Electronics > Beginners
How to elliminate low frequencies (RC Filters and co)
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bitman:
I'm "playing" with filters, practicing the formula and trying to apply them to practical use cases.  One of the things I'm trying to do is to aplify the signal that the filter passes through, but I don't want anything on the lower frequences coming through. Since filters do not block lower/higher frequencies but just makes them smaller, is there any way to block unwanted frequences completely or how would you approach the issue of only letting frequencies higher than let's say 100Khz through to the circuit you want the frequency to control. In other words, frequencies lower than 100Khz would be considered noise.

Is this "just" a game of averages where making the signal low enough that it has no real impact on the subsequent circuit?
rstofer:
Filters can have very slow roll-off like a single pole RC filter at -20 dB per decade or something MUCH steeper like the Chebyshev Filter.

https://blog.bliley.com/filter-typology-face-off-a-closer-look-at-the-top-4-filter-types

There are also crystal filters which are very narrow in a pass band.

In the end, the idea is to reduce the amplitude of the unwanted frequencies to the point where they don't unduly influence the output.

And then there is the entire topic of Active Filters (op amps start to show up)

https://en.wikipedia.org/wiki/Active_filter
mikerj:

--- Quote from: bitman on December 13, 2018, 12:37:33 am ---Is this "just" a game of averages where making the signal low enough that it has no real impact on the subsequent circuit?

--- End quote ---

Effectively yes, you choose your filter design so that out of band signals are attenuated sufficiently for your application.  The perfect filter that you describe is commonly known as a "brick wall filter" and it's not implementable in practice.  However multi-pole filters with sharp roll off and high stop band attenuation are realisable, though they often introduce their own problems.  In the analog world component tolerances can have a significant impact the performance of multi-stage filters, introducing excessive ripple or peaking.
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