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    Chap 4:Radiation

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    Agenda Radiation Coupling between Distant Devices.

    Superposition of Multiple Sources.

    Design for Radiated EMC.

    Cabinet Shielding.

    Absorption Loss and Reflection Loss.

    Effects of Shield Apertures.

    Waveguide Vents.

    Shield Penetration by Wires and Cables.

    Treatment of Low-frequency leads.

    Treatment of High-frequency leads.

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    Radiation Coupling Electric and magnetic couplings between closely

    spaced devices can be analysed separately.

    Signal from a source can be coupled to distant device

    by means of radiated emission. Electric and magnetic

    fields are interrelated. Basic radiation structures: electric dipole and current

    loop (magnetic dipole).

    Current distribution over an antenna surface can be

    regarded as a collection of infinitesimally small dipolesand loops.

    Total radiated field is the superposition of individual

    dipoles and loops.

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    Short Dipole

    dl

    E

    Er

    x

    y

    z

    r

    Io

    jkrr

    oS ea

    jkrrr

    jka

    jkrr

    dlIE

    sin

    11cos

    112

    4 3232

    jkroS ea

    rr

    jkdlIH

    sin1

    4 2

    Intrinsic Wave Impedance (eta),

    Phase Constant,c

    k

    2

    At Near Field region (kr > /2)

    jkroS ea

    r

    jklIE

    sin

    4

    jkroS ea

    r

    jklIH

    sin4

    Donut

    shape

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    Electric-field Source

    For kr >> 1, Zw= independent of frequency and distant from

    the source. Both ESand HSinversely proportional to r. For kr > ESdominant. Zwvaries with frequency and location ( r and ). ESinversely

    proportional to r3and HSinversely proportional to r2.

    ajkr

    lIE oS

    1

    4 3

    ar

    lIH oS

    1

    4 2

    (Farad)

    11

    rC

    CkrH

    EZ

    S

    S

    w

    dl

    E

    H

    x

    y

    z

    r

    = 90o

    Io

    At Far Field region,

    At Near Field region, consider = 90o,

    S

    S

    wH

    EZ

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    Small Current Loop

    A

    H

    Hr

    x

    y

    z

    r

    Io

    At Near Field region (kr > /2)

    jkr

    ro

    S eajkrrr

    jka

    jkrr

    AjkIH

    sin

    11cos

    112

    4 3232

    jkroS earr

    jkAjkI

    E

    sin1

    4 2

    jkr

    ro

    S eajkr

    ajkr

    AjkIH

    sin

    1cos

    12

    4 33

    jkroS ea

    r

    AjkIE

    sin1

    4 2

    jkroS ea

    r

    jkAjkIH

    sin4

    jkroS ea

    r

    jkAjkIE

    sin4

    Donut

    shape

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    7

    Magnetic-field Source

    (Henry)rL

    LkrH

    EZ

    S

    S

    w

    At Far Field region,

    At Near Field region, Consider = 90o,

    S

    S

    wH

    EZ

    ajkr

    AjkIH oS

    1

    4 3

    a

    r

    AjkIE oS

    1

    4 2

    For kr >> 1, Zw= independent of frequency and distant from

    the source. Both ESand HSinversely proportional to r. For kr

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    Real and Reactive Powers

    377

    Zw()

    1/2 r/

    magnetic

    source

    electric

    source

    100.01

    near field far field

    Periodic storage and

    return of energy between

    electric field and circuit

    (capacitive).

    Energy dissipated as

    electromagnetic wave

    (pure resistance).

    Periodic storage and

    return of energy between

    magnetic field and circuit

    (inductive).

    Wavelength

    +

    -

    High-Z source

    Low-Z source

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    Superposition of Electric Sources

    CM current radiates through dipole structure. E-field oriented in one particular direction if dipole

    currents are in-phase linearly polarised. Generally, tip of Etotaltraces out an ellipse.

    IC ICEtotal= 2EC

    I I

    Etotal

    Htotal k

    E

    E

    E

    t

    E

    t

    t=0t1

    t1

    t2 t2

    k

    E

    E

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    Superposition of Magnetic Sources

    ID

    Etotal

    II

    Htotal

    Htotalk

    k

    HH

    Etotal Htotal

    k

    linearly polarised

    elliptically polarised

    DM currents radiate through current loops.

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    Practical Antennas

    Current along a dipole is not constant. Total field is the

    superposition of fields due to many small dipoles.

