Effectiveness of Capacitive Layers and Decoupling Capacitor'

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Effectiveness of Capacitive Layers and
Decoupling Capacitor.

• Ingvar Karlsson
• Applied Technologies
• Ericsson AB
• Ingvar.Karlsson_at_era.ericsson.se

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Current injected into a PCB
i
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Simplified resoning of the behaviour
2Dt
Dt
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Equations for Voltage, Current and Impedance
d
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Validation of Theory by use of TDR and TDT
Test Board
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TDR Results
TDR result with no short circuit in hole C.
TDR result with a short circuit at hole C.
Calculated TDR result with short circuit and
without short circuit in hole C.
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TDT Results
TDT result with no short circuit in hole C.
TDT result with a short circuit in hole C.
Calculated TDT results with short circuit and
without short circuit in hole C.
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Input Impedance of a Via
abs(Z)
imag(Z)
d
real(Z)

• Power planes have an inductive behaviour
• Z ? d

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Permittivties Effect on the Impedance
Low benefit of a high er material
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Gain Factor of a Decoupling Capacitor

• M Voltage noise with decoupling / Voltage noise
  without decoupling.

U
r 100 mm
LC 0
LC 0.8 nH
r 10 mm
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Transient Responce with Decoupling Capacitor
tr 0.1 ns, r 10 mm
tr 1 ns, r 50 mm
tr 1 ns, r 10 mm
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Conclusion

• No need for decoupling capacitors.

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Conclusion

• The benefit of a decoupling capacitor is strongly
  reduced for high transient current noise (high
  di/dt).
• The most important way to reduce high frequency
  power noise on a PCB is to reduce the distance
  between power and ground planes. The higher edge
  rate of the signals (high di/dt) the more
  important is it to have a small distance between
  the power and ground planes.
• Decoupling capacitors are needed on the board to
  reduce low frequency power noise as variation in
  power consumption.

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Conclusion cont

• Decoupling capacitors have a reduction effect on
  planar resonances within PCBs. It is important
  to have decoupling capacitors spread over the PCB
  to minimize the planar resonance effect.
• High permittivity material have only minor effect
  on noise reduction.