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Filtering PWM/BAM Noise — Parallax Forums

Filtering PWM/BAM Noise

bradharperbradharper Posts: 64
edited 2011-01-31 10:14 in General Discussion
I'm currently hearing PWM/BAM noise in an audio signal when ramping up/down the brightness of an LED. I'm new to signal processing (as well as electronics), and wanted to poll the experts here on the most effective solution.

Most of these are very basic questions, but I want to solidify my assumptions before I waste anymore of mine or anyone else's time.

1) Is this noise merely a result of the switching required to generate the PWM/BAM signal?
2) Would the cog/pin (C5/P7) relationship of my DAC/signal output enhance/suppress the noise?
3) Is a low-pass filter the most effective approach to suppress noise of this nature?
4) What's the best way to filter in terms of preserving the upper frequency band of my audio, and doing so with a minimal part count and footprint (I have very limited board real estate to work with).

Audio Circuit Schematic: 
            ||             ||
   P7 --o---||-----GND ----||------------ 7.4V
        |   ||             ||     |
        |  10nF           .1uF    |
        |   ___       ||    .-----o----.
        '--|___|------||----|  TDA7052 |-------1W 8ohm SPK
                      ||    '----------'
            4.7k      1uF

(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

Again, lots of new concepts here for me... I know I have lots of requisite knowledge to integrate, but am I on the right path to finding a solution? Any thoughts, questions, insights appreciated.

I appreciate everyone's time.

Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2011-01-31 09:12
    Move your 10nF cap to the other side of the 4.7K resistor. That will form a low-pass filter. Get rid of the cap on the speaker outputs. (You are using both speaker outputs, right?) Also be sure to follow the datasheet recommendation regarding the power supply filter and bypass caps.

    -Phil
  • bradharperbradharper Posts: 64
    edited 2011-01-31 09:28
    Phil, there was an error in my schematic. The .1uF is the bypass cap (I don't have the 220 though). And yes, using both the +/- outputs on the TDA.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2011-01-31 10:14
    The large filter cap is important. Go to RadioShack and get one. (100uF should actually be enough.) Again, move the 10nF cap as I recommended.

    -Phil
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