201305.pdf 43279KB May 08 2013 11:07:04 PM

More documents

Recommendations

Info

SIGNAL INTEGRITYBY howard johnson, phdpredict the size of a reflected signal:Why reflections happenThrow a rubber ball hard against a concrete wall. The ballbounces. You have just experienced a reflection. Reflectionshappen in propagating systems at those points where theconditions of propagation change.The best way to understand reflections is to track themovement of energy. When the ball reaches the wall, itcannot continue on its original course. Neither can the ball simply stop,because the energy associated with its incoming path cannot just disappear;it must go somewhere.Nature solves this problem by dividingthe incoming energy among theavailable outgoing modes of propagation.Those modes include the ball’sretreating along its incoming direction,an acoustic wave (bonk!) generated inthe air, a slight movement of the concretewall, and a residual thermal agitationof both rubber and concrete.The proportions of energy containedin each outgoing mode are determinedby the law of conservation of momentumin combination with the variousproperties, called boundary conditions,of each individual outgoing mode. Thereflecting-ball problem is tricky, becausethe ball can arrive at any incidentangle, it can carry spin, and there areINCOMINGENERGYREFLECTEDENERGYmultiple avenues for energy dissipation.A transmission line supports onlytwo modes of operation: A signal eithergoes straight down the line in one directionor comes back in the other. That’sit. In a typical digital application, noother modes of operation are possible.When a traveling wave encounters aload at the end of a transmission structure,only three entities are involved:the incoming power, the fraction ofthat power that is dissipated in the load,and the remaining power, all of whichreflects back toward the source.The solution to the problem ofallocating power among those threemodes is the highly vaunted “reflectioncoefficient” formula, which is used toCONCRETEWALLFigure 1 The wall converts a portion of the incoming energy into sound, heat,and vibration.SOUND,HEAT,ANDVIBRATIONV= Z –Z R L C.V IZ L+Z CThe reflection formula expresses theratio of reflected voltage to incidentvoltage, as observed at the end of atransmission line, computed as a functionof the load impedance at the end,Z L, and the characteristic impedance ofthe transmission line, Z C.If the end of a transmission structureis left open-circuited, making an infiniteload impedance, that load drawsno current and therefore dissipates zeropower. In that case, 100% of the incidentpower reflects, creating a reflectedsignal just as large as the incident waveform.If you imagine electrical currentas being loosely analogous to physicalvelocity in the rubber-ball example,the concrete wall did the same thing.When struck, the wall hardly moved,absorbing very little energy, and therebyreflecting almost perfectly.If you reduce the endpoint loadimpedance to successively lower values,the load begins to draw more current,dissipating more power and creating aprogressively weaker reflected signal. Inthe physical world, you can test that ideaby throwing your ball at three things: awooden fence, a wall of hay bales, anda wet sheet hanging out to dry. Whenthe mechanical impedance of the wallbest matches the physical properties ofthe ball, the ball reflects least strongly.Electrically, a load impedance thatcompletely absorbs all incoming powercreates zero reflection. That is the idealarrangement for an end terminator.That particular impedance, if you canfind it, is defined as the characteristicimpedance of the transmission structure.There is no other definition. Allformulas for characteristic impedanceare merely approximations of this ultimatemeaning.EDNHoward Johnson, PhD, of Signal Consulting,frequently conducts technical workshopsfor digital engineers at Oxford Universityand other sites worldwide. Visit hisWeb site at www.sigcon.com, or e-mail himat howie03@sigcon.com.20 EDN | MAY 2013 [ www.edn.com]
