Connectors, Cables, and Electromagnetic Compatibility (EMC)

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Connectors, Cables, and Electromagnetic
Compatibility (EMC)
Chris Allen (callen_at_eecs.ku.edu) Course website
URL people.eecs.ku.edu/callen/713/EECS713.htm
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Connectors

• Purpose
• Transmit signals ( DC levels) across board
  boundaries while preserving signal fidelity
• Issues factors that can affect signal fidelity
  (Tr , noise level)
• Crosstalk (inductive)
• Series inductance
• Electromagnetic interference
• Ground continuity
• Capacitance

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Connectors

• Consider the connection between two
  printed-circuit boards
• Assume the connector length, H, is electrically
  shorti.e., H ltlt l, l vp Tr
• Consider the return current path both current
  paths X and Y share the same ground pin
• Part of the magnetic flux from signal X is
  enclosed in signal Ys circuit path
• ? The result is mutual inductance between X and
  Y, LXYmutual inductance ? crosstalk
• Estimate of LXY includes contribution by signal
  and return current
• LXY has a componentfrom the current overlap
• LXY has a componentfrom the ground pins
  inductance

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Connectors

• To first order
• wherea distance from signal X to signal Y
  (inches)b distance from signal Y to ground pin
  (inches)c distance from signal X to ground pin
  (inches)D diameter of connector pin (inches)H
  pin length in connector (inches)LXY mutual
  inductance between loops X Y (nH)

Overlap
Ground pin
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Connectors

• Example
• Consider a standard connector with 100-mil pitch,
  500-mil connector pin length, 10-mil pin diameter
• Find LXY for case when X, Y, and return path are
  in adjacent pins
• a 100 mils
• b 100 mils
• c 200 mils
• H 0.5
• D 0.010

80 of LXY is from ground pin 20 from overlap
Inductance from the ground pinis typically
larger than the overlap term
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Connectors

• The induced signal, E, is
• Half of the crosstalk signal is launched in the
  forward direction, half in the reverse direction,
  therefore the induced voltage is LXY/(2RTr)
• Consider GaAs gates (Tr 150 ps) with R Zo
  50 ?for LXY 9.37 nH, the crosstalk will be
  0.62
• The coupled signal is 60 of the full signal
  swingmostly due to the shared ground pin
• Lesson learned mutual inductance is mostly
  responsible for crosstalk generated by connectors

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Connectors

• How to reduce crosstalk in the connector?
• Increase Tr ? may adversely affect system
  performance
• Increase Zo and R
• Decrease LXY
• Now explore ways to reduce LXY
• Recall that 80 if LXY came from the ground pin
  term
• ? changing the pin spacing has little effect
• To reduce LXY we can provide parallel ground pins

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Connectors

• After adding a ground pins, reduces LXY
• For this case the return current through the
  lower ground pin is reduced 50 the mutual
  inductance due to overlap similarly reduced.
• Likewise the addition of a second ground pin
  reduces the mutual inductance due to ground-pin
  inductance is reduced by 50.
• However benefits from adding further ground-pin
  are reduced.

Ground pin
Overlap
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Connectors

• To reduce LXY further, separate X Y with
  intervening ground pins
• For this case ground pins are added between the X
  Y signal pathsthe ground-pin inductance
  becomes less significant than the overlap
• LXY is reduced by approximately 1/(1N2)
• For N 5
• Adding ground pins outside X Y does little to
  reduce crosstalk
• Summary use plenty of extra ground pins
  separate signal pins as much as possible

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Series inductance

• Just as we saw with PC board traces, a
  disruption in the return current path results
  in an increased inductance
• While the return current path follows the path
  of least inductance, a portion may pass through
  GND1 or GND3
• The result will be a larger current loopthat
  introduces series inductance
• Longer risetime
• Potential crosstalk
• Radiation of energy electromagnetic
  interference (EMI)

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Series inductance

• Guidelines for reducing series inductance
• Use extra ground pins in connectorsprovides low
  impedance return path
• Place all connectors with common elements close
  togetherreduces loop area
• Provide a low impedance ground on both
  boardsreduces loop area

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Ground continuity beneath connector

• Ground pins in connector provide a return path
  for the signal current
• Placement of the ground pins and how it connects
  to the ground plane is important
• The large slot in the ground plane causes the
  return current to take a longer path

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Ground continuity beneath connector

• A preferred layout provides path for the return
  current

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Ground continuity beneath connector

• Should also consider reassigning the pins on the
  connector to avoid having all ground pins on the
  far side

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High-speed signal connectors

• Custom connectors have been designed for
  high-speed applications
• Characteristics
• Internal ground structures
• Low-impedance ground path
• Deliberately increased pin capacitance to balance
  the pin inductance

