12/13/2015© X2Y Attenuators, LLC1

Common Mode Filters Test comparisons, X2Y ® versus CM Chokes and PI Filters 12/13/2015© X2Y Attenuators, LLC2

Common Mode and EMI 12/13/2015© X2Y Attenuators, LLC3  Most EMI compliance problems are common mode emissions.  Only 10’s of uAs in external cables are enough to violate EMC standards. Figure 1, FCC Emissons Limits

Common Mode Noise Model 12/13/2015© X2Y Attenuators, LLC4  The E field developed between any lead exiting a shielded enclosure and the enclosure outer skin radiates  Ability to radiate depends on: –Power in the noise source –Coupling efficiency between the effective antenna structure and the surrounding space  The power leads and the case form the antenna Figure 2, Common Mode Noise Model

Common Mode Noise Model 12/13/2015© X2Y Attenuators, LLC5  To Reduce Radiation: –Reduce the noise potential between the case and leads, AND/OR –Reduce the coupling efficiency to surrounding space Figure 2, Common Mode Noise Model

Reduce Noise Potential  Reduce HF current in product –Rarely an option  Decrease shunt impedance to case –Optionally insert series impedance between the noise source and the EMI shunt 12/13/2015© X2Y Attenuators, LLC6 Figure 3, Reduce Noise Potential

Reduce Coupling  Reduce antenna efficiency –Change the cable length  Keep cables short  Avoid odd multiples of quarter-wavelengths to dominant noise sources –Cable routing / shielding  Route cables to minimize radiated noise pick-up  Mismatch antenna impedance –Increase driving impedance to much higher than the antenna characteristic impedance* –Decrease driving impedance to much lower than the antenna characteristic impedance * Antenna impedance may be anywhere from 10’s to 100’s of Ohms 12/13/2015 © X2Y Attenuators, LLC Confidential, Internal X2Y ® use ONLY 7 Figure 4, Reduce Antenna Coupling Efficiency

Differential Noise 12/13/2015© X2Y Attenuators, LLC8  Voltage(s) between multiple leads that form an antenna in the area between. Figure 5, Differential Noise Radiation

Mode Conversion  Occurs when individual filters are not matched  Differential signal energy converts into common-mode energy  Common-mode energy converts into differential energy  Avoid by matching filters throughout stop- band 12/13/2015© X2Y Attenuators, LLC9

CM Chokes as EMI Filters  Ideally, CM chokes work by increasing the noise source impedance, mismatching it to the antenna.  A CM choke is a 1:1 transformer where the primary and secondary are both driven. –Both windings act as both primary and secondary. –Current through one winding induces an opposing image current in the other winding –For K close to 1.0, effective impedance is:  Z ≈ 2πF*L MAG 12/13/2015© X2Y Attenuators, LLC10 Figure 6, Common Mode Choke Operation

CM Chokes as EMI Filters  In an ideal 1:1 transformer, primary and secondary currents are identically equal and opposite –Real transformers, currents match to between +/-1% and +/-5% over the optimum design frequency range. –Real CM chokes work over a limited frequency range due to parasitic capacitance and stray leakage inductance.  For a given core material; –the higher the inductance used to obtain lower frequency filtering, the greater the number of turns required and consequent parasitic capacitance that defeats high frequency filtering, and leakage inductance. 12/13/2015© X2Y Attenuators, LLC11 Figure 7, CM Choke Parasitic Capacitance

CM Chokes as EMI Filters  Mismatch between windings from mechanical manufacturing tolerance causes mode conversion. –A percentage of signal energy converts to common mode, and vice-versa. –This gives rise to EMC issues as well as immunity issues. 12/13/2015© X2Y Attenuators, LLC12 Figure 8, CM Choke Mode Conversion

CM Chokes as EMI Filters  CM chokes have one really good application: – Signals must be passed that operate in the same frequency range as CM noise that must be suppressed.  Otherwise, CM chokes are: – Large – Heavy – Expensive – Subject to vibration induced failure. 12/13/2015© X2Y Attenuators, LLC13 Figure 9, CM Choke Mode

