay times. process of understanding Sspeed digital cir- and controlling the cuits and systems, is The Core Issues of Signal Integrity collective effects of “real often poorly defined. As a Let’s try to group the various causes of sig- world” behaviors on an result, the effects that the nal integrity problems into related areas. ideal digital signal, to term attempts to describe These are listed according to the approximate maintain reliable, error-free may not be completely degree of difficulty: circuits and systems understood by the engi- 1. Transmission line effects—losses and neers designing and test- reflections in interconnecting traces, including ing those systems. In this article, we will try to package leads, vias and connectors, as well as identify the behaviors that collectively can be impedance matching to the active devices. defined as signal integrity. 2. Coupling effects—crosstalk between sig- The word integrity is defined as: “whole- nal lines, or between signal and clock lines. ness; completeness; having unimpaired 3. Ground currents—Signal return cur- action.” Thus, signal integrity deals with the rents (the “other side” of transmission lines); factors that cause a deviation from an unim- propagated signal, clock and noise; plus DC paired digital signal. Signal integrity is usual- “ground bounce.” ly described as relating mainly to the effects of 4. Power integrity—DC supply distribution physical structures on ideal digital signals, but and decoupling; plus unwanted signal or clock other effects are also involved, such as clock propagation through power distribution cir- signal timing, power distribution and noise. cuits. The magnitude of the effects of physical 5. Electromagnetic Interference (EMI)— structures is frequency dependent, increasing External noise ingress, self-interference, con- with higher operating frequencies. Signal trol of radiated emissions. integrity is a relatively recent concern, 6. Circuit design issues—Clock distribu- becoming a critical element in digital design tion, timing errors, logic process sequencing. as clock speeds have increased. Digital signals This last item on the list is certainly not must now be handled like microwave signals, the least important, but was put in that posi- since the bandwidth required to carry those tion because it should be a familiar issue for a digital signals extends well into the micro- digital designer. And there are several other wave frequency range. factors that might have been included on this Likewise, high speed reduces the margin of list, such as manufacturing tolerances and error for clock timing signals. Accurately dis- temperature, humidity and aging effects. tributing the system clock to multiple circuits on a large p.c. board requires an unprecedent- The General Definition ed level of precision. At high speeds, timing Signal integrity can be defined as the net errors are magnified, both in timing pulse effect of all the impairments to a digital sig- alignment and in rise/fall times that can cause nal’s waveform as it travels between the active 52 High Frequency Electronics High Frequency Design SIGNAL INTEGRITY devices in a circuit or system. The from the signal trace to ground shorter wavelengths and increased goal is to remove or greatly reduce (shunt), which acting alone would AC reactances noted above. In addi- those impairments. attenuate the signal. Most inductive tion, the increasingly rapid fluctua- effects are due to the length of the tion of high frequency signals means Visible Effects of Signal trace (series), where increased reac- that they contain more energy. When Impairments tance also affects signal attenuation. that energy is high enough, we can When the data stream is evaluat- Combined capacitance and induc- discern its effects, which include cou- ed end-to-end, poor signal integrity tance can also create resonances. At pling, radiation and surface waves. will result in excessive data errors. low frequencies, those resonances are As we learned in school, these propa- Examination of the digital signal usually well above the bandwidth gating electromagnetic waves are waveform with a high speed oscillo- occupied by the digital signal, but described by Maxwell’s equations, scope may show reduced amplitude, with increasing frequency they may which will bring us to the next topic: droop or tilt; slowed rise/fall times; directly affect the digital waveform. preventing impairments. increased noise floor; and missing Typical effects include overshoot and pulses or split pulses (missing seg- ringing. Robust Signal Integrity Design ments of pulses). Eye diagram analy- Transmission line behavior—At If we are to avoid signal integrity sis will show a closed eye with poor, high frequencies, the length of an problems in high speed digital cir- or non-existent, logic state detection interconnection will eventually cuits, all of the above characteristics margin. become an appreciable fraction of a must be considered. Past practices in The next step is identifying the wavelength. Over that length, the the tens to low hundreds of Mbps causes of the observed problems. As applied waveform will undergo tran- effectively handled frequency-depen- individual sources of impairment are sitions in magnitude and phase, dent effects and basic transmission identified and corrected, smaller defined by the distributed inductance line behavior. Today, electromagnetic effects will become apparent, and capacitance of the trace, ground analysis is essential. Eventually, all significant problems type and dielectric material. Circuit EM analysis is remarkably accu- should be reduced to levels that boards are transmission lines, with a rate, and there are a wide range of result in reliable data transmission characteristic impedance established computer tools available. Unfortu- throughout the system. by the above parameters. nately, the mathematics of EM analy- If the input impedance of a digital sis is complex, requiring manipula- However... device matches the characteristic tion of large matrices and large-scale It is much better to avoid the type impedance of the transmission line, integrations. These calculations are of troubleshooting noted above! all energy will be absorbed by the then repeated over many discrete Signal integrity problems are best device, and none will be reflected observation locations, with many fre- handled at the earliest stages of back toward the source. Thus, the quency steps. Even with powerful design. This is where digital design impedance of the p.c. board trace is computers and multi-core tech- and microwave design combine to important, as is the terminating niques, large problems cannot be develop interconnections with mini- impedance of the device. solved quickly.At a recent conference, mal degradation over a sufficient Reflected energy resulting from a a scientist at a major computer com- bandwidth to carry high speed digital mismatched condition will cause sig- pany noted that EM analysis of a typ- signals. nal degradation. Note that those ical motherboard or backplane reflections are not caused only by the required more than 24 hours compu- Underlying Principles termination; irregularities in the tation time. With this time require- Let review the areas of concern transmission line can create localized ment, it would be impractical to use listed previously. Items 1, 2 and 3 are variations in its characteristic any type of trial-and-error method to related, since they all are types of impedance, referred to as discontinu- solve signal integrity issues. impairments due to the high frequen- ities. These may include bends, tran- Current design efforts are focused cy effects of physical structures. sitions to vias, thickened areas where on more “bite size” problem areas. Classic high frequency effects include: components are soldered, and any Connector manufacturers have per- Frequency-dependent behavior— other physical deviation from a uni- formed analyses of their products and Capacitance and inductance are fre- form line structure. And as frequency developed standardized p.c. board quency-dependent. Capacitive reac- increases, the magnitude of the devi- patterns. Users can duplicate those tance decreases with increasing fre- ation increases. patterns and have predictable perfor- quency, while inductive reactance Electromagnetic effects—With mance where the connector interfaces increases. Most capacitive effects are increasing frequency, we have the to their circuitry. Integrated circuit 54 High Frequency Electronics Stay Visible! vendors are beginning to provide per- formance data out to the pins of the package, not just functional data on the logic operations. Printed circuit board design tools are addressing the issue with proven solutions for common structures: Advertise in BGA and other standard IC connec- tions, ground plane “fill” areas, power High Frequency distribution traces, etc. These tools are currently in development, with Electronics! some well established, but others less reliable for a user’s specific applica- tion. At large OEMs with sufficient resources, designers have developed similar techniques in-house, break- Don’t disappear

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