How Return Loss Gets its Ripples
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Slide -1 Return Loss Ripples How Return Loss Gets its Ripples an homage to Rudyard Kipling Dr. Eric Bogatin, Signal Integrity Evangelist, Bogatin Enterprises www.beTheSignal.com @beTheSignal Downloaded handouts from www.beTheSignal.com Fall 2011 www.beTheSignal.com Bogatin Enterprises, a LeCroy Company 2012 Slide -2 Return Loss Ripples 45 Minute Speed Training • Virtually every measured or simulated return loss (S11) shows “ripples” • Sometimes, insertion loss also shows ripples • Why? • What does it tell us about the interconnect? S21 Differential Response, dB 0 Differential Response 0 -10 -20 -30 S11 -10 -20 -30 -40 0 -40 0 1 2 3 4 5 6 freq, GHz 7 8 9 10 1 2 3 4 5 6 7 8 9 10 freq, GHz Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -3 Return Loss Ripples 2 Port S-Parameters Applies to single-ended and differential S-parameters Magnitude and phase Detector 50Ω V source DUT Z 0 = 50Ω ~ • Z0 = 50Ω 50Ω S11: Return loss magnitude/ phase detector It is the reflected signal Impedance mismatch from 50 ohms throughout the interconnect A little about losses • Transparent interconnect: S11: large, negative dB S21: small, negative dB S21: Insertion loss It is the transmitted signal Impedance mismatches throughout the interconnect Losses Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -4 Return Loss Ripples Connection Between Insertion and Return Loss S21 Vincident S11 Vreflected How is S11 related to S21? S11 + S21 = 1 Vtransmitted There is no such thing as conservation of voltage There is conservation of energy Energy in a wave ~ V2 In a lossless interconnect 2 S 21 = 1 − S11 Bogatin Enterprises, a LeCroy Company 2012 2 1 = S11 + S 221 + losses Sources of loss: - conductor - dielectric - coupling (cross talk) - radiation www.beTheSignal.com Slide -5 Return Loss Ripples For Lossless Interconnect: Insertion Loss from Return Loss dB(S21_sim) When plotted in dB, relationship between S11 and S21 looks unusual 0.0 -0.5 -1.0 2 S 21 = 1 − S11 -1.5 -2.0 -2.5 -3.0 -3.5 -4.0 -4.5 -5.0 -50 -45 -40 -35 -30 -25 -20 -15 -10 -5 0 dB(S11_sim) S11 of < -10 dB yields S21 > -0.5 dB Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -6 Return Loss Ripples How Reflections Result in Return, Insertion Loss Ripples Z0 < 50 Ohms S21 50 Ω 50 Ω At low frequency ALL interconnects are transparent When Len << ¼ λ ½ 0 min S11 0 0 0 max S21 S11 0 • When Len << ¼ λ Reflections from front and back, 180 deg out of phase No net reflection, all transmitted waves in phase and add S11 large negative dB, S21 nearly 0 db Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -7 Return Loss Ripples When Len = ¼ λ, S21 Lowest, S11 Highest Z0 < 50 Ohms S21 When Len << ¼ λ ½ 0 min S11 0 0 0 max S21 S11 0 When Len = 1/4 λ ½ max S11 ¼ ¼+0 ½ ½+0 min S21 ¾ • When Len = (½ x n + ¼ ) λ Max S11, min S21 Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -8 Return Loss Ripples When Z0 ≠ 50 Ohms, Multiple Reflections From The Terminations Cause Ripples Z0 < 50 Ohms S21 When Len << ¼ λ ½ 0 min S11 0 0 0 max S21 S11 0 When Len = 1/4 λ ½ max S11 ¼+0 ½ ½+0 ¼ min S21 • When Len = n x ½ λ Reflected waves from front and back subtract ¾ Minimum reflected signal Transmitted waves all in phase, S21 max • As frequency increases, insertion, return loss increase, decrease When Len = ½ λ ½ ½+0 min S11 1 1+0 ½ max S21 1½ Longer distance between reflections, shorter the frequency between high and low Larger the impedance difference, the larger the modulation Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -9 Return Loss Ripples Shorter the Distance, Farther Apart the Ripples ∆f = 1 2TD 3 inches is ~ 1 GHz spacing Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -10 Return Loss Ripples Every ½ λ, Another Dip ∆f = 1 2TD • Frequency spacing is a measure of distance between discontinuities • Case 1: TD = 1 nsec Freq between dips is 1/2nsec = 500 MHz • Case 2: TD = 0.5 nsec Frequency between dips is ½ x 2 GHz =1 GHz Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -11 Return Loss Ripples Larger the Impedance Difference, Larger the Modulation Dips Z0 = 40 Ohms Z0 = 30 Ohms Z0 = 20 Ohms When return loss is less than ~ – 13 dB, insertion loss impact is < 1 db Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -12 Return Loss Ripples Add Some Jazz • Impedance mismatch on the ends • Add attenuation • Add multiple discontinuities • Fine features increase, general structure stays the same Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -13 Return Loss Ripples Two Uniform, Tightly Coupled Stripline Traces Bogatin Enterprises, a LeCroy Company 2012 • What are the features of S11, S21? • What does it say about impedance variations? • How would you interpret the results? www.beTheSignal.com Slide -14 Return Loss Ripples The Pop Quiz • What are the features of S11, S21? • What does it say about impedance variations? • Why does spacing between ripples change with frequency? • How would you interpret the results? Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -15 Return Loss Ripples Extra Credit • Why does S11 increase at higher frequency? • What happened to the ripples? Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -16 Return Loss Ripples Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com Slide -17 Return Loss Ripples For More Information www.BeTheSignal.com SI Library Webinars, feature articles, presentations, hands on labs Class schedules Blog: www.beTheSignal.com/blog @beTheSignal Published by Prentice Hall, 2009 Bogatin Enterprises, a LeCroy Company 2012 www.beTheSignal.com
