low pass filter

[shankar]

Anyone got an idea of how a filter can have a roll off rate greater than 20 db per decade using a single pole or more than 40 db per decade using a double pole.

[MP]

6 db per stage is pretty much the standard. In most cases Q=Fc/BW or BW=Fc/Q, where BW is bandwidth, Q is amplitude and Fc is the cutoff frequency (or center frequency if the filter is band pass). As you can see, you have to give up bandwidth or Q. Cascading several low order stages produces a high order filter, which is capable of what you want to accomplish. I have seen some strange non-standard versions of filters but they usually are lacking some item. I would recommend to stay with Butterworth, Chebychev or Salen Key as they are the proven standards. I have posted a state variable filter tutorial somewhere here on the forum which shows you how to calculate all of the parameters. A search should bring it up. If not, let me know and I will dig it up for you.

MP

[Kevin Weddle]

You forgot to mention the Bessel characteristic. By the way, these are just characteristics of the filter. Your filter will exhibit something close to one of these. Bessel is the most linear. Butterworth exhibits 20db per decade, which is a little steeper rolloff. The bessel is more like a one to one with frequency. This is desireable because the voltage can be linked to frequency directly. For instance, in the first decade, a drop of 1db might correspond to 100 hz and a drop of 2db will correspond to 200 hz. In the Butterworth, a drop pf 1db will correspond to 80 hz and a drop of 2db will correspond to 120hz.

[MP]

Kevin, the steeper roll-offs are only obtainable with cascaded design. The single pole concept will not give you much roll-off.

MP

[audioguru]

Kevin,
With a single-stage filter, there is no way to control the filter characteristic: a Bessel or Butterworth will both be the same, with a droopy 20dB/octave roll-off.
With a multiple-stage filter, a Bessel has a droopy, broad and undefined corner frequency, while a Butterworth uses feedback to achieve its sharp, steep and precise corner frequency.