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I know that we can build a chip (or circuit) or use FPGA to have re-configurable digital filters. But how about analog filters, can we have re-configurable analog filters? thanks

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can we have re-configurable analog filters?

Yes. The knob you turn on your grandma's kitchen or living room radio changes the tuning of an oscillator by changing the capacitance of a component. Any stable oscillator is essentially a filter for its resonant frequency.

Also, plenty of other examples: Tunable RC filters with adjustable R; mechanically tunable metal spring filters; tunable microwave cavity resonators with tuning screws, PIN diode-based LC filters... the list goes on.

Basically, take any analog filter topology and make one of the components adjustable, you get a reconfigurable filter.

Is that useful? Well, sometimes it is, if you need to keep your passband adjustable (though that is more commonly done via octave filter banks) or need to adjust your ADC's anti-aliasing filter (though that is more commonly done using high-rate ADCs and digital filtering / sampling methodology).

Generally speaking, you avoid doing complicated things with filters in analog domain as far as possible, really. Analog filters are always a trade-off, and digital filtering hence has replaced them wherever feasible.

Can you get as much freedom as with a digital filter? Practically never. Something like a RRC is pretty trivial in digital, but will need a lot of work to even remotely approximate in analog domain, it will be dependent on temperature, and costly. The closest you'd get to an analog RRC would probably be a bucket-brigade filter, and that's almost a digital filter: discrete in time, but continuous in amplitude.

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  • $\begingroup$ I am thrilled with your professional answer, thanks very much $\endgroup$ Jun 23, 2020 at 12:01
  • $\begingroup$ It may be worthwhile to mention "tone" knobs found on powered audio equipment as well as entirely-passive electric guitars. $\endgroup$
    – supercat
    Jun 23, 2020 at 20:27
  • $\begingroup$ You can also use switched capacitors filters as programmable filters too $\endgroup$
    – Ben
    Jun 23, 2020 at 21:45
  • $\begingroup$ @Ben there's a large semantic overlap between switched cap and bucket brigade filters. (I'd say they are essentially the same thing; or, the switched capacitor filter is the dominant implementation of a bucket-brigade filter; you can do the same using e.g. switched lengths of transmission line, but I'd call that "slightly exotic") $\endgroup$ Jun 23, 2020 at 22:35

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