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RC Low-Pass Filter

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Recipe #3 — RC low-pass filter

A resistor and capacitor can form a simple filter that allows lower-frequency changes through more easily while reducing higher-frequency content.

RC LOW-PASS FILTER IN R = 1 kΩ OUT C = 0.1 µF low frequencies pass more easily; high-frequency noise is shunted toward ground

Example values

R = 1 kΩ and C = 0.1 µF.

fc = 1 / (2πRC)
fc ≈ 1.59 kHz

The cutoff frequency is not a brick wall. The response changes gradually around it.

Real-world uses: removing high-frequency noise from a sensor signal, smoothing a control voltage, reducing hiss or RF pickup, or forming the low-frequency branch of a simple crossover.

Why the capacitor helps remove high-frequency noise

A capacitor's reactance decreases as frequency rises. Higher-frequency signal components therefore find an easier path through the capacitor toward ground.

Speaker crossovers use the same idea — with different parts and power levels

A passive loudspeaker low-pass branch commonly uses an inductor in series with the woofer rather than this small-signal RC arrangement. The underlying purpose is the same: keep unwanted high frequencies out of the low-frequency driver.

Try these yourself

Which frequencies does a low-pass filter favor? RC LOW-PASS FILTER IN R = 1 kΩ OUT C = 0.1 µF low frequencies pass more easily; high-frequency noise is shunted toward ground
A low-pass network passes lower frequencies more readily and increasingly attenuates higher frequencies.
What practical job can an RC low-pass filter do?
A low-pass filter can smooth or remove unwanted high-frequency content.
With R = 1 kΩ and C = 0.1 µF, the cutoff is closest to:
fc = 1/(2πRC) ≈ 1.59 kHz for 1 kΩ and 0.1 µF.
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