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How to calculate filter order from graph?

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Syafiq

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Hi

How do I calculate the filter order from a graph according to the formula "log(f/fc)" ? My fc is 20kHz, but I am unsure of what "f" represents. Does it represent the input frequency? Thanks!
 
Fc is the Filter CutOff frequency.

f is the independent frequency at which you wish to evaluate the filter response.

ie If f=10Fc, you are talking about a frequency one decade above the cutoff freq, which if it is -40db down from the response at Fc, means you have a second order filter.
 
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Fc is the Filter CutOff frequency.

f is the independent frequency at which you wish to evaluate the filter response.

ie If f=10Fc, you are talking about a frequency one decade above the cutoff freq, which if it is -20db down from the response at Fc, means you have a second order filter.
Let's say my Fc = 20kHz, and I am doing a 3rd order filter. With this, how do I find my actual "db/decade" based on my graph? I understand ideally a 3rd order filter will be -60db/decade, however I would like to make a comparison whether I actually achieved that value.
 
filter.jpg
 
If you measure the slope of the roll-off, once the roll-off is flat (log-log scale), then the dB/octave or db/decade will tell you the filter order. Each added filter order generates an additional roll-off of 6dB/octave or 20dB/decade of frequency.
 
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