czech,
If you are referring to stricly passive RC filters, then here are some disadvantages:
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Can not implement the classic filter types, such as Butterworth, Chebyshev, Legendre, Bessel, etc.
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Maximum "Q" is 0.5.
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If you include active RC filters, then here are some advantages:
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Filter characteristics do not depend on load resistance.
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Capacitors are phsically smalller than inductors at low frequencies.
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The main disadvantage of active RC filters is that the gain-bandwidth product must be much higher than the maximum frequency of interest. This limits their applicaltion to relatively low frequencies.
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Disadvantages of LC filters:
~ Must be designed for specified source and load reistances.
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~ Inductors that are close to ideal inductors are difficult to design, so the final design must be "tweaked" to allow for finite Q, distributed capacitance.
~ Inductors for low frequency applications are physically large.
Regards,
Kral