LOWPASS
HIGHPASS
Passive RC-Filters have a slope of 6 dB per Octave. The cutoff frequency 
is the frequency at which the incoming signal is attenuated by 3 dB.
USAGE
Enter any two values (Frequency , Resistance 
, Capacitance 
)
Computed fields are highlighted in blue
Use the locks to protect fields
Arrow buttons (or keyboard up/down) jump to next/previous E-series values for R and C; hold Shift for fine (E96) steps
In the frequency field you may also enter values in cycles (use the unit s, ms, ns and so on).
What are RC-Filters used for?
A highpass filter is commonly used to remove low frequency content from a signal, for example in audio circuits to remove rumble or the DC (0 Hz) offset from a signal. This is very commonly used in audio circuits that are operated from a single supply voltage (e.g. +9VDC in guitar pedals) as opposed to the more professional bipolar power supplies with ±12 VDC or more. In an active single supply system with 9V all signals need to stay within 0 and 9V – since audio signals swing positively and negatively this would cut the negative half off1, that means these circuits needs to offset (or bias) the incoming signals by 4.5 V (half the supply voltage) so it can swing as freely as possible. At the output of a circuit this has to be removed again and for this passive RC highpass filters are commonly used.
Passive lowpass filters are used to remove high frequency content from a signal. This is for example used in the Tone control of a electrical guitar to shape the sound, but is much more commonly used to remove potentially harmful frequency content from signals with fast transitions for example to keep electromagnetic emissions low. Another use is to smooth out signals, remove high frequency noise and the likes.
Since the part count on passive RC filters is very low, this is an extremely common circuit.
- This is a simplification for the purpose of explaining the concept, in reality clipping behavior depends massively on the circuit used[↩]