# More questions about RC Filters

Discussion in 'General Electronics Chat' started by danielb33, Nov 23, 2012.

1. ### danielb33 Thread Starter Member

Aug 20, 2012
105
0
I posted a question earlier today similar to this, it was answered will but after reading I have more questions.

what is the cutoff (or corner) frequency for RC filters? My understanding is we have a basic equation for capacitors reactance, or resistance at difference frequencies. As frequency increases, reactance decreases. But when the frequency reaches a certain magnitude, the reactance increases again. Is this the cutoff frequency??? See the link below for a website showing the R-C Filter cutoff (or corner) frequency automatically after entering R and C values.

http://www.muzique.com/schem/filter.htm

Thanks for the help!

2. ### Audioguru Expert

Dec 20, 2007
10,668
1,190
The "cutoff frequency" of a filter is the frequency where the response has started to drop and has an amplitude that is -3dB (0.707 times) the output at frequencies where the filter has no effect.

The calculation is simple: f= 1 divided by (2 x pi x R x C.

3. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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*sigh*
$
f=\frac{1}{2 \cdot \pi \cdot R\cdot C}
$

4. ### danielb33 Thread Starter Member

Aug 20, 2012
105
0
The "cutoff frequency" of a filter is the frequency where the response has started to drop and has an amplitude that is -3dB (0.707 times) the output at frequencies where the filter has no effect.

When you wrote "the response" are you talking about the capacitors response to frequency? What amplitude? Honestly that confused me more than it helped lol. Maybe you can explain in little more in depth, breaking the above statement down to concepts like capacitance and voltage (i.e., the amplitude of some voltage or some current). This way I can actually picture what you are saying.

If you are feeling really generous you could write a little about -3dB. I keep reading dB everywhere but have never heard of this used in my schooling. We used %, not dB. Not sure why people use this, seems more confusing.

Thanks again for the responses.

5. ### Audioguru Expert

Dec 20, 2007
10,668
1,190
I am not a teacher so I will not teach you the basics that you can learn yourself.
You seem to know nothing about "frequency response".
You also know nothing about a simple RC filter.

Decibels are used for sound because they are logarithmic like our hearing's response to loudness. Percent is not used for loudness.

6. ### danielb33 Thread Starter Member

Aug 20, 2012
105
0

Decibals are not just for "loudness" as you wrote. They are simply a logarithmic ratio used for many things...in electronics we never really talk of decibels for sound but power or amplitude ratios.

Nov 13, 2012
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8. ### danielb33 Thread Starter Member

Aug 20, 2012
105
0
Thank you, those videos helped.