LEDrivIRHigh-Voltage Buck Control ICs forConstant LED Current RegulationLineEMI FilterIRPLLED7 Demo BoardRectifier IC and Switches LEDsICIRS2980 Features• Internal high voltage regulator• Hysteretic current control• High side current sensing• PWM dimming with analog or PWMcontrol input• Free running frequency with maximumlimiting (150kHz)LEDrivIR IRS2980Current (mA)IRPLLED7 Demo BoardLED Current vs Input Voltage39038037036035034033032031060708090100110120130140150160170180Input VoltageLED Current (mA)IRS2980 Benefits• Low component count• Off-line operation• Very simple design• Inherent stability• Inherent short circuit protectionDemo Board Specifications• Input Voltage 70V to 250V (AC)• Output Voltage 0V to 50V (DC)• Regulated Output Current: 350mA• Power Factor > 0.9• Low component countPartNumberPackageVoltageGateDrive CurrentStartupCurrentFrequency• Dimmable 0 to 100%• Non-isolated Buck regulatorIRS2980S SO-8 450V +180 / -260 mA
Page 1 and 2: Primary LogoMAY 2013www.edn.comIssu
Page 3: MAY 2013www.edn.comIssue 5Teardown:
Page 6 and 7: The Trusted Leaderfor Any Measureme
Page 8 and 9: www.mouser.comThe Newest Products f
Page 11 and 12: onlineJOIN THE CONVERSATIONComments
Page 13 and 14: aND DirEctorPatrick Mannion1-631-54
Page 15 and 16: Baseband & RFMIMO & FadingRohde & S
Page 17 and 18: Your work is fast-paced.Now there
Page 19 and 20: Analog Devices, MathWorks offerengi
Page 21: pulseTI introduces first eight-chan
Page 25 and 26: Three banks of plastichousings cont
Page 27 and 28: CST STUDIO SUITE 2013Make the Conne
Page 29 and 30: DESIGN FORMANUFACTURING&YIELDWITHOU
Page 31 and 32: the DFM issues that provide the hig
Page 33 and 34: 10 MHz Distribution Amplifiers• S
Page 35 and 36: such as diagnosis-driven yield anal
Page 37 and 38: FastSPICE simulators employ aggress
Page 39 and 40: CELEBRATINGTHEARTOFENGINEERINGEDN A
Page 41 and 42: help people witha damaged skeleton,
Page 43 and 44: Value Instruments:The quality you e
Page 45 and 46: ANALOG ICs: AD9670/71OCTAL ULTRASOU
Page 47 and 48: which often burden other aspects of
Page 49: SPECIAL SECTIONTOP 25GLOBALDISTRIBU
Page 52 and 53: We Do Business Your WayCatalogOnlin
Page 54 and 55: Special SectionUlysses and theSiren
Page 56 and 57: Special SectionNew dynamicsAt the s
Page 58 and 59: Special SectionDid lessons from the
Page 60 and 61: Special Sectionseeing how useful th
Page 62 and 63: Special Sectionvs.Resolving designc
Page 64 and 65: Special SectionAmerica’sdeclared(
Page 66 and 67: Special SectionTips for designengin
Page 68 and 69: Special SectionTOP 25 Global Electr
Page 70 and 71: Special SectionDuelingswordsin theE
Page 72 and 73:
Special SectionThe anti-counterfeit
Page 74 and 75:
special sectioninventory and decidi
Page 77 and 78:
designideasreaderS SOLVe deSIGN PrO
Page 79 and 80:
ANALOGINTEGRATIONISN’T FOREVERYON
Page 81 and 82:
INNOVATORS THINK STATUS-QUOIS LATIN
Page 83 and 84:
Congratulationsto the 2013 ACE Awar
Page 85 and 86:
advertisementA Robust 10MHz Referen
Page 87 and 88:
designideasCLASSICSOriginally publi
Page 89 and 90:
Top GunS.Ip&E SoLuTIonS FRoM AVnETY
Page 91 and 92:
productroundupSenSorS/TranSducerSPr
Page 93 and 94:
minated with either a cable in a co
Page 95 and 96:
No other oscilloscope can touch it.
Page 97 and 98:
No other oscilloscope can touch it.
show all

201305.pdf 43279KB May 08 2013 11:07:04 PM

Create successful ePaper yourself

Delete template?

Save as template?