Can you help me interpret this spectrum analysis noise plot?

Discussion in 'Wireless Internet' started by Danny D'Amico, Dec 18, 2013.

  1. Nonsense, when you quit learning you're dead. You know how to learn
    which is a quality many people lack. ^_^

    TDD
     
    The Daring Dufas, Dec 21, 2013
    #61
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  2. Danny D'Amico

    miso Guest

    If they run the front end AGC using data from the back end ADC, I see
    two scenarios. One, if there is sufficient filtering on a per channel
    basis (not likely), then the out of band energy will not make it to the
    ADC, but the front end could be overloaded. Assuming crappy front end
    filtering, the strong signal is setting the front end gain, making it
    likely that the desired signal gets a reduced level.

    I bet a WISP would benefit from a good front end low loss filter
    (excluding channel overlap), but it would no longer be frequency agile.
     
    miso, Dec 22, 2013
    #62
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  3. Yep. The front end usually has a ceramic bandpass filter that passes
    the entire 2.4 to 2.483 GHz band. Anything within the bandpass can
    overload (desensitize) the receiver front end. After the A/D
    converter, the digital filter and demodulator will only decode the
    desired channels (usually 3-4 channels for direct sequence spread
    spectrum) which eventually drives the AGC. If the interfering signal
    is outside of these channels, the AGC never kicks in.

    I've seen a few block diagrams (which I can't seem to find again),
    that show a digital AGC detector immediately after the front end A/D
    converter. It was a simple digital level detector, where anything
    over some present digital level, would start AGC action. As such, any
    signal that makes it through the front end bandpass filter will
    produce AGC gain reduction. In other words, a strong wi-fi signal
    anywhere in the band will produce effects similar to desensitization
    without actually causing the front end amplifier to conduct or go
    non-linear.

    I'm not sure which of these two methods is considered best practice.
     
    Jeff Liebermann, Dec 22, 2013
    #63
  4. Danny D'Amico, Dec 22, 2013
    #64
  5. I have no idea if that's high, low, or typical for your location.
    Actually, I'm surprised that Airview doesn't show more junk, noise,
    interference, and other stations at your altitude, so I guess it would
    be "low".
     
    Jeff Liebermann, Dec 23, 2013
    #65
  6. OK. Thanks. I don't have any idea what's a "good" Airview report, versus
    a noisy one. It *seems* low to me; but I wouldn't even know what low is.
     
    Danny D'Amico, Dec 23, 2013
    #66
  7. These might help.
    <https://www.google.com/search?tbm=isch&q=airview>
    Compare the Airview screens with yours. The middle graph (waveform
    view) is the one that shows other stations and interference best.
     
    Jeff Liebermann, Dec 23, 2013
    #67
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