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.