    Radiation is most efficient when dipole length is half-

    wavelength.Current need not flow in a loop.

    ~VS

    RS

    I/OVN

    ICM=VN(f)/Rrad(f)

    ICM

    |I|

    z

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    VS

    Radiation Resistance

    Radiation resistance, Rrad= Prad/ |Irms|2 Reciprocity Principle applies.

    For loop antenna, radiation is efficient when loop

    circumference is close to one wavelength.

    Zin

    ~VS

    RS

    Rloss

    Rrad

    jXin

    Transmit

    antenna

    Zin

    Ri

    Rloss

    Rrad

    jXin

    Receive

    antenna

    ~

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    Radiated Emission

    In digital circuits, radiation spectrum is wide due to harmonics.

    Radiation in all directions, no dominant polarisation vector.

    If signals are synchronised, radiation pattern may become directional.

    Total power of fields that add coherently may be greater than the sum of

    their individual powers. Monopole, dipole and current loop above ground plane - use method of

    images. Constructive and destructive interference at alternate locations.

    Source Receiv er

    Image

    ~

    ~

    Rrad= (Rrad)dipole/ 2

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    Design for Radiated EMC Use dedicated return paths for clock leads and sensitive

    circuits (Emission and immunity).

    Keep loop areas as small as possible (place return paths

    close to incident paths, use thin dielectric for striplines).

    Reduce current and voltage on long lines (compatible withreliable operation).

    Use shielded cables or balanced twisted-pairs for long and

    critical interconnections.

    Apply bypass capacitor on low-frequency analog signal leads. Prevent oscillation in MHz range: proper feedback stability,

    decoupling/filtering to improve PSRR, minimise parasitic

    feedback.

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    Digital Design Use as low clock frequency as practicable - use ground grid.

    Above 30 MHz, ground plane is essential.

    Reduce number and length of leads carrying synchronous

    periodic waveforms, such as clock signals.

    Use slowest rise-time compatible with reliable operation -increase series impedance using resistor or ferrite bead.

    Low-loss inductor tends to cause ringing - less useful.

    Shunt capacitor is not desired because it reduces dv/dt at the

    expense of increased di/dt on power lines - worsen emission.di/dt

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    Impedance Matching

    Important for high-speed digital design.

    Severe ringing may affect data transfer if it exceeds the noise

    margin.

    Resonance - peak in harmonics at certain frequency range,

    increase radiation.

    clZo /

    Zin

    ZS ZL Peak at ringing frequency

    f

    A

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    Backplanes Buses which drive several devices/boards carry much higher switching

    currents - higher radiation.

    Synchronous excitation cause higher EMI field at certain directions.

    High speed buses - use ground plane/distributed ground returns (grid).

    Incorporate a ground pin next to every high speed clock, data, and

    address pin. Higher frequency LSB shall have dedicated ground track. Use buffer for fan out.

    NG NG Good

    buffer

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    I/O Ground Use a clean ground for I/O connection to external circuits.

    Apply filter on the connection between I/O ground and signal ground if

    necessary.

    Screen the cables.

    Minimise ground noise voltages using low-inductance ground layout

    (ground plane/grid). Ensure logic currents do not flow through I/O ground.

    ~VS

    RS

    I/OVN

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    Cabinet Shielding

    Used when source emission or susceptibility cannot be

    sufficiently reduced by other techniques.

    Electromagnetic wave incidents on a conductive material

    causes current to flow in the shield. The reflected wave is

    actually the re-radiated wave.

    The shield attenuates the original field as it penetrates the

    shielding material - attenuation loss.

    A portion of wave energy is reflected and never get through

    the shield to reach the receiver - reflection loss. High permeability shield provides low reluctance path for the

    flow of magnetic flux - redirection.

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    Skin Depth

    |E|

    Eo

    z

    Eo/e

    rtj

    oS eEE rtjoS e

    EH

    where

    = + j= propagation constant

    = attenuation constant

    = phase constant

    r

    oS eEE ro

    S eE

    H

    If is not zero, the magnitudes of ESand HS

    decrease as the wave penetrates the material.

    jeff effj

    For a conductor,

    jeff

    jj 12

    21Skin depth,

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    Absorption Loss

    For an absorption loss of 80 dB at 10 MHz, the required thickness of:Aluminium = 0.236 mm (9.3 mil)

    Stainless steel = 0.078 mm (3.1 mil)

