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MC2833 distorts sound

Discussion in 'Electronic Components' started by [email protected], May 10, 2007.

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  1. Guest

    Hi
    Does anyone know why the FM transmitter IC distorts the sound signal
    at the output?
    I am using the MC2833 FM transmitter with a crystal that has a rated
    primary frequency of 7.3728MHz at 11th harmonic (88.4736 MHz) trying
    to capture it with an FM radio. The carrier frequency is very strong;
    however, the sound is distorted.
    Please post any helpful hints.

    Thanks
     
  2. How about the fact that you are 26 kHz. away from the center of the band?
    That is severely mistuned. I'd expect a great deal of distortion.

    Jim
     

  3. Is it overmodulated?


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  4. Guest

    Thanks for the reply

    I think the MC2833 has a 400KHz bandwidth...
     
  5. Guest

    Thanks for the reply

    Overmodulation is not the case...I'm still trying different things
     
  6. First, how do you know it's not the receiver that's causing the distortion?

    Second, since you want the signal at 88MHz, you are likely using an FM
    broadcast band receiver. Those are intended to receive relatively wide
    deviation signals.

    The MC2833 is designed for narrow deviation. Admittedly the multiplication
    to 88MHz will multiply the deviation, but it still may be a stretch. So
    the "distortion" may actually be that the deviation is too small for
    the receiver. Or, in trying to get enough deviation, the deviation of
    the transmitter is not linear.

    Michael
     
  7. Huh?

    I don't follow what Jim is saying, but the "bandwidth" of the IC is somewhere
    past 50MHz.

    If by "bandwidth" you mean deviation, 400KHz at 11MHz is outrageously
    large for a crystal controlled transmitter. The datasheet says 10KHz
    at 16MHz is the maximum deviation.

    Michael
     
  8. Overloaded input?
     
  9. Guest

    You're not on a FM broadcast frequency. Digitally tuned receivers will
    not lock onto a random frequency.
     
  10. Excuse me, why would you "think" that? Saying that a device has thus and
    such a bandwidth is a completely bogus statement; it means nothing besides
    letting us know that you don't have a clue. Besides, these are the "sci"
    newsgroups where we try to have a modicum of professionalism. We don't
    "think", we have datasheets that we know how to read.

    1. You are multiplying a 7.37 MHz. crystal up and using the 11th harmonic,
    hoping to come up with an 88.5 MHz. signal for an FM receiver to hear. As I
    said, you are 26 kHz. away from the receiver's center frequency. At this
    mistune of the receiver, I would expect some of the distortion that you are
    hearing.

    2. The modulation deviation of this device at 16 MHz is 3, 5, and 10 kHz.
    (min, typical, max). Since the "variable reactance" device is a varicap in
    series with an external inductor, I'd expect the maximum deviation of this
    device to be roughly half of these values at 8 MHz., perhaps a little less
    at 7.37. Let's say, 1.5, 2.5, and 5 kHz. respectively -- just for
    discussion.

    3. So typically in a x11 multiplier, you would expect around 28 kHz. maximum
    deviation for a x11 multiplier using the device you have chosen. But the FM
    broadcast standard is 75 kHz.. You therefore have a very underdeviated
    signal if you stay within the limits of the device ou have chosen or a very
    overmodulated and distorted signal if you try and drive the modulator into
    saturation.

    4. Short answers ... off channel and overmodulated -- and it ain't gonna
    get any better with this device.

    Jim
     
  11. Guest

    Well said Jim, the guy's out to lunch.
     
  12. Guest

    Thanks Jim

    Very impressive lecture...I'll have a second look at the datasheet.
     
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