I've designed both types of filters for years and can't really explain the difference in clear simple terms. I'm hoping somebody here can help me out with that. First, some working terminology: RF filter - a filter with a controlled input and output impedance, usually 50 ohms, to match the characteristic impedance of the transmission lines feeding it Signal processing filter - a filter with a high input and low output impedance, can be an active filter, usually used at frequencies below RF (e.g. audio) A good way to answer the question is in that "intermediate" area, say at 50Mhz. At this frequency both types of filters are practical. Of course cost is always a consideration but the heart of this question is why one would be needed as compared to the other. There are a lot of subtle things to consider, which I think is the reason I've struggled to come up with clear, concise answer. My replies will only serve to point out some of the subtleties that get overlooked in overly simplistic answers.