    Although stainless steel has a much lower conductivity than aluminium, it makes

    a better shielding material (in term of absorption loss).

    z

    eE

    EA

    z

    o

    o 686.8log20

    21

    ro rCu Cu= 5.82 107S/m

    Material r r rrBrass 0.26 1 0.26

    Aluminum 0.66 1 0.66Gold 0.70 1 0.70

    Stainless steel 0.02 300 6

    Nickel 0.25 50 12.5

    Iron (cast) 0.18 60 10.8

    Iron (pure) 0.18 4000 720

    rr

    o

    o

    f

    16.15

    1

    F/m,1036

    1

    H/m,104

    9

    7

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    Reflection LossE

    i

    Er

    E1

    E1a

    E1r

    E1ra

    E2 E2a

    E2r

    E2ra

    E3

    z

    metallic shield

    E1t

    E2t

    E3t

    ........321 tttt EEEE

    MRAE

    E

    t

    i log20SEness,EffectiveShieldingorlossTotal

    dB686.8

    log20

    zeA z

    dB30log204

    log20

    S

    W

    Z

    ZR

    dB01log20 2 zeM

    Absorption Loss

    Reflection Loss

    Internal reflection

    Energy is redirected, not dissipated - standing wave within

    shielded cabinet or resonance may interfere circuit

    operation. for 100 MHz is 3 m - shield usually not at far field. At near field, metallic shield has very high reflection loss

    on E-field source, but very low loss on H-field source.

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    Diversion of Magnetic Field

    High permeability material diverts magnetic flux lines.

    Used around magnetic recording head, motor and transformer.

    (Mumetal, r= 20,000, Permalloy, r= 2,500) 2 layers of cylindrical shell (with adequate spacing) can give a

    better shielding effect than 1 shell with the same total thickness. Presence of air-gaps cause leakage flux to get into the

    enclosure.

    B

    Bi

    B

    Bi

    B

    Bi

    1 2 3

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    Shielding of Low-Frequency H-Field

    No magnetic shielding effect if r= 1 (copper, aluminium).rof magnetic materials decreases with frequency -

    diversion of magnetic field become ineffective.

    Saturation due to strong magnetic field causes rtodecrease - degrades the shielding effectiveness.

    For time-varying magnetic field, eddy current flows in the

    shield and opposes the external field.

    At high frequencies, absorption loss takes effect. To cover wide frequency range, high-permeability material

    with moderate conductivity is preferred (steel and iron).

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    Current Flow in a Shield

    EM wave incidents on a shield induces a current on the

    metal surface. The induced current must be allowed to

    flow smoothly for wave to be reflected efficiently.

    The presence of a slot (openings in a shield for

    ventilation, fan, wiring, access door) diverts the current

    and reduces the shielding effectiveness.

    The length of slot perpendicular to direction of current

    flow determines the amount of radiation leakage. Thewidth of the slot has little effect on the radiation.

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    Effects of Slots

    = +

    J+Ja

    JJ

    aJ

    a

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    Slot Antenna Theory

    The slot and the complementary

    dipole (consisting of a perfectly

    conducting flat strip of the same

    geometry) has the same radiation

    pattern; electric and magnetic fieldsinterchange.

    ~ ~

    .XX.E

    H

    H

    E

    E

    H

    H

    E

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    Leakage from Slots

    Apertures on a shielding plate can act as effective radiator as

    dipole antennas whose conductor dimensions are those of

    the aperture. Transmitted field strengths are very high for slot length of odd

    multiple of /2.

    0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60 1.8 2.0

    0.1

    0.2

    0.3

    0.4

    0.5

    0.6

    r

    H

    H

    i

    t

    /l

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    Treatment of Slots

    All slots must be shorter than /2 at the highest operatingfrequency.

    Overlapping panels without electrical bonding or continuity

    does not eliminate the effects of the slot.

    For panel that must be opened for servicing, use multiple

    screws at frequent intervals around a lid in order to break up

    the long slot. Shorter antennas in a row tend to radiate less

    efficiently than a long one.

    Use plating of tin, nickel, or cadmium instead of paint for themating surfaces.

    Use conductive paint/caulk/tape to bond all joints.

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    Treatment of Slots

    Use high-conductivity metallic gaskets (wire knit mesh or

    beryllium copper finger stock) to close the gaps.

    Gaskets should be placed on the inside of any securing

    screws so that the EM wave will not radiate from the screw

    holes.