Amphenol NeXLev Characteristics Ball grid array
attach for high density surface mount Handles
data rates up to 12 Gb/s High signal density Up
to 145 real single-ended signals per linear
inch Approx 30 lb mating force for 300 I/O module
Amphenol NeXLev Mezzanine Connector
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High-speed signal connectors

• Custom connectors have been designed for
  high-speed applications

Designed for 25 Gb/s Performance 85 and 100 ohm
impedance High signal density Up to 84
differential signals per inch
Routing out in two layers. First layer shown in
blue, second layer shown in red.
Amphenol XCede Backplane Connector
Double ground vias are spaced 1.56 mm apart
providing a wide secondary routing channel.
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Flex circuits as connectors/cables

• Flexible circuits is a technology for assembling
  electronic circuits by mounting electronic
  devices on flexible plastic substrates, such as
  polyimide
• Can support controlled-impedance transmission
  lines (e.g., 50 ?) in microstrip or stripline
  geometries
• Low observed crosstalk (lt 2 in microstrip)
• Different types of flex circuits are available

Single Layer
Double Layer
Multiple Layer
Rigid Flex
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Flex circuits as connectors/cables

• Flexible circuits is a technology for assembling
  electronic circuits by

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Ribbon cables

• In digital systems, multiple signals are passed
  between boards
• For wide-bandwidth signals, controlled-impedance
  transmission lines are typically used
• At low frequencies, ribbon cables are often used
• Ribbon cables, like most other cable geometries,
  have limited frequency response due to the skin
  effect
• As a result, the risetime becomes degraded
  (longer Tr) and this becomes worse with cable
  length
• Crosstalk can also be significantAs in the
  connector, this depends on placement of ground
  conductors for return the current path
• Can be reduced by introducing extra grounds
• G S G G S G G S G

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Ribbon cables

• Alternative assignments of conductors within
  ribbon cable

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Ribbon cables

• To reduce the radiation from the cable, various
  techniques can be applied
• Twist differential wire pairs
• Shield the entire cable with conductive layer
  (e.g., foil)
• Add a choke
• Flat ribbon cable with periodically twisted
  adjacent wire pairs are commercially available

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Ribbon cables

• Shielded ribbon cable
• Continuous conductive shield contains the
  electric field and provides return current path
  for any stray currents
• Ensuring ground continuity at the connectors is
  essential to achieve the desired performance

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Ribbon cables

• Balanced and unbalanced lines
• A balanced line is a pair of similar conductors
  that have equal impedances along their length and
  have equal impedances to ground and to other
  circuits.
• Conductors in an unbalanced line have dissimilar
  conductors or do not have equal impedances to
  ground or other circuits
• Balanced and unbalanced circuits can be
  interconnected using a transformer called a balun
• Compared to unbalanced circuits, balanced
  circuits have better rejection of external noise

Balanced
Unbalanced
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Electromagnetic compatibility (EMC)

• EMC pertains to the ability of a device,
  equipment, or system to function satisfactorily
  in its electromagnetic (EM) environment without
  introducing intolerable EM disturbances to
  anything in that environment
• The two main aspects of EMC are
• function satisfactorily EM susceptibility (EMS)
• without producing intolerable disturbances EM
  interference (EMI)
• EMI standards are set by regulatory agencies
• Commercial products in the USFederal
  Communication Commission (FCC), Part 15
• Military products in the USMIL-STD-461E
• EMS standards are set according to the
  application
• Regulated medical devices, military
• Unregulated commercial products (market driven)

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FCC regulations for digital devices

• For high-speed digital systems, EMS
  (susceptibility) is a measure of how well the
  system will perform its function in the presence
  of EM signals
• example, next to an AM or FM radio transmitter,
  near an airport with a powerful radar
• EMI for digital devices is regulated by FCC for
• An unintentional radiator (device or system)
  that generates and uses timing signals or pulses
  at a rate in excess of 9,000 pulses (cycles) per
  second and uses digital techniques inclusive of
  telephone equipment that uses digital techniques
  or any device or system that generates and uses
  radio frequency energy for the purpose of
  performing data processing functions such as
  electronics computations, operations,
  transformations, recording, filing, sorting,
  storage, retrieval, or transfer.
• fclk ? 9 kHz

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FCC regulations for digital devices

• The FCC divides digital devices into two classes
• Class A marketed for use in commercial,
  industrial, or business environment
• Class B marketed for use in the residential
  environment
• Personal computers fall under Class B
• Class B regulations are stricter than Class A
• FCC regulations have the force of lawfines
  and/or jail for willful violation
• Two forms of emissions are regulated
• Conducted emissions
• Radiated emissions
• Conducted emissions pertain to currents passed
  through the units AC power cord and connected to
  the power grid.
• The concern is enhanced radiation due to the
  large antenna represented by the AC
  distribution network