X2Y ® Capacitors  Nearly Ideal Shunts: –When properly applied, X2Y ® capacitors filter CM noise by both attenuating source energy, and mismatching antenna impedance. –The key is very low, and matched inductance. –Proper application must mind inductance in the common path: G1/G2 terminals 12/13/2015© X2Y Attenuators, LLC14 Figure 10, X2Y Capacitor as EMI Filter

X2Y ® Capacitors  X2Y ® capacitor shunts between A, B, and G1/G2 attachments. –Component inductance is very low:  ≈110pH from each A or B to G1/G2.  Low impedance shunt serves two purposes: –Divides noise voltage –Mismatches external antenna impedance  Reflects inside noise back inside  Reflects external noise: EFT/ESD back towards outside. 12/13/2015© X2Y Attenuators, LLC15 Figure 11, X2Y Capacitor as EMI Filter

X2Y ® Capacitors  Performance is typically limited by external capacitor wiring inductance: –L3A/L3B, L4A, L4B  Maximize performance by minimizing L3x, and L4x inductances. –Follow X2Y ® mounting guidelines.  L1x, and L2x inductance is OK and even beneficial when balanced. –Limitation on L2 is to keep connection close to egress. 12/13/2015© X2Y Attenuators, LLC16 Figure 11, X2Y Capacitor as EMI Filter

12/13/2015 © X2Y Attenuators, LLC Confidential, Internal X2Y ® use ONLY17 X2Y ® Capacitors  Example, Good Circuit 1 Mounting Practice –Minimize, L3A, L3B  Connect internal A, B pad connections near base of pads  Connect external A, B pad connections near base of pads –Minimize L4A, L4B:  Connect through minimum length, maximum width connections to chassis edge.  G1 immediate connection to Chassis metal  G2 via to wide polygon on PCB layer 2 12/13/2015© X2Y Attenuators, LLC17 Figure 12, Good X2Y CM Mounting Practice

12/13/2015 © X2Y Attenuators, LLC Confidential, Internal X2Y ® use ONLY18 X2Y ® Capacitors,  Circuit 1 Mount Practices to Avoid –Avoid introducing unnecessary  Common inductance between I/O traces and the A or B pads of the X2Y capacitor  Common inductance between the G1 / G2 pads of the X2Y capacitor and chassis –Any of the above practices impairs performance at high frequency 12/13/2015© X2Y Attenuators, LLC18 Figure 13, Poor X2Y CM Mounting Practice

Example, Single Board Computer Power Feed  Single board computer –68HC11 processor  5uH CM choke tested  PI filter w/ 5uH CM choke tested  Seven values of X2Y ® capacitors tested –47pF, 100pF, 220pF, 330pF, 470pF 560pF, 1000pF 12/13/2015© X2Y Attenuators, LLC19

Comparative Performance 1MHz – 500MHz  CM chokes and PI Filters –Both exhibit similar performance  Filter cut-off ≈ 32MHz  Attenuation effective to about 450MHz  Parasitic capacitance completely defeats CM choke and PI filter above 450MHz –No attenuation compared to no filter case 12/13/2015© X2Y Attenuators, LLC20

Comparative Performance 1MHz – 500MHz 12/13/2015© X2Y Attenuators, LLC21 CM chokes and PI Filters

Comparative Performance 50MHz –1GHz  X2Y ® capacitors effective to 1GHz and beyond  Capacitance value determines low frequency rejection  Very small X2Y ® caps (47pF) superior solution vs. CM chokes or PI filters down to 300MHz  470pF and larger X2Y ® caps superior to choke based filters over all frequencies  X2Y® 1000pF vastly better radiated emissions than 5uH CM choke or PI filter 12/13/2015© X2Y Attenuators, LLC22

Comparative Performance 50MHz –1GHz, 47pF 12/13/2015© X2Y Attenuators, LLC23 47pF Superior to CM choke Above 300MHz GSM ambient

Comparative Performance 50MHz –1GHz, 100pF 12/13/2015© X2Y Attenuators, LLC24 100pF Superior to CM choke Above 150MHz

