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Audio Noise Problem - Need HELP

Discussion in 'Electronic Design' started by Jacques St-Pierre, Feb 17, 2007.

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  1. Hello,

    I did design a product that include a analog and a digital section. It's a
    form of digital sound recorder. Many design restriction are in consideration
    that I will not describe here. Just to say, that I must work with off the
    shelves low cost parts and it must run a long time on batteries.

    The product is operational, but I did try to add some improvement and I run
    in some strange behavior that you may give me help.

    Until now, both section (A&D) was running on the same switching supply. The
    model I was using was not very good and it switching frequency was low
    enough to give us trace of it in the analog signal. So I replace it by a
    higher frequency switcher with good result. So trying to push it, I decide
    to use two supply's, one for each section. The analog section only drawn
    25ma while the digital section drawn 80ma, So we talk very small supply
    using National LM2672 switcher. The two supply option, give me the option to
    power down the analog section while not use.

    It work, but I found a 4khz tone in my recording. I can not figure out where
    it come from except that if I stop the switcher supply for the analog
    section and use the digital power all over, the tone disappear. I have good
    scopes & tone generators, but not much more sophisticate equipments. I try
    to see the tone on my analog signal without success, I can see it on the
    scope, so it is very low level, but it is audible when played back.

    That 4khz tone can not be the switcher frequency that run around 260khz, but
    I suspect it can be a interference between both supply, maybe on the A/D
    chip that use both supply. Anyway, I can not find a simple way to get rid of

    Any suggestions may be welcome to correct my problem. If I can not find any
    solution, I will have to go back to the one supply solution and find a way
    to clean the digital +5vdc to be use for the analog section.


  2. Ecnerwal

    Ecnerwal Guest

    I'd guess that there's 4khz difference between the frequency the analog
    supply section is running at, and the frequency of the digital supply.
    That is, they are nominally operating at 260 kHz, but one is perhaps
    operating at 258 and the other at 262, so you are getting a difference
  3. Maybe the 2 switchers are at 4kHz offset, and you hear the difference (mix)?
    This could happen if any non-linearity causes multiplication.
    In that case use _one_ oscillator for both switchers.
    Could also be an interference of an harmonic of one with the other one
    if frequencies are very different.
    Or use 2 analog supplies.
    Screens.... low impedances, filtering, all needed.
  4. Eeyore

    Eeyore Guest

    It's a beat frequency no doubt.

    Make the 2 switching frequencies at least 25kHz apart.

  5. Genome

    Genome Guest

    Since the other turbo-nutter barstards are so fixated on the cause of the
    problem and have totally failed to bother to download the data sheet and
    look at it they have failed to mention that the IC you are using.....

    Has a synchronisation pin on it....... Page 13)

    It's not guaranteed to solve your problem but it might help.


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