National Communications Forum 2000 Session T01A

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Circuit Analysis II - EE4114
STOP-BAND FILTERS
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Stop-Band Filter

• Stop-band filters can be constructed using a
  low-pass and a high-pass filter.
• Rather than the cascaded configuration used for
  the pass-band filter, a parallel arrangement is
  required.

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Stop-Band Filter

• Stop-band filters can be constructed using a
  low-pass and a high-pass filter in a parallel
  arrangement.
• A low-frequency f1 can pass through the low-pass
  filter
• High-frequency f2 can use the parallel path.
• f0 in the reject-band is higher than the
  low-pass critical frequency and lower than the
  high-pass critical frequency, and is therefore
  prevented from contributing to the levels of V0
  above 0.707Vmax.
• The characteristics of a stop-band filter are
  the inverse of the pattern obtained for the
  pass-band filters

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Stop-band filter using a series resonant circuit
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Stop-band filter using a parallel resonant network
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Circuit Analysis II - EE4114
DOUBLE-TUNED FILTERS
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Double-Tuned Filter

• Some network configurations display a pass-band
  and a stop-band characteristic, these networks
  are called double-tuned filters.

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Double-Tuned Filter

• Fig. (a) the parallel resonant circuit will
  establish a stop-band for the range of
  frequencies not permitted to establish a
  significant VL.
• The greater part of the applied voltage will
  appear across the parallel resonant circuit for
  this frequency range due to its very high
  impedance compared with RL.
• For the pass-band, the parallel resonant circuit
  is designed to be capacitive (inductive if Ls is
  replaced by Cs).

• The inductance Ls is chosen to cancel the
  effects of the resulting net capacitive reactance
  at the resonant pass-band frequency of the tank
  circuit, thereby acting as a series resonant
  circuit. The applied voltage will then appear
  across RL at this frequency.

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Double-Tuned Filter

• Fig. (b) the series resonant circuit will still
  determine the pass-band, acting as a very low
  impedance across the parallel inductor at
  resonance.
• At the desired stop-band resonant frequency, the
  series resonant circuit is capacitive.
• The inductance Lp is chosen to establish
  parallel resonance at the resonant stop-band
  frequency.

• The high impedance of the parallel resonant
  circuit will result in a very low load voltage VL.

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