harmonic d istortion meter

The following circuit is an improved version