Predicting and Controlling Common Mode Noise from High Speed Differential Signals Bruce Archambeault, Ph.D. IEEE Fellow, inarte Certified Master EMC Design Engineer, Missouri University of Science & Technology Adjunct Professor bruce@brucearch.com May 2014
Why Control Common Mode Noise in Differential Pairs? Common Mode Noise is inevitable in differential pairs Skew Rise/fall time mismatch Asymmetry in channel Common mode noise is a big problem in EMC! Common mode noise can increase differential crosstalk May 2014 Bruce Archambeault, PhD 2
Common-Mode Noise on PCB Differential microstrip pair Differential driver Noise (crosstalk) Common-mode current Noise (emissions) Noise (emissions) Multilayer PCB May 2014 Bruce Archambeault, PhD 3
Common Mode Noise Due to Skew Small amounts of skew create significant common mode nose As little as 1% of bit width for skew can have significant EMI effects As little as 10% of bit width skew creates CM signal of equivalent amplitude as initial signals May 2014 Bruce Archambeault, PhD 4
0.6 Individual Channels of Differential Signal with Skew 2 Gb/s with 50 ps Rise and Fall Time (+/- 1.0 volts) 0.4 0.2 Voltage 0-0.2-0.4 Channel 1 No Skew 10 ps 20 ps 50 ps 100 ps 150 ps 200 ps -0.6 5.0E-10 1.0E-09 1.5E-09 2.0E-09 2.5E-09 3.0E-09 Time (seconds) May 2014 Bruce Archambeault, PhD 5
0.6 Common Mode Voltage on Differential Pair Due to In-Pair Skew 2 Gb/s with 50 ps Rise and Fall Time (+/- 1.0 volts) 0.4 Amplitude (volts) 0.2 0.0-0.2-0.4 10 ps 20 ps 50 ps 100 ps 150 ps 200 ps -0.6 5.0E-10 1.0E-09 1.5E-09 2.0E-09 2.5E-09 3.0E-09 3.5E-09 4.0E-09 4.5E-09 5.0E-09 Time (seconds) May 2014 Bruce Archambeault, PhD 6
Common Mode Voltage on Differential Pair Due to In-Pair Skew 2 Gb/s with 50 ps Rise and Fall Time (+/- 1.0 volts) 110 105 100 95 10 ps 20 ps 50 ps 100 ps 150 ps 200 ps Level (dbuv) 90 85 80 75 70 65 60 0.0E+00 1.0E+09 2.0E+09 3.0E+09 4.0E+09 5.0E+09 6.0E+09 7.0E+09 8.0E+09 9.0E+09 1.0E+10 Frequency (Hz) May 2014 Bruce Archambeault, PhD 7
Common Mode from Rise/Fall Time Mismatch Small amounts of mismatch create significant CM noise Not as significant as skew, but harder to control! May 2014 Bruce Archambeault, PhD 8
0.6 Example of Effect for Differential Signal with Rise/Fall Time Mismatch 2 Gb/s Square Wave (Rise/Fall = 50 & 100 ps) 0.4 Channel 1 Channel 2 T/R=50/100ps 0.2 Voltage 0-0.2-0.4-0.6 0.0E+00 2.0E-10 4.0E-10 6.0E-10 8.0E-10 1.0E-09 1.2E-09 1.4E-09 1.6E-09 1.8E-09 2.0E-09 Time (Seconds) May 2014 Bruce Archambeault, PhD 9
0.2 0.15 Common Mode Voltage on Differential Pair Due to Rise/Fall Time Mismatch 2 Gb/s with Differential Signal +/- 1.0 Volts T/R=50/100ps T/R=50/150ps T/R=50/200ps 0.1 0.05 Level (volts) 0-0.05-0.1-0.15-0.2 0 5E-10 1E-09 1.5E-09 2E-09 2.5E-09 3E-09 3.5E-09 4E-09 4.5E-09 5E-09 Time (seconds) May 2014 Bruce Archambeault, PhD 10
100 Common Mode Voltage on Differential Pair Due to Rise/Fall Time Mismatch 2 Gb/s with Differential Signal +/- 1.0 Volts 95 90 85 T/R=50/55ps T/R=50/100ps T/R=50/150ps T/R=50/200ps Level (dbuv) 80 75 70 65 60 55 50 0.0E+00 2.0E+09 4.0E+09 6.0E+09 8.0E+09 1.0E+10 Frequency (Hz) May 2014 Bruce Archambeault, PhD 11
Common Mode from Amplitude Mismatch Small amounts of mismatch create significant CM noise Harmonics are additive with other sources of CM noise May 2014 Bruce Archambeault, PhD 12
Common Mode Voltage on Differential Pair Due to Amplitude Mismatch Clock 2 Gb/s with (100 ps Rise/Fall Time) Nominal Differential Signal +/- 1.0 V 0.06 0.04 0.02 Amplitude (volts) 0.00-0.02-0.04 10 mv Mismatch 25 mv Mismatch 50 mv Mismatch 100 mv Mismatch 150 mv Mismatch -0.06 0.0E+00 5.0E-10 1.0E-09 1.5E-09 2.0E-09 2.5E-09 3.0E-09 3.5E-09 4.0E-09 4.5E-09 5.0E-09 Time (Seconds) May 2014 Bruce Archambeault, PhD 13
