Example 1: A 100 Hz square wave is applied to a low-pass filter with a cut-off frequency at 600 Hz. The filter input and output signals are displayed on the oscilloscope screen channels 1 and 2 respectively. The spectrum analyzer shows the power spectrum of the input signal on channel 1. You can see that the output signal still looks like a square wave but it is distorted.
The next screenshot shows the power spectrum of the signal displayed on Channel 2. We can see that harmonics beyond the cut-off frequency are sharply attenuated. In fact the only harmonic we can see is at 600 Hz. Since the signal power of the 5th harmonic (900 Hz) is approximately - 20 dBW and this harmonic is not visible at all in the output spectrum, the attenuation at that frequency must be at least 30 dB.
Nevertheless, it is interesting to see that a square-wave like signal can be obtained with as little as four harmonics.
Example 2: In this example, the input signal is noise with a uniform power spectrum. That is, the signal power is constant at all frequencies. The filter input and output signals are displayed on the oscilloscope screen channels 1 and 2 respectively. The filter used is a 12th low-pass filter with a cut-off frequency of 1 kHz. We can see that the output signal (channel 2) is a cleaner version of the input signal. Much of the high frequency content has been filtered out. The output spectrum shows the attenuation beyond the cut-off frequency.
Example 3: In this example, we have passed a 100 Hz random signal through a 5th order low-pass filter with a cut-off frequency of 300 Hz. The filtering can be observed in both time and frequency domain: Harmonics beyond 300 Hz drop abruptly and no harmonics beyond 600 Hz are visible. In the time domain, the output signal still resembles the input signal, however the high frequency oscillations in the input have been filtered out.