Comparative Performance 50MHz –1GHz, 220pF 12/13/2015© X2Y Attenuators, LLC25 220pF Comparable/Superior to CM choke Above 50MHz

Comparative Performance 50MHz –1GHz, 330pF 12/13/2015© X2Y Attenuators, LLC26

Comparative Performance 50MHz –1GHz, 470pF 12/13/2015© X2Y Attenuators, LLC27

Comparative Performance 50MHz –1GHz, 560pF 12/13/2015© X2Y Attenuators, LLC28

Comparative Performance 50MHz –1GHz, 1000pF 12/13/2015© X2Y Attenuators, LLC29

Comparative Performance 50MHz –1GHz 47pF, 1000pF 12/13/2015© X2Y Attenuators, LLC30

Comparative Performance 50MHz –1GHz, 1000pF 12/13/2015© X2Y Attenuators, LLC31  X2Y® 1000pF vastly better radiated emissions than 5uH CM choke or PI filter

X2Y® Capacitor Selection  X2Y® capacitors operate as shunts. –X2Y’s attenuate all energy above cut-off frequency –Select value large enough to block offensive HF noise –Select value small enough to pass required signal energy 12/13/2015© X2Y Attenuators, LLC32

X2Y® Capacitor Selection Methods  METHOD 1 - Pass Source Signals w/ Known T RISE / T FALL –Establish T RISE / T FALL  C <= T RISE_10%_90% _MIN /(2.2*Z SOURCE ) –Example: CAN BUS 1Mbps  T RISE_10%_90% <= 50ns  Z SOURCE = 120 Ohms / 2 = 60 Ohms  C MAX <= 50ns/(2.2*60 Ohms)  C MAX <= 380pF  Recommended value = 330pF  T RISE_10%_90% <= 44ns –T RISE_10%_90% / T FALL_90%_10% < 5-10% of bit period is usually OK  5% –C <= 1/(44*Bit_Frequency*Z SOURCE ) –CAN BUS »C <= 1/(44*1MHz*60Ohms) <= 380pF  10% –C <= 1/(22*Freq*Z SOURCE ) 12/13/2015© X2Y Attenuators, LLC33

X2Y ® Capacitor Selection  Method 2 – Known Cut-off Frequency –Noise cut-off frequency F CO is known, source impedance Z SOURCE  C <= 1/(2π*F CO *Z SOURCE ) –Example: Switching power supply harmonic suppression  F CO = 2MHz  Z SOURCE = transmission line impedance 1 Ohm  C MIN >= 1/(2π*2MHz*1 Ohm) = 1/1.26E7 = 80nF  Recommended minimum value = 100nF 12/13/2015© X2Y Attenuators, LLC34

Summary  Most EMI problems are Common Mode  Reduce common mode by attenuating driving voltage and/or mismatching antenna impedance –Properly mounted X2Y ® caps do both  Select X2Y ® capacitor values based on known source impedance and either required signal pass-band ( sets max value ), or required noise stop-band ( sets min value ) 12/13/2015© X2Y Attenuators, LLC35

X2Y vs. CMC 12/13/2015© X2Y Attenuators, LLC36 ApplicationX2Y Part UsedComments CanBus, EMI filtering pF NPO 100VX2Y less overshoot on signal than 5uH CMC, lowered radiated emissions 5uH CMC, power filterX2Y nF 100VX2Y much lower radiated emissions 10/100 BaseT, EMI suppression pF NPO 100VPass 100BaseT requirements, BER testing Telecom, Power Filter nF X7R 16VSteve checking w/Siemens for this info Data / Signal lines uF X7R 50VX2Y on PCB at I/O, replaced 51uH CMC Input filter, Instrumentation amplifierX2Y nF 50VSee MT-070 TUTORIALMT-070 TUTORIAL The following X2Y® components are in production replacing common mode chokes in various application uses.

12/13/2015© X2Y Attenuators, LLC37 Johanson X2Y ® Products Scott Muller Johanson Dielectrics Direct: Cell: E-Fax: Steve Cole X2Y Product Manager Johanson Dielectrics, Inc. TEL