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RC low-pass filter

AC circuits and filters · Intermediate · about 15 minutes · part of Zen’s Electronics Design Mastery

The capacitor’s reactance falls as frequency rises, shorting high frequencies to ground. Below f_c the signal passes; above it the output falls 20 dB per decade and lags by up to 90°.

RC low-pass filter: the circuit on the simulator's bench

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How it works

fc = 1 / (2πRC) = 159.2 Hz

|H| = 1 / √(1 + (f/fc)²) = 0.705 (-3.0 dB) at 160 Hz

φ = −arctan(f/fc) = -45.2°

Try it on the bench

  1. Select the AC source and type other frequencies: 16 Hz (passes), 160 Hz (−3 dB, −45°), 1600 Hz (about a tenth).
  2. Compare the yellow input and blue output on the scope. The output lags.
  3. Open Tools… → Bode plotter and run a sweep: read the −3 dB corner off the curve and compare it with fc in the theory. Change a part and run again; the two curves overlay.

Parts on the bench

In the course

Zen’s Electronics Design Mastery, module “AC & Reactance”.

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