    Gasket

    (incorrect position)

    Gasket

    (correct position)

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    Waveguide Theory

    Hollow-tube waveguide has a high-pass response. The cut-off frequency depends on its cross-sectional dimensions.

    EM field below fcwill be attenuated and its amplitude will

    decrease rapidly with distance.

    b

    a

    TE10

    TE11

    or TM11

    /c

    jkb

    n

    a

    mmn

    2

    2222

    ,22

    b

    n

    a

    mcfmnc

    rtj

    oS eEE

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    Waveguide Vents

    To allow air flow into the shielded enclosure, opening

    can be formed using a number of small waveguides

    welded together in a honeycomb fashion.

    If the waveguide cross-section is small enough that the

    dominant mode (TE10) cut-off frequency fcis higher than

    the highest frequency generated by the equipment, then

    all radiated emissions will be attenuated.

    The attenuation is proportional to the length of the

    waveguide. No conductor should pass through the opening. (Fiber-

    optic cable may be used.)

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    Honeycomb Vents

    air flow

    front view side view

    a

    cfc

    2

    ac

    fff

    c

    cc

    22 2210

    a

    leA l 3.27log20

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    Shield Penetration by Wires & Cables

    Wire passing through a hole into a shielded enclosure

    may cause more harms than those due to the hole. The

    wire can conduct noise current through the hole and then

    re-radiates the wave into the air space.

    A waveguide with a wire inside may have zero cut-offfrequency (similar to a coaxial cable), hence gives no

    attenuation.

    At high frequencies, current over a wire tends to crowd in

    an annulus at the wire surface due to skin effect.

    rw

    rw

    rw

    rw

    21 skin depth of Cu is 38 m at 3 MHz

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    Common-mode Current

    A fraction of the current may return via the outer surface of

    the shield conductor.

    Coupling from the outer surface to ground can give rise to a

    common-mode current (I2I1). A cable carrying common-mode currents through a hole on a

    shielding cabinet behaves as if a noise source were

    connected between the shield and the cable at the hole.

    I1

    I2

    I3

    I1

    I2

    I3

    ~ VN

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    Treatment of Low-Frequency Leads

    Zs1

    IN1

    Zsn

    INn

    Cb1

    Cbn

    Leads that do not intentionally carry RF signals may act as antenna to

    radiate RF noise generated from nearby circuits.

    Connect bypass capacitors between the shield and every wire at the

    penetration holes. Ground wire that enters the shielding cabinet should be connected

    directly to the shield at the penetration point. To avoid undesirable dc

    current from flowing in the shield, use bypass capacitor, if necessary.

    Zs1

    IN1

    Cb1

    Cbn

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    Treatment of Low-Frequency Leads

    The reactance of each bypass capacitor must be much lower

    than the respective slot-antenna driving-point impedance Zm

    (form by the wire and the hole).

    Series inductance LCof bypass capacitor makes the filtering of

    RF noise less effective above the self resonant frequency.

    Feed-through capacitor must be mounted directly in the hole.

    dielectric

    Filter connectors are also available.

    The connector body must be well

    bonded to the shield for effective

    bypassing of RF noise and avoid

    cross-talk between wires.

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    Treatment of Low-Frequency Leads

    If driving point impedance Zmis small, use series inductance

    to impede the RF noise from conducting through the hole.

    Use ferrite bead inductor to reduce shunt capacitance.

    Inductance of the choke may decrease due to saturation effectof dc current. To prevent saturation, use common-mode choke.

    ferrite bead

    conducting sleeve

    ferrite bead

    conducting sl eeve

    Zs1

    IN1

    Zsn

    INn

    Lb1

    Lbn

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    39

    Treatment of Low-Frequency Leads

    For extreme cases, filters containing pi, tee, or laddernetworks are available.

    When lossless capacitor or inductor is used, RF energy is

    blocked from conducting out of the shielding cabinet but not

    dissipated. The RF energy contained within the shield is likelyto couple to other leads. Magnetic material with moderate

    conductivity increases absorption loss and dissipate the RF

    energy as heat, provide better shielding.

    ferrite

    dielectric

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    Treatment of High-Frequency Leads

    Cannot bypass or block the RF signals.

    For long cables, most radiation is due to common-mode

    currents.

    Use common-mode chokes to reduce common-modecurrents.

    Outer conductor of coax cable must be bonded all around

    the hole.

    ferrite

    coax cable

    shield

    ferrite