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Residual AM and PM?

Discussion in 'Electronic Basics' started by billcalley, Nov 28, 2005.

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  1. billcalley

    billcalley Guest

    Hi all,

    I cannot seem to find any strong explanations for "residual AM",
    "residual PM", and "residual FM" in any of my electronic's books or
    Online. What are the causes and symptoms of these particular effects?
    Any help is most appreciated!

    Thank You,

  2. Don Bowey

    Don Bowey Guest

    Bad design, I imagine.

  3. Gee, I can't imagine that Google would fail in this arena.

    Anyway, residual AM means that one gets some amplitude modulation of a
    signal when the intent was FM or some other type of modulation. This can
    happen when power supplies are not stiff or when the modulators (the
    circuits that combine the intelligence signal with the carrier signal) are

    Residual FM means that one gets some frequency modulation of a signal when
    the intent was AM. This one is easier as it usually is caused by non-stiff
    power supplies.

    QUAM modulators have some interactions and subtle effects that I cannot help
    you with here. QUAM is an acronym for Quadrature Amplitude Modulation. I
    am certain that you can Google for this.

    Good luck!
  4. Mark

    Mark Guest

    instead of residual PM or FM.... look up "phase noise"

  5. Tim Shoppa

    Tim Shoppa Guest

    "residual AM" is the unwanted amplitude modulation when you
    frequency-modulate an oscillator. This can be because your FM is tuning
    outside the bandpass of some element or simply because of
    gain-bandwidth product for very wide deviations. You eliminate it by
    running the signal through limiter stages.

    "residual FM" is the unwanted frequency shift caused as you attempt to
    do amplitude modulation. Cause is usually too much coupling of the
    modulation into voltage-variable-reactances in the tank. The solution
    is usually to add more buffering and isolation between the oscillator
    stage and the modulator stage.

    "residual PM" is an unwanted phase shift as you amplitude-modulate a
    stage or as the input level to a discriminator changes. Solution is
    usually better limiting/isolation/bypassing/power supply regulation
    (this is often not too different than residual FM).

    The above definitions of residual AM and FM are pretty much as
    described in old radio texts. Unwanted PM most often occurs in a
    comparator stage (boundary between analog and digital).

  6. billcalley

    billcalley Guest

    Thanks Tim, Charles, and Mark -- very much appreciated!

    Best Regards,

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