Over on YouTube Paul from ‘Tall Paul Tech’ has uploaded a video that explains and demonstrates the effect of square wave harmonics using a HackRF and RTL-SDR.
Paul starts by using GNU Radio to explain how square waves are created from a set of sine waves, and how a signal can essentially turn into a square wave if it is too strong and clipping the analog to digital converter (ADC). The result for your SDR is that harmonics of strong signals can show up at incorrect frequencies.
Later in the video, Paul shows this effect using a HackRF and RTL-SDR.