90 Common Mode Voltage on Differential Pair Due to Amplitude Mismatch Clock 2 Gb/s with (100 ps Rise/Fall Time) Nominal Differential Signal +/- 1.0 Volts 80 70 10 mv Mismatch 25 mv Mismatch 50 mv Mismatch 100 mv Mismatch 150 mv Mismatch Level (dbuv) 60 50 40 30 20 0.0E+00 1.0E+09 2.0E+09 3.0E+09 4.0E+09 5.0E+09 6.0E+09 7.0E+09 8.0E+09 9.0E+09 1.0E+10 Frequency (Hz) May 2014 Bruce Archambeault, PhD 14
0 Mode Conversion Example For High Speed Connector (Simulation Data) -10-20 Scd21 (db) -30-40 -50-60 Uncompensated Port3,Port1 Uncompensated Port4,Port2 Compensated Port3,Port1 Compensated Port4,Port2-70 0 5 10 15 20 25 Frequency (MHz) May 2014 Bruce Archambeault, PhD 15
Compensation Added extra trace length before connector to compensate for connector pin mismatch End-to-end SI sees the improvement EMC does not see the improvement May 2014 Bruce Archambeault, PhD 16
Via Symmetry Effect on Common Mode Conversion May 2014 Bruce Archambeault, PhD 17
Cable Shielding Important Different cables have different amounts of shielding Likely to vary with frequency May vary from vendor to vendor May 2014 Bruce Archambeault, PhD 18
Measured Shielding Effectiveness for Various USB Cables Courtesy of Dana Bergey, FCI May 2014 Bruce Archambeault, PhD 19
How Much is Too Much? Start with source amplitude Near end will have full amplitude 5 Gb/s May 2014 Bruce Archambeault, PhD 20
How Much is Too Much Skew? Most of skew comes from PCB differential trace pair mismatch Can be caught during PCB EMC rule checking For example, BoardCheck rule requires differential pair length to match May 2014 Bruce Archambeault, PhD 21
Based on TUHH formula May 2014 Bruce Archambeault, PhD 22
Example I/O Cable @ 10 Gb/s Fundamental Harmonic at 5 GHz Source = 97 dbuv S cd21 = -16 db (from plot for 10% skew/bit-width) Assume cable shielding 15 db CM noise on external cable = 71 dbuv Rule-of-thumb limit > 1 GHz = 1 mv (60 dbuv) CM over limit by 6 db! May 2014 Bruce Archambeault, PhD 23
Options Fix skew on PCB May not be possible due to routing constraints Improve shielding Costly? Add filter before I/O connector Discrete filters expensive and may distort intentional signal Use Electromagnetic Band gap (EBG) filter? May 2014 Bruce Archambeault, PhD 24
EBG Filter Larger Bandwidth Design Three EBG are designed to resonate around the central design frequency of 8 GHz Port 1 g Port 2 Port 3 g Port 4 g w w w a 1 a 2 a 3 g g g g f 0 -df f 0 f 0 +df Courtesy of Prof Orlandi, Univ L Aquila f res = f TM10-10 % f res = f TM10 + 10 % f res = f TM10 = 8 GHz May 2014 Bruce Archambeault, PhD 25
Example EBG Filter Results (Measure and Simulation Comparison) S cc21 Grade = 2, Spread = 2 S dd21 Courtesy of Prof Orlandi, Univ L Aquila Grade = 2, Spread = 2 May 2014 Bruce Archambeault, PhD 26
Example EBG Near Field Results Fullwave Simulation in Microwave Studio with Different EBG Filter Locations P 1 P 4 P 2 P 3 Traces P 3 Traces P 1 P 4 P 2 No Radiation EBG Traces Traces No Radiation P 1 P 4 P 2 P 3 No Radiation EBG P 1 P 4 P 2 P 3 EBG Courtesy of Prof Orlandi, Univ L Aquila May 2014 Bruce Archambeault, PhD 27 27
SI Concerns for CM Some devices require CM to be below a specified amount Often given in time domain Beware of out-of-band CM S dc21 can convert external common mode to differential mode noise May 2014 Bruce Archambeault, PhD 28
Summary Many asymmetries can cause common mode noise When specifying the amount of CM conversion that is allowable must include source amplitude and expected shielding While EMC is primary concern, SI immunity can be a consideration May 2014 Bruce Archambeault, PhD 29