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Discussion in 'Electronic Basics' started by thejim, Feb 6, 2006.

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  1. thejim

    thejim Guest

    I know that IF frequency is the difference between the incoming signal
    and the local oscilator frequency in a frequency synthesizer.
    My question is why in a frequency synthesizer we need the IF frequency>
  2. You need a frequency synthesizer to generate all those different local
    oscillator frequencies, so that, no matter what frequency you are
    wanting to receive, the IF frequency is the same. The IF gain section
    has fixed tuning.
  3. Don Bowey

    Don Bowey Guest

    The I.F. is at either the _sum_ or the _difference_ frequency; it's up to
    the designer.

    The mixing (multiplication) of the frequencies that results in the sum and
    difference, is not done in the synthesizer; it is done in the Mixer, a.k.a.
    Converter stage.
    In a receiver using an I.F., a synthesizer is commonly used to generate the
    variable (tuning) frequencies, whereas the fixed frequency for the Mixer is
    typically generated by a crystal oscillator.

    If you elect to design a Direct Conversion receiver, you use only the
    variable frequency tuning oscillator (or synthesizer) and demodulate
    received signals directly to baseband audio - there is no I.F.

  4. You don't need any mixing and thus any intermediate frequencies in some
    synthesizers. Google for "direct digital synthesis" (DDS) to see how this
    can work. Also, single-loop phase-locked-loop (PLLs) synthesizers use only
    a reference, a divider, a phase comparator and a voltage controlled
  5. Don Bowey

    Don Bowey Guest

    Even a Direct Conversion receiver needs a mixer (demodulator).

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