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Does energy of the signal lose after going through the filter?

  1. Oct 26, 2013 #1
    This is not a homework, just a problem that I can't figure out. Please help.

    Assuming that I have a low pass filter with cutting frequency, fc and a signal with many different frequencies (with some below fc and some above fc) is applied to the input of the filter. As we know, the high-frequency components will be attenuated.

    My question: does energy of the signal lose after going through the filter?

    How about the the filter is parallel LC? In this case, with the assumption that L, C are ideal (no energy dissipated in these components) then the energy of the signal at input and output of LC filter has to be the same. However, some high-frequency components of the signal are destroyed now. How is that (energy at input and output are equal) possible?
  2. jcsd
  3. Oct 26, 2013 #2


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    They are not equal. If I receive a broad range of transmitted frequencies, then the attenuated frequencies give up their energy to my filter and the output could be much, much less than the input.

    Note that the frequencies which are passed are attenuated to some degree as well.
  4. Oct 26, 2013 #3


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    There are two ways in which a filter can work. It can dissipate the power at certain frequencies in its resistive components or it can present a bad mismatch to the source at those frequencies and 'reflect' the energy.
  5. Oct 26, 2013 #4
    I once worked for a company (many years ago) that manufactured high pass, low pass, and bandpass filters. We called this loss you're talking about the insertion loss.
  6. Oct 27, 2013 #5


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    Insertion loss refers to the loss of signal at the maximum in the pass band. I thought the OP was asking about where the losses arise outside the pass band. I wonder which it was?
  7. Oct 27, 2013 #6


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    generally not that specific
    from wiki

    In telecommunications, insertion loss is the loss of signal power resulting from the insertion of a device in a transmission line or optical fibre and is usually expressed in decibels (dB).

    Insertion loss is a figure of merit for an electronic filter and this data is generally specified with a filter. Insertion loss is defined as a ratio of the signal level in a test configuration without the filter installed (|V1|) to the signal level with the filter

    for me insertion loss is the loss produced by the insertion of any passive device into a circuit

    Last edited: Oct 27, 2013
  8. Oct 27, 2013 #7


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    what the OP is referring to isn't really insertion loss, as you pointed out sophieC

    rather it seems from his poorly worded question, what happens to the signal outside the passband
    I think we agree on that :smile: ....
    and you answered it in post #3

  9. Oct 27, 2013 #8


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    Note, I'm more familiar with this sort of question/paradox in mechanical systems not electrical, but the basic principles and the math are the same.

    Before you start worrying about "where the energy went", make sure you are talking about the right quantity, especially if you are considering ideal components like an LC filter with zero resistance. Remember that energy or power is only transmitted by the parts of the voltage and current that are in phase with each other. If your filter has a complex impedance, what you make sure you are talking about power in watts, and not Volt-Amps.

    For example, if you have a constant input voltage source and vary the frequency, that does not usually mean you have a constant input power source.
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