Low Pass Filter basics

toyo22r: What determines the response type? (ie. butterworth, bessel, etc.) ==================== Response has to do with damping... this is controlled by Q of the coil Q=omegaL/R, f=1/(2*pi*R*C)

Sorry. Too early. that f was for an RC filter. For the LC filter f=1/(2*pi*sqrt(LC))

Active or passive ?

Graham

Not alone. The load on the filter affects the damping too. ( in the case of an active filter the gain ).

Graham

The various response types emphasise certain filter features, such that one chosen style may offer a little extra benefit in a particular filtering job. (You) make the choice based on knowing what flavours are available (there's about half a dozen kicking about) and the particular properties that they each advertise. Feature wise, there's not a great deal to mug up on but I can't offhand suggest a [useful] filter website as I've not seen one and most filter textbooks lose the plot after about page 1. At the end of the day, a LP filter is a LP filter and a design that gives components calculated for a Butterworth style response (feature: flat response upto it's cutoff) can be OK for maybe 90% of all day to day filtering needs. All the rest is just niceties :) regards john

Look here:

Chapter 3 has a nice explanation of antialias filters, and gives a good description of the different types of filters.