how to increased the roll off rate

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naspek

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hey there...
i wanted to modified my 1st order Butterworth low pass filter with K=1.
i want to increased the roll off rate.. but, after changing the resistor value,
the capacitor value and fo, i the roll of rate still the same..
how can i increased my roll off rate?
 

To change the roll-off rate you need to add an extra RC stage.

An nth order pole has a slope of -20*n dB/decade.
What you are doing is changing the critical frequency and not the slope..

google bode plot concepts!
 

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    naspek

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abbeyromy said:
To change the roll-off rate you need to add an extra RC stage.
An nth order pole has a slope of -20*n dB/decade.
What you are doing is changing the critical frequency and not the slope..
google bode plot concepts!
Click to expand...

Just two small correction/supplements:
1.) An "extra RC stage" means in practice: A second order network instead of 1st order, because the series connection of two first order stages does allow a maximum pole Q only below 0.5 (Butterworth: Q=0,707)

2.) Not an "nth order pole" but an nth order transfer function has the mentioned slope of -20*n dB/dec.

_________

And - naspek - please don't speak anymore from a first order Butterworth response!
 

if i add another RC network, the roll off rate will be -40dB/decade..
which is decreasing from -20dB not increasing. am i right?
 

naspek said:
if i add another RC network, the roll off rate will be -40dB/decade..
which is decreasing from -20dB not increasing. am i right?
Click to expand...

Is -5 larger or smaller than -1 ?

A slope of -40dB/dec is smaller than -20dB/dec but the corresponding roll-off rate is larger, because the roll-off rate is considered to be positive! (The word "roll-off" contains already the negative slope). That´s the whole story.
But check your task! Are you interested in another roll-off rate only or do you like to design a filter with a certain transfer function resp. frequency response (like Butterworth, for example) ?
 

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    naspek

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@LvW
Thanks for the corrections!!
 

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