1, the square wave is rich in higher harmonics, which is difficult to be filtered out: after filtering, the higher harmonics of the square wave itself will also be attenuated, and the waveform will be deformed: the rising edge and falling edge will be gentle, and the vertex will become smooth. 2. The shaping circuit should be used, that is, the hysteresis comparator is used again to generate a new square wave after filtering. 3. The interference signal in the picture is not necessarily carried by the square wave itself, but is probably introduced by the transmission line of the oscilloscope.