Basic fundamentals of -3dB .........!

Discussion in 'General Electronics Chat' started by mishra87, May 13, 2016.

  1. mishra87

    Thread Starter Member

    Jan 17, 2016
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    Can anybody let me understand the what is -3dB attenuation in electrical and electronics...!!!
     
  2. bertus

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  3. MrChips

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    A bel is the logarithm (base-10) of the power ratio.

    1:1 power ratio is 0 bel.
    1:10 power ratio is 1 bel.
    1:100 power ratio is 2 bel.
    1:1000 power ratio is 3 bel.

    There are 10 decibels in 1 bel.

    Hence,
    1:1 power ratio is 0dB.
    1:10 power ratio is 10dB.
    1:100 power ratio is 20dB.
    1:1000 power ratio is 30dB.

    1:2 power ratio is 3dB.

    How did we arrive at this?

    [​IMG] which makes

    [​IMG] and

    [​IMG]

    (with thanks to Papabravo, post #5)

    In filter circuits, one wishes to identify the roll-off (cut-off, corner or knee point) frequency in the frequency response curve. When the power is reduced to 50% this is the -3dB point, i.e. the gain is -3dB (i.e. the signal is attenuated by 3dB).

    250hz_low_pass.png

    For a voltage V into a load resistor R, the power into the load is calculated as

    P = \frac{V^2}{R}

    Hence the voltage drop at the cut-off frequency is 1/√2 = 0.707 or 70%

    When the voltage drops to 50% this represents a power gain of -6dB.

    (Note that we need to be careful with our choice of words. When someone says "-3dB attenuation" we know what they mean, i.e. a loss of 3dB. Semantically, "-3dB attenuation" means a "gain of 3dB". The person really means to say "3dB attenuation" or "-3dB gain".)
     
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  4. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    One of the things I was missing for the longest time is the fact that you are looking for a point that is reduced by 3dB.

    Take MrChips graph. The highest point is 0, this is maximum power. The point where signal is at half power is 0dB-3dB=-3dB. However, if you have signal that has maximum power at 5dB, then the point where signal has half power is at 5-3=2dB. Another example, you have signal that has maximum power at -7dB, then the signal will have half its power at -7dB-3dB=-10dB.
     
  5. Papabravo

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    I think your basic question was why is 3 dB so special?
    Well it is because:

    log_{10} (2) =0.3010 which makes

    10 \times log_{10} (2) \;\approx\;3 and

    10 \times log_{10} \left ( \frac{1}{2}\right)\;\approx\;-3
     
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  6. MrChips

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    I was just about to add that to my post #3. Papabravo beats me to it.
     
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  7. crutschow

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    Mar 14, 2008
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    -3dB is the point where a signal is reduced to 1/2 the power (1/√2 of the voltage) of the base signal level.

    It also happens to be the frequency that you calculate for a simple resistor and capacitor low/high pass filter from its RC time-constant (called the corner frequency or 1/2πRC) which happens to be the -3dB point in it's response curve.
    By convention, most filters are characterized by this -3dB corner frequency point (among other characteristics).
     
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  8. Papabravo

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    Feb 24, 2006
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    Continuing...
    The reason it is called a "corner" frequency is because the ideal piecewise linear approximation of the typical lowpass response actually has a 'corner' at about the 3 dB point. (I'm not sure it is exact, but if not it is close.)
     
  9. MrChips

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    For a basic RC low-pass filter, the -3dB point has some significant characteristics.
    The corner frequency occurs at ω = 1/RC

    i.e. 2 \pi f = \frac{1}{RC}

    At this point the reactance of C matches the reactance of R.
    The resultant impedance

    Z = \sqrt{R^2 + {X_c}^2}

    V_{out} = V_{in} \frac{X_c}{Z}

    From this you can see where the 70% voltage drop appears.

    The phase shift is -45° at the -3dB point.

    RC Low Pass Filter response.gif

    Reference: http://www.electronics-tutorials.ws/filter/filter_2.html
     
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  10. mishra87

    Thread Starter Member

    Jan 17, 2016
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    I did not understand
    (Note that we need to be careful with our choice of words. When someone says "-3dB attenuation" we know what they mean, i.e. a loss of 3dB. Semantically, "-3dB attenuation" means a "gain of 3dB". The person really means to say "3dB attenuation" or "-3dB gain".)

    In either cases it is written " -3db attenuation " but different meaning in terms of loss and gain. ..!!!
     
  11. MrChips

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    We know what the writer means when they say "-3dB attenuation" but this is incorrect.
    Two negatives make a positive. "-3dB attenuation" is literally translated to "3dB gain" but this is not what the writer is trying to express.

    The writer should have said "-3dB gain" or "3dB attenuation".
     
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  12. johnmariow

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    May 4, 2016
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  13. mishra87

    Thread Starter Member

    Jan 17, 2016
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    Thanks for your valuable time and information.

    I wanted to understand of -3db because it's frequently used term in electronics.
    actually I was totally blank about this term 3db and it's meaning . I wanted to understand the complete theory of 3db whether it's in terms of gain or loss. I have frequently seen in explain it's Always written -3db that's why I wrote the same.
     
  14. wayneh

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    Sep 9, 2010
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    I have no problem understanding the relative meaning of decibels, but I always struggle with the absolute. For instance my home theater receiver dutifully reports the volume setting in decibels. If the wife is out, I can watch a movie at -20dB and have it be gloriously loud. The AV gear heads talk about 0dB being "reference level". What the heck does that mean?
     
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  15. MrChips

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    Aha! That is a darn good question.
    In the telephony, recording and broadcasting industry there is a standard reference level of 1mW into 600Ω.
    VU meters are calibrated so that the needle shows 0dB when 1mW into 600Ω is applied. This is equivalent to a voltage input of 0.775V into 600Ω.
     
  16. Lestraveled

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    The dB system is a relatitive system and has no absolute value unless referenced to something else. For instance dBm is dB referenced to 1 milliwatt. dBV is db referenced to 1 volt.
     
  17. wayneh

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  18. MrChips

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    owwwww!

    105dB_SPL is loud!

    You may want to set your volume control to maybe -60dB.

    Noise_HR_EN.jpg
     
  19. wayneh

    Expert

    Sep 9, 2010
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    Well I think the idea is that 105dB is the loudest peaks. Most content would be 20dB below that, or 85dB. Since I'm usually 25 below that, I guess I'm at 60dB.
     
  20. mishra87

    Thread Starter Member

    Jan 17, 2016
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    Thank you all for your comment !!!
    Let me go through all the comment and links given by you people and try to understand ..!!!!!!!!!
     
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