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25th March 2012, 17:20 #1
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Cutoff frequencies from Frequency response curve
While calculating the cutoff frequencies using the Frequency response curve, why do we draw a line 3dB below the curve to mark the Cutoff frequencies? Why is it 3dB and not something else?

25th March 2012, 17:50 #2
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Re: Cutoff frequencies from Frequency response curve
A 3 dB reduction in power is equal to 1/2 of the original value. I don't know the historical origin, but marking the point where signals are reduced by 1/2 or more, seems like a good point of reference.

25th March 2012, 17:50

25th March 2012, 17:54 #3

25th March 2012, 18:48 #4
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Re: Cutoff frequencies from Frequency response curve
Dineshkumar005,
While calculating the cutoff frequencies using the Frequency response curve, why do we draw a line 3dB below the curve to mark the Cutoff frequencies? Why is it 3dB and not something else? .
RatchLast edited by Ratch; 25th March 2012 at 19:33. Reason: Changed 10*log(1/√2) to 20*log(1/√2)
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25th March 2012, 18:48

25th March 2012, 19:03 #5
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Re: Cutoff frequencies from Frequency response curve
The 3dB point indicates 1/2 the power of the signal. (Not the voltage)

25th March 2012, 19:32 #6
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Re: Cutoff frequencies from Frequency response curve
klystron,
The 3dB point indicates 1/2 the power of the signal. (Not the voltage) .
Ratch

25th March 2012, 19:32

25th March 2012, 20:12 #7
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Re: Cutoff frequencies from Frequency response curve
i am doing project on frequency resonse analyser card..... and i dont know any thing about these Can anyone hel me in this?

26th March 2012, 02:11 #8
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26th March 2012, 02:59 #9
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26th March 2012, 03:16 #10
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Re: Cutoff frequencies from Frequency response curve
Decibels is a way to quantify the measure of something to a known reference level. You can measure decibels (dB's) of any magnitudebased quantity... voltage, current, power, resistance, etc. Most often people will talk about the 3 dB point of a filter, which is the point at which the output signal is attenuated by 1/2.
To calculate dB's of voltage, current, resistance, etc... you use 20*log(x)
To calculate dB's of power, you use 10*log(X).
When someone says a filter has 20 dB of rejection at 100 MHz away from the center frequency, they are saying that a signal at that particular frequency will go in at some power X, and come out of the filter at a power level of 0.01*X. 20 dB = 10*log(?), so ? = 0.01.
For more information, start reading up here: http://en.wikipedia.org/wiki/Decibel
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