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Amplitude distortion and phase distortion

Discussion in 'Electronic Design' started by SaMPEI, Jul 18, 2005.

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  1. SaMPEI

    SaMPEI Guest

    In a monodimensional,linear,time-invariant system there is a
    relationship between amplitude distortion and phase distortion?if
  2. The Phantom

    The Phantom Guest

    It is the will of Bode.

    Go read "Network Analysis and Feedback Amplifier Design", H. W. Bode
  3. The term distortion is not really used in enginerring.
    In signal theory, there is the linear transmission
    function, which doe not creates harmonics, and there
    are the non-linear cases with harmonics.
    The linear transmission function relates the amplitude
    and phase response of the system to the input signal.
    The non-linear cases are not nearly as good covered
    with theory as are the linear ones.

  4. Joseph

    Joseph Guest

    I have read that a Fir filter has linear phase so is amplitude response
    hasn't distortion in passband(no oscillation is present):this
    affirmation is true?
  5. Not all FIR filters have a linear phase vs frequency relationship.

    If a linear phase vs frequency relationship is required it is
    usual to use a FIR.

    A filter with a linear linear phase vs frequency relationship
    delays a time domain signal without changing its shape. The delayed
    signal will be distorted if it has harmonic content that lies outside
    the range of frequencies where the linear phase relationship is

  6. To start with, get rid of this distortion stuff. Linear
    systems do not have distortions, by definition. When
    distortion comes in, they are in the nonlinear regime,
    eg clipping. This is not covered by the linear theory.

    There is amplitude ratio and phase shift. One pair
    for each frequency.

    A FIR is a forward propagating multiply-add-register
    set. It has thus a finit impulse response.
    Depending on the coefficient, whatever amplitude
    and phase response can be achieved.

  7. Mark

    Mark Guest

    homework problem?

  8. Jon

    Jon Guest

    By phase distortion, I assume you are referring to a deviation from
    linear phase. There is a relationship, but it depends on the transfer
    function of the system. For example, an all-pass filter has an output
    whose amplitude does not vary with frequency, but its phase shift does.
  9. Jon

    Jon Guest

    A FIR filter can be made to have linear phase. Any FIR filter whose
    coefficents are symmetrical with respect to the center coefficent wil
    have linear phase. For example, let A(c) = the coefficient of the
    middle sample. Then if A(c-1) = A(c+1), A(c-2) = A(c+2), etc the filter
    will have linear phase. All FIRs do not necessarily have linear phase.
    No analog system can be made to have linear phase.
  10. Rich Grise

    Rich Grise Guest


  11. If you are only talking about waveform distortion, this is true.

    The terms 'phase distortion' and 'frequency distortion' (meaning limited
    bandwidth) also exist. These days they are less commonly used
    expressions, but they do need to be excluded from your definition.
  12. Ban

    Ban Guest

    There seem to be different meanings of 'linear phase' if used by digital and
    analog guys respectivly.
    In digital is meant a filter without any phase shift (exept a certain
    unavoidable constant delay).
    An analog guy will understand 'linear phase', that the phase is increasing
    linearly with frequency. Unadvertedly this will lead to the same as above, a
    constant delay for all frequencies.
    The thing is, the digital guy forgets about the delay and is proud that his
    filter has no phase change at all. And the analog guy forgets about the
    filter and concentrates on the delay...
    The analog loves OTOH a *minimum* phase filter. This type is related to the
    OP-question, because in the analog world every filter needs a phase change
    to perform its function. For a filter of 1st order this will be +90° for a
    HP or -90° for a LP at the -3dB points and +/-180° at infinite/zero. A 2nd
    order filter has double of these values. Most natural processes (pendulum,
    mass/spring etc.) behave as minimum phase systems.
    A minimum phase filter has the shortest possible delay. Sometimes a very
    important attribute.
    We can calculate the phase from the amplitude in a frequency plot for a
    minimum phase system, this might be helpful in certain processing
  13. Ban

    Ban Guest

    a 1st order filter has 45° at the corner frequency and 90° at infinite,
    what I wrote was for a second order filter.
  14. Poly Chrome

    Poly Chrome Guest

    check a good filter book.
    linear phase = constant delay
    delay is the derivative of phase
  15. Ban

    Ban Guest

    Yes, exactly what I wrote.
    For a digital guy the meaning is not constant delay, but a filter with an
    amplitude function without any impact on the phase. This is indeed
    impossible in the analog domain.
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