- An RC filter is very soft, a single (real) pole. The roll-off is gradual, and the asymptote is -20dB/dec.
- You can chain RCs, but the roll-off gets even softer. An RCRC filter (equal values) has two poles, but the effect of each RC on the other causes the poles to spread apart, by a factor of 3 or so IIRC. Effectively it's a cutoff at Fc then another cutoff at 3Fc. So the roll-off is very gradual indeed. You eventually get a -40dB/dec asymptote.
- You can mitigate the interaction effect, by scaling the values geometrically. You might do an R1-C1-R2-C2 filter with R2 = 10*R1 and C2 = 0.1*C1. This puts the poles within about 10% of each other, almost as sharp as a single RC but twice as effective.
- This doesn't take you very far of course, as three stages you need 100x the values, etc.
- Aside: you can actually get voltage gain from a passive (RC) filter. The trick is to use phase shift to your advantage (it is a non-minimum phase filter). The maximum gain per stage is very small, a few percent. Just a neat curiosity.
- Active filters use gain elements to raise the Q factor of RC networks, or transform them into other impedances (negative resistance, inductance, etc.). This is also an aside, as you've specified passive.
- And finally, that leaves inductors. An LC filter can have voltage or current gain (just not power gain), can have complex poles (giving sharper cutoffs, peaked or ringing response, etc.), but requires impedance matching for consistent response -- the network is part of a power transfer system, and that transfer is completely dependent on the source and load impedances.
You can also mix them, so you have RLC filters with responses somewhere inbetween; some power is lost in the filter, and the cutoff may be softer, but maybe it's more tolerant of impedance mismatch. Resistance of course is unavoidable in any filter*, but we generally design LC filters with low enough component losses (high Q inductors, capacitors) that the insertion loss is usefully low.
*Yes even superconducting filters! There are still loss mechanisms even in otherwise-ideal conductors. The trick is, superconductors are fine at DC, but at AC, all bets are off. In the extreme, consider a puck of YBCO: it's black, and doesn't change color with temperature. It's not superconducting at optical frequencies at least. It actually follows that, somewhere between DC and light, there is a cutoff where resistance takes over, and indeed there's an obvious quantum-mechanical reason for that. They can be quite excellent though: Q factors of 10^7 or so, better than most quartz crystals. Optical resonators too (built from dielectric supermirrors).
Tim