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Pre-cancelling mixer products (DAC sample selection)

Discussion in 'Electronic Basics' started by Hobbyist, Nov 7, 2003.

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  1. Hobbyist

    Hobbyist Guest

    I'm working on a hobby project where I need to get a "phase locked"
    pair of sinewaves (25Mhz and 27Mhz) from point A to point B. I've
    considered several approaches and I've concluded that the most
    appropriate way is going to be "analog modulating" an LED with a
    signal generated by a DAC. For a first test I'm thinking 1350 samples
    (i.e. 25 * 27 * 2) fed to a 54 MSPS converter. I'm picturing a modest
    bandpass filter (26Mhz +/- 1Mhz) after the DAC but basicly the DAC
    output will directly modulate the current flow through the transmitter
    LED.

    My problem comes in generating the sample values. I generated a plot
    of a simple sin25MHz( t ) + sin27Mhz( t ) for each sample point. The
    resulting "mixer products" were obvious. My hobby level books
    (magazines mostly) say they are (f1 + f2), (f1 - f2), (f1 + 2f2), etc.
    My first question is: Are these "mixer products" new information which
    can be removed from my DAC output or are they inherently required if
    I'm going to want to recover the individual source signals at the
    receiver?

    If I can remove the "mixer products", then I was thinking it would be
    a lot easier if I didn't generate them in the first place. I thought
    an easy way to do this (for the first few "influential" products at
    least) would be to add additional "compliment" waves to the initial
    suming process so that each product would be "cancelled" out of the
    sample stream. Does this make sense? Could anyone provide (or direct
    me to) a simple description of how to determine the correct phase and
    amplitude of the "compliment" wave for each "mixer product"? (It's not
    at all clear from my initial plot)

    Thanks for any and all comments or suggestions,
    Hobbyist
     
  2. John Larkin

    John Larkin Guest

    Obvious, but nonexistant. If you plot the sum indicated, it may *look*
    like there are additional frequencies, but there really aren't, at
    least not unless you then pass the sum through a nonlinear process. A
    DAC is nonlinear, a bit, and the LED will be too, but these shouldn't
    be too serious.
    Again, you need a mixer (a nonlinear device) to create the additional
    frequencies. Linear summing (as, say, with resistors) is not 'mixing'
    in this sense. Audio folks consider linear summing to be 'mixing', but
    RF jocks know that a true mixer is a multiplier.

    John
     
  3. Bob Masta

    Bob Masta Guest

    As a curious aside to this, note that the same effect
    occurs when you linearly add two audio sine waves
    (as in an audio-type "mixer"). If you look at the
    sum on a scope, you can see the non-existant
    difference frequency as peaks in the overall waveform.
    The curious part is that you can hear this "beat
    frequency", even though it isn't really threre.
    Yep, you do need a nonlinearity for this to happen,
    but it's in the auditory system inside your head!


    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
     
  4. Fred Abse

    Fred Abse Guest

    Picking a mixer device with as near a square law characteristic as
    possible will reduce the unwanted products. JFETS are quite good in this
    respect.
     
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