# Cutoff frequencies from Frequency response curve

1. ## 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?

2. ## 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.

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3. ## Re: Cutoff frequencies from Frequency response curve

Originally Posted by 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?
By the way - there are many filter responses with a cut-off frequency that is NOT defined for the 3dB attenuation point (e.g. Chebyshev responses)

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4. ## 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? .
For a single section filter (only one storage element), that is the frequency where the resistance equals the reactance of the storage element (L or C). Let's take an example of a low pass filter where the resistance is connected to the voltage source and the output is taken from across the capacitor. For lower frequencies, the reactance of the cap is high, and most of the supply voltage is across the cap. Assuming R = 1, at the cutoff frequency where R=Xc, we get abs(-j/(1-j)) = 1/√2 . Expressed is dB's we get 20*log(1/√2) = -3dB.

Ratch

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5. ## Re: Cutoff frequencies from Frequency response curve

The -3dB point indicates 1/2 the power of the signal. (Not the voltage)

6. ## Re: Cutoff frequencies from Frequency response curve

klystron,

The -3dB point indicates 1/2 the power of the signal. (Not the voltage) .
You are correct, sorry for the typo. I should have written 20*log(1/√2) = -3dB.

Ratch

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7. ## 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?

8. ## Re: Cutoff frequencies from Frequency response curve

Originally Posted by sanjays87
i am doing project on frequency resonse analyser card..... and i dont know any thing about these Can anyone hel me in this?

9. ## Re: Cutoff frequencies from Frequency response curve

Originally Posted by klystron
The -3dB point indicates 1/2 the power of the signal. (Not the voltage)
What this actually mean?

10. ## Re: Cutoff frequencies from Frequency response curve

Originally Posted by surz90
What this actually mean?
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.