An RC filter simulator, with a scope and a Bode plot

An RC low-pass filter is one resistor and one capacitor: the signal goes in through the resistor, and the output is taken across the capacitor. Slow signals get through, because the capacitor has time to charge to each new value. Fast ones do not, because the capacitor cannot follow them. The frequency where the two meet is the corner, f = 1 / (2 pi R C).

The circuit above is a 16 k resistor and a 100 nF capacitor, so the corner is at 1 / (2 pi x 16000 x 100e-9), which is 99.5 Hz. A function generator drives it with a 100 Hz sine, a two-channel oscilloscope watches the input and the output, and a multimeter reads the output’s true RMS. At the corner the output is 0.707 of the input, 3 dB down, and an eighth of a cycle behind it, and that is what the scope shows.

Step the generator’s frequency with the buttons on its face. At 20 Hz the two traces sit almost on top of each other. At 500 Hz the output is down to about a fifth and lags by nearly 80 degrees. At 2 kHz it has all but gone. That is the filter’s whole behavior, measured by hand the way it is on a bench.

Then let the simulator do the sweep. Open Analysis in the editor’s more menu and choose Frequency response: drive the generator, watch the joint between the resistor and the capacitor, and sweep from 1 Hz to 100 kHz. What comes back is a Bode plot, gain in decibels and phase in degrees against frequency on a log axis. It is flat below the corner (a fraction of a decibel under 0 dB, which is the scope’s and the meter’s own inputs loading the capacitor), crosses -3 dB and -45 degrees at 99.5 Hz, and falls at 20 dB a decade above it. The engine linearizes the circuit at its operating point and solves it at every frequency, the way an AC analysis in SPICE does, and a Download CSV button hands you every point.

Change the capacitor to 10 nF and run the sweep again: the corner moves up a decade, to 995 Hz. Add a second RC stage after the first and the slope doubles to 40 dB a decade.

The same frequency response works on op-amp circuits, LC tanks and anything else on the canvas with a signal generator in it.