Theory and design of waveform purer

To reduce harmonics in the AC voltage signal, a precise bound-pass RC filter named ‘waveform purer’ is designed with a double-T network in the negative feedback loop. This filter keeps fundamental frequency unvaried, in both phase and amplitude, but greatly reducing harmonics. Related principles are discussed, including the ‘bridge balance principle’ of double-T network and the ‘three time constants principle’. Experiment result shows that output of waveform purer has a very high precision, with an uncertainty about parts of 10⁻⁸ V.