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- 25th March 2012, 18:20 #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

below the curve to mark the Cutoff frequencies? Why is it 3dB and not something else?__3dB__

- 25th March 2012, 18:20

- 25th March 2012, 18:50 #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, 18:54 #3

- 25th March 2012, 19:48 #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 20:33. Reason: Changed 10*log(1/√2) to 20*log(1/√2)

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- 25th March 2012, 19:48

- 25th March 2012, 20:03 #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, 20:32 #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, 21:12 #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?

- 25th March 2012, 21:12

- 26th March 2012, 03:11 #8

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- 26th March 2012, 03:59 #9

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- 26th March 2012, 04:16 #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 magnitude-based 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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