TDE

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TDE Course 7

• The attenuation of the capacitive and inductive
  perturbations

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Course content

• The attenuation of the capacitive coupled
  perturbations (PACC)
• The attenuation of the inductive coupled
  perturbations(PACI)

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The principle of the capacitive coupling

• Near filed area specific perturbations where the
  perturbative energies are concentrate in the
  electric field.

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PACC1, PACC2

• PACC1. Reducing of the voltage value which
  appears at suscepter can be obtained by
  decreasing of the following factors
• The pulsation of the perturbative voltage
• The evaluation impedance
• The coupling capacitance (parasitic).
• PACC2. Is recommended to action in perturbator
  circuit for filtering the high order harmonics
  which accompany the signals or for attenuation of
  the square signals fronts.

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PACC3

• Is recommended to put in parallel with the
  evaluation impedance of the suscepter circuit a
  capacitor with a higher value that the value of
  the coupling capacitance.

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PACC4

• The coupling capacitance can be minimize by
  adopting some geometrical measures to move away
  the suscepter circuit against the perturber
  circuit..

For a plane condenser we have for capacitance the
following formula
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Examples of capacitive coupling in equipment
construction

• Between signal traces and metallic housing if the
  devices
• Between signal traces and a.c. power traces.
• Between switching transistors for switching
  suppliers, the radiator on which them are
  mounted, device housing, other traces from the
  device
• Between the transformers winding (can appear
  parasitic capacitances up to 100pF-1nF)
• Between the signal traces on the PCB boards with
  digital circuits (mostly CMOS, these circuits
  functioning in general at higher voltages and
  having high input impedances).

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Values examples for parasitic capacitances

• Between a 500 K? resistor terminals - 1,5 pF
• Between two stranding wires -75 pF/m
• Between the central wire and the shield of a
  shielded cable - 100pF/m
• Between neighbors terminals of a connector - 3
  pF
• Between the relay coil winding and housing - 50
  pF
• Between a potentiometer cursor and its housing -
  20 pF.

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The capacitance between two parallel cylindrical
traces
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The illustration of the near field area effect
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The capacitance of a cylindrical trace place over
a ground plane
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The capacitance between two cylindrical traces
with rectangular section - case I
If d/wltlt1, than KC1
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Fringed factor KC
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The capacitance between two cylindrical traces
with rectangular section - case II
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The capacitance between two cylindrical traces
with rectangular section - case II - continuation
If dgtgtw
If the traces have different widths
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The relative equivalent permittivity

• The relative equivalent electrical permittivity
  is the result of placing of conductors in no
  homogenous dielectric environments. The value of
  the parameter is calculated in function of the
  relative permittivity of the substrate and the
  geometrical dimensions of the condensers

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The capacitance of a rectangular section trace
place over a ground plane
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The capacitance between two traces with
rectangular section placed over a ground plane
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PACC5

• To reduce the capacitive coupling between two
  circuits we can use a shielding procedure.
• The shield against the capacitive coupling must
  be realized by metals with a very good electrical
  conductivity (Cu, Al) electric shields.

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Principles for realizing of electric shield
precincts
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The realization of the shields around the
conductors which transmit signal at distance
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PACC6

• To protect the signals transmitted at high
  distances against the capacitive coupling is
  recommended them consolidation by local
  amplification.

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PACC7

• Is recommended to replace the voltage
  transmission with current transmission
  (transmission in current loop).

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The principle of the inductive coupling

• Near filed area specific perturbations where the
  perturbative energies are concentrate in the
  magnetic field. The coupling mechanism can be
  modeled considering the mutual inductance between
  those two circuits

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PACI1

• To diminish the inductive couplings is
  recommended to action in perturber circuits for
  decreasing the speed variation of the currents
  and to reduce the mutual inductance between
  circuits..

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The mutual inductance between two rectangular
loop parallel disposed
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The mutual inductance between two rectangular
loops disposed in a random manner
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The mutual inductance between two circular traces
placed over a ground plane
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The mutual inductance between two traces with
rectangular section placed over a ground plane
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PACI2

• To reduce the inductive coupling is recommended
  to
• Change the geometrical disposal (increasing of
  distance d) of the circuits which influence each
  other
• Decrease the common parallel length of the
  circuits (decreasing of dimension l)
• Reduce the receiver loops surface

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PACI3

• To reduce the inductive coupling is recommended
  that the suscepter loop to be disposed parallel
  with the flux direction. In the next picture is
  illustrated two situations which are frequently
  used in practice, situations in which the
  magnetic flux generated by the perturber circuit
  doesnt intersect the surface of the neighbor
  loop.

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PACI4

• A practical procedure to obtain a good
  attenuation of the inductive coupling is to twist
  the signals wires (wire stranding). This
  operation is indicated both for perturber
  circuit, situation in which the generated
  magnetic fluxes annul each other, and for
  suscepter circuit, situation in which the induced
  electromotor voltages in neighbor loops annul
  each other. The efficiency of the procedure
  increase with the number of twists per unit
  length.

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PACI5

• Another antiperturbative procedure that can be
  implemented is shielded.
• At lower frequencies, under 50 KHz, have
  efficiency the shields made by ferromagnetic
  materials. This shields will be connected, as
  electric shields, at the referential potential at
  only one extremity.
• At higher frequencies, upper 50 KHz, we can use
  ordinary electric shields. Those shield the
  magnetic field by consummation of the
  perturbative energies by the effect of the
  turbionar currents.

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Principles for precincts realization for magnetic
shielded
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Comparison between different interconnecting ways
for magnetic shields
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Example

• For the MSP43032x circuit the producer indicate
  the way for connecting the transducers. In
  situation in which the transducer is shielded
  (left picture) is recommended to use a shielded
  cable with shield connected at the transducer
  shield and at the reference potential of the
  measurement circuit. If the transducer is not
  shielded (right picture) is more efficient to use
  twisted shielded wires. In this case the wire
  shield is not connected at the reference
  potential of the measurement circuit. The
  realization of a such of connection would
  determine the coupling perturbations in the
  shield to arrive at the input of the measurement
  circuit.

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Example - continuation
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Individual tasks

• Study the course material
• Solve the problems for Homework 7
• Search on Internet a material about capacitive
  crosstalk or inductive crosstalk and comment its
  content.