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FCC regulations for digital devices

• FCC regulation on conducted emissions are
  concerned with the frequency range from 450 kHz
  to 30 MHz
• Although the emission to be controlled is
  current, the limits are in volts
• A line impedance stabilization network (LISN)
  in-line with the AC power cord converts the
  interfering current to a measurable voltage

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FCC regulations for digital devices

• Line impedance stabilization network (LISN)
• Purpose
• prevent noise externalto test from
  contaminatingmeasurement
• present constant impedance(in frequency and site
  to site) to the product between phase ground
  and betweenneutral ground
• IP VP/50, IN VN/50
• 50 ? represents input to spectrum analyzer
• 1 k? (R1) is to provide discharge path for C1
  when 50 ? is not present
• FCC limits for conducted emissions
• Class A digital devices Class B digital devices
• 0.45 to 1.705 MHz, 1000 ?V or 60 dB?V 0.45 to 30
  MHz, 250 ?V or 48 dB?V
• 1.705 MHz to 50 MHz, 3000 ?V or 69.5 dB?V

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FCC regulations for digital devices

• Second form of emission is radiated emission
  electric and magnetic fields
• Regulation requires measurement of E-field only
• Measured field strength in ?V/m or dB?V/m
• From 30 MHz to 40 GHz
• Both V H polarizations with respect to ground
  plane of test site
• Measured at a specific distance
• To be measured in open-field test site over a
  ground plane with tuned dipole
• Difficult to accomplish so preliminary tests
  are made with
• Broadband antennas (biconical or log-periodic
  antennas)
• In a semi-anechoic chamber

100-kHz BWquasi-peak detector
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FCC regulations for digital devices

• FCC limits for radiated emissionsfrom digital
  devices
• Class A _at_ 10 m distance30 to 88 MHz 39
  dB?V/m88 to 216 MHz 43.5 dB?V/m216 to 960
  MHz 46 dB?V/mabove 960 MHz 49.5 dB?V/m
• Class B _at_ 3 m distance30 to 88 MHz 40
  dB?V/m88 to 216 MHz 43.5 dB?V/m216 to 960
  MHz 46 dB?V/mabove 960 MHz 54 dB?V/m
• In the far field, field strength goes as 1/r
• so at 200 MHz, while both classes
• have limit at 43.5 dB?V/m (150 ?V/m)
• Class B limit is tighter by 10/3 or 3.33
• due to the closer measurement
• distance

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FCC regulations for digital devices

• How difficult is this level to achieve?
• ExampleTTL low-power Schottky logic (Tr ? 6 ns,
  I ? 8 mA) test circuit using coplanar strip
  transmission lineThis simple circuit violates
  both Class A and B limits

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Radiated emissions

• In high-speed digital circuitsradiated emissions
  gt conducted emissions
• Radiated emission come from currents on wires
  (PCB traces)
• Current type is key in determining radiated
  emission level
• Two current types
• differential-mode current
• common-mode current
• Consider two parallel conductorseach carrying
  current I1, I2
• These currents can be decomposed
  intodifferential-mode current, IDcommon-mode
  current, IC

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Radiated emissions

• ID, differential-mode currents equal amplitude,
  opposite direction
• Differential currents desired currents on
  transmission line
• Common-mode currents undesired
• IC not predicted in transmission line analysis
• Typically, IC ltlt ID
• however IC produces larger radiated emissions
  than ID
• Why?
• From Faradays law that in the far field

Predicted in transmission line analysis
In far field, differential-mode fields cancel
In far field, common-mode fields add
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Radiated emissions

• Example
• Consider a 1-m long ribbon cable, with a 50-mil
  pitch
• When carrying a signal with 30-MHz, 20-mA
  differential current
• it produces a radiated emission of 100 ?V/m _at_ 3 m
• To produce the same radiated emission level with
  common-mode current requires 8 ?A (30-MHz
  frequency _at_ 3 m distance)
• 20 mA / 8 ?A 2500 or 68 dB of difference
• Conclusion The radiation efficiency of
  common-mode currents is more than 1,000,000 times
  greater than that of differential-mode currents
• How are common-mode currents created?
• Structural asymmetries, large loop areas

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Radiated emissions

• How to quantify the radiated emissions
• Consider the conductor pair
• Assumptions
• Sinusoidal currents
• Conductor spacing (S) is small, S ltlt ?
• First the radiation from differential-mode
  current
• At a point in the plane of the conductorsat a
  distance d from the midpoint
• ED increases with current (ID), frequency (f),
  length (L), and separation (S) and decreases
  with distance (d)

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Radiated emissions

• Radiation from differential-mode current
• Note the term L?S loop area A
• The parameter of interest is
• where K1 1.316 x 10-14/d
• letting d 3 m (Class B)
• makes K1 4.39 x 10-15
• Thus ED/ID increases with frequency at 40
  dB/decade

