Making A Low Pass Filter with a Capacitor?

Question

If I want to make a low pass filter which significantly attenuates signals above 5kHz using a 60 ohm resistor and a capacitor, what value capacitor, in microfarads (ΩF), should I use?

billrussell42 · Accepted Answer

Depends on what frequencies you want to pass through the filter, and how much you want to attenuate the high frequency signals.

Series R, shunt C is the circuit.

fc = 1 / 2πRC
will give you the cutoff frequency, where the gain starts to roll off. 
if you pick a fc of 1 kHz, the C = 1 / 2π(1k)(60) = 2.6 µF

starting with that, at 5kHz, the attenuation is
Xc/R = 1/R2πfC = 1/ (60)(2π5000)(2.6µ) = 1/5 
or output = 0.2 volts for 1 volt input.

As you can see, with a single pole filter, the roll off is slow.

You can increase the C for more attenuation, but that lowers the cutoff frequency, and we have no idea of the parameters, such as frequencies that must pass unattenuated.

Making A Low Pass Filter with a Capacitor?

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