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Discussion in 'Electronic Basics' started by Bill, Sep 25, 2003.

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  1. Bill

    Bill Guest

    In frequency terminology what means IF?
    And how to calculate or measure bus capacitance and is 470pF is limit
    capacitance for all buses or jut I2C?

  2. Jack Ass

    Jack Ass Guest

    Can't help you with the capacitance, but I have seen the "IF" term used for
    frequency mixers. In this context it means "intermediate frequency". Mixers
    have three ports, the two inputs signals are typically called "RF"
    (radiofrequency) and "LO" (local oscillator), and the output "IF". The
    reason for this notation is that the most common use of a mixer is to
    demodulate a signal of lower frequency (IF) that rides on a much higher
    frequency carrier (RF).

    There might be other contexts in which "IF" means another thing, though.
  3. Fine so far...
    In a superhetrodyne receiver, the incoming RF is converted to a
    (usually lower) intermediate frequency (IF), where it is amplified and
    band-limited (to reduce interference from signals on adjacent
    frequencies), then fed to the demodulator.
  4. Jack Ass

    Jack Ass Guest

    You are of course right... That is how a superheterodyne works, and it has
    to be the most widespread circuit in which a frequency mixer is used. My
    apologies for the misinformation.

    The one application I was referring to (direct demodulation) is the one I
    have been using mixers for in the last years... I work with lab
    instrumentation, and there it is very comon to use lock-in amplifiers and
    mixers to directly demodulate a signal that rides a carrier of the same
    frequency as the LO. Typically one uses some function generator to provide
    the modulation signal for the experiment -could be modulation of a laser
    beam, a microwave source...- and also to provide the LO signal for the
    mixer, which is later use to demodulate and recover the signals obtained
    from the experiment. Physicists use this kind of phase-sensitive detection
    setup -old and well known in electronics- to increase the sensitivity of
    their detection schemes.
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