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Radiated emissions

• Radiation from differential-mode current
• Recall that for digital signalsfor f gt fCLK, the
  signal power spectral density changes at 20
  dB/decadefor f gt Fknee, the signal power
  spectral density changes at 40 dB/decade

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Radiated emissions

• Radiation from differential-mode current
• Combining the spectral response of
  differential-mode current radiation with the
  digital signal spectrum yields the expected
  spectral characteristics radiated by a digital
  differential-mode current
• To reduce ED we can
• Reduce ID (technology limited)
• Reduce Fknee (increase Tr)
• Reduce the loop area, A
• From a practical perspectivereducing the loop
  area, A, is most realistic

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Radiated emissions

• Now the radiation from common-mode current
• At a point in the plane of the conductorsat a
  distance d from the midpoint
• EC increases with current (IC), frequency (f),
  length (L),and decreases with distance (d)
• Note
• linear frequency, not f 2
• no dependence on separation (S)
• numerical scale factor is 8 orders of magnitude
  larger
• Can simply into the form
• where K2 1.257 x 10-6/d or for d 3 m, K2
  4.19 x 10-7
• EC/IC increases with frequency at 20 dB/decade

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Radiated emissions

• Radiation from common-mode current
• Combining the spectral response of common-mode
  current radiation with the digital signal
  spectrum yields the expected spectral
  characteristics radiated by a digital common-mode
  current
• To reduce EC we can
• Reduce IC
• Reduce Fknee (increase Tr)
• Reduce the length, L

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Radiated emissions

• Other differences in the radiation from
  common-mode and differential-mode currents
• Radiated fields from differential-mode
  currentsare largest in the planecontaining the
  currentsand have nulls in the planeof symmetry
• ED is smaller for reduced S
• Radiated fields from common-mode currentsare
  uniformly large in all directions
• EC is independent of S

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Radiated emissions

• Returning now to the example
• fCLK 10 MHzTr 6 ns (TTL LS)
• L 7 (178 mm)S 165 mils (4 mm)
• ?V 4 VZo 400 ? ? ID 10 mA
• Fknee 210 MHz ? Tr 2.4 ns
• Spectral shape indicates radiation is from
  common-mode current

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Radiated emissions

• Returning now to the example
• fCLK 10 MHz, Tr 6 ns, L 7 (178 mm), S
  165 mils (4 mm)
• ?V 4 V, Zo 400 ? ? ID 10 mA
• Assume IC 100 ?A
• f ED max EC max
  .
• 10 MHz fCLK 3 ?V/m (9 dB?V/m) 746 mV/m (117
  dB?V/m)
• 200 MHz Fknee 1249 ?V/m (62 dB?V/m) 14.9 V/m
  (143 dB?V/m)

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Reducing radiated emissions

• To reduce EMI we can
• Reduce the common-mode currents, IC ? EC
• Reduce the loop area, A ? ED
• Reduce Fknee ? ED and EC
• To reduce Fknee (increase Tr)
• We can insert a low-pass filter in the signal
  path if this does not impair the circuit
  performance
• To reduce common-mode currents
• We can use a common-mode choke
• For currents where signal and return pass
  through the magnetic choke (e.g., 1, 2), the
  presence of the choke has no effect.
• The number of turns 0 in this inductor.
• For currents where the signal passes through the
  choke but the return does not (e.g., 3, 4), the
  choke increases the path inductance.
• Number of turns 1 in this inductor.

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Reducing radiated emissions

• To enhance the chokes effectivenesswe can use
  multiple windings around the choke
• We can reduce the loop area by using shielding
• Signal conductors surrounded by a good conducting
  shield provides a low-impedance return path for
  the signals
• If signal and return conductors are both
  surrounded by good conductor shields, the
  common-mode current will induce a return current
  in the shield
• Radiated emissions are thus reduced
• Breaks in the shield can cause significant
  radiated emissionsespecially at the cable ends
  where the shield and connector join
• However multiple conductors within a shield may
  experience crosstalk

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Reducing radiated emissions

• Common-mode chokes suppress EC but do not help
  with ED
• Recall that ED varies as a function of ?
• We can take advantage of this feature in
  suppressing ED
• Consider a twist in a conductor pairfor L ltlt l
  Tr/vp vp/(2 Fknee) ?knee/2
• In the far field, the ED fields tend to cancel
• Similarly, coupling into an adjacent straight
  wire will also cancel? reduces crosstalk
• This technique, twisting wire pairs, does not
  help reduce radiation from common-mode currents
• For adjacent twisted pair lines, crosstalk also
  cancels if twisted in a like direction and at
  similar intervals