Hello Hal,
I wonder how much of that market area is now done in software?
What was the upper limit on the useful frequency of switched cap
filters?
AFAIK the LTC1068 can be clocked to above 5MHz and if you'd design a
filter with a clock/CF ratio of 25 you could manage 200kHz. Not
impossible with a digital system but it does get expensive. However, the
LTC1068 would also cost you over $5/1k.
So nowadays it often looks like this: Audio is done in firmware while
things above 100kHz are handled by ye olde LC circuitry, clever mixing
and so on. Active filters have crept up into those regions as well but
power consumption can become an issue.
It all depends on what you want to do. There really is no way around the
fact that discrete inductors show around 10% tolerance and capacitors
with tolerances under 5% are expensive. Many times we analog guys first
take a look around, see what's there in filters for consumer gear.
Ceramic IF filters and so on. Then we try to come with a mixing scheme
that lets us use those filters since the generation of carrier
frequencies is inexpensive and precise.