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What is the advantage in bandpass modulator than lowpass modulator?

Discussion in 'Electronic Design' started by boki, Oct 7, 2003.

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  1. boki

    boki Guest

    Hi, All:

    What is the advantage in bandpass modulator than low-pass modulator?

    Could you please explain more on 10.7MHz frequency point relationship?
    How about the reciver or LNA ... something else, as long as the
    advantage useful than low-pass.

    Thanks a lot!

    Boki.
     
  2. Tom Bruhns

    Tom Bruhns Guest

    Do you mean "bandpass filter" and "low-pass filter"? That might make
    more sense. If so, um, isn't it obvious that a bandpass filter is
    able to reject frequencies below its lower cutoff, in addition to
    rejecting frequencies above its upper cutoff?

    10.7MHz as an IF frequency is simply historic. IFs are chosen,
    generally, to be reasonably easy to filter and to provide reasonable
    image rejection capability. The higher the IF, the easier it is to
    provide image rejection filtering but (traditionally) the more
    difficult and expensive to provide the proper IF bandpass filtering.
    Sometimes multiple IFs are used to get both image rejection and
    (narrow) bandpass filtering.

    If you used a lowpass filter at the IF, you'd generally let a lot of
    signals come through that you really wanted to reject. What effect
    they have depends on the rest of the circuit design, especially the
    detector.

    My answers are to the questions I think you may have been asking, but
    I'm sorry to say that the questions were not terribly clear to me.

    Cheers,
    Tom
     
  3. boki

    boki Guest

    How about:
    Bandpass modulator makes signals are digitized in the IF band without
    being down converted to the baseband.
     
  4. Bill Sloman

    Bill Sloman Guest

    That would imply that you are asking "what are the advantages of
    direct digital synthesis of the modulated RF waveform, over the
    modulation of the RF carrier by the modulating signal".

    The answer is that if you do it right, you can get in it all into one
    - largely digital - chip, and avoid the problems produced by higher
    order non-linearities in an analog modulator.
     
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