Upload
koleksi-soalan-percubaan
View
240
Download
0
Embed Size (px)
Citation preview
8/13/2019 Slides Chap4
1/40
1
Chap 4:Radiation
8/13/2019 Slides Chap4
2/40
2
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.
8/13/2019 Slides Chap4
3/40
3
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.
8/13/2019 Slides Chap4
4/40
4
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
8/13/2019 Slides Chap4
5/40
5
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
8/13/2019 Slides Chap4
6/40
6
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
8/13/2019 Slides Chap4
7/40
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
8/13/2019 Slides Chap4
8/40
8
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
8/13/2019 Slides Chap4
9/40
9
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
8/13/2019 Slides Chap4
10/40
10
Superposition of Magnetic Sources
ID
Etotal
II
Htotal
Htotalk
k
HH
Etotal Htotal
k
linearly polarised
elliptically polarised
DM currents radiate through current loops.
8/13/2019 Slides Chap4
11/40
11
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
8/13/2019 Slides Chap4
12/40
12
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
~
8/13/2019 Slides Chap4
13/40
13
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
8/13/2019 Slides Chap4
14/40
14
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.
8/13/2019 Slides Chap4
15/40
15
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
8/13/2019 Slides Chap4
16/40
16
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
8/13/2019 Slides Chap4
17/40
17
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
8/13/2019 Slides Chap4
18/40
18
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
8/13/2019 Slides Chap4
19/40
19
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.
8/13/2019 Slides Chap4
20/40
20
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,
8/13/2019 Slides Chap4
21/40
21
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
8/13/2019 Slides Chap4
22/40
22
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.
8/13/2019 Slides Chap4
23/40
23
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
8/13/2019 Slides Chap4
24/40
24
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).
8/13/2019 Slides Chap4
25/40
25
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.
8/13/2019 Slides Chap4
26/40
26
Effects of Slots
= +
J+Ja
JJ
aJ
a
8/13/2019 Slides Chap4
27/40
27
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
8/13/2019 Slides Chap4
28/40
28
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
8/13/2019 Slides Chap4
29/40
29
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.
8/13/2019 Slides Chap4
30/40
30
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)
8/13/2019 Slides Chap4
31/40
31
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
8/13/2019 Slides Chap4
32/40
32
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.)
8/13/2019 Slides Chap4
33/40
33
Honeycomb Vents
air flow
front view side view
a
cfc
2
ac
fff
c
cc
22 2210
a
leA l 3.27log20
8/13/2019 Slides Chap4
34/40
34
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
8/13/2019 Slides Chap4
35/40
35
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
8/13/2019 Slides Chap4
36/40
36
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
8/13/2019 Slides Chap4
37/40
37
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.
8/13/2019 Slides Chap4
38/40
38
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
8/13/2019 Slides Chap4
39/40
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
8/13/2019 Slides Chap4
40/40
40
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