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# -3 dB point of a cutoff frequency

March 2, 2011 | 10:43 pm

HI,
working with a low-pass filter I realized the amplitude response at the cutoff frequency is not 0.707 but 0.98.
Could someone explain why this value is so different respect what we learned in many articles on filters???
Thanks
fv

March 2, 2011 | 10:49 pm

How are you measuring that?

March 3, 2011 | 6:02 pm

The peak amplitude (out) at a whatever cut frequency.

March 3, 2011 | 6:29 pm

why should it be 0.707?

isnt 0.707that what we should exspect from a onepole filter
at one octave _above the cut frequency?

:)

March 3, 2011 | 7:50 pm

My question is how are you isolating the frequencies at the cutoff so that you are only measuring their amplitude? If you’re using something like peakamp it will measure the amplitude of the entire signal.

March 3, 2011 | 8:24 pm

what i’ve learned at school is -3dB at the center frequency and a -6/dB per octave slope for first order filter. So at twice the center frequency it should be -9dB…

But it seems, that these values are dependant of the center frequency for [onepole~] implementation…

Here’s something to play with…

– Pasted Max Patch, click to expand. –

March 4, 2011 | 10:00 am

so, as @mudang showed in its patch, – 3dB point of the cutoff frequency is only around 1 Hz…
when we increase the fc values, the "-3dB point" decreases, becoming 2 dB, 1 dB and so on…

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