"Tim Shoppa" wrote in message
ups.com...
Are you perhaps really talking about RC active filters to replace
crystal or mechanical IF filters? I have always been VERY PESSIMISTIC
about this, usually even few percent tolerance capacitors (not cheap)
cause your passband/ultimate rejection to be way way worse than what
you expect.

I'm not as pessimistic as you, but I share your disappointment to some extent.
However, if a filter doesn't need to be in "active duty" 100% of the time,
having it self-calibrate seems potentially viable, at least for lower order
filters.

Different filter topologies have different tradeoffs, but
RC active filters when you want the Q to be 100 and want even mediocre
ultimate rejection are economically unfeasible in every analysis I've
done.

I'm usually after active filters for the sake of space, and consider Q100
difficult to achieve at HF with any physically small design.

I do enjoy the occasional posts from people whose SPICE simulations show
ultimate rejections in the couple hundred dB ballpark. :-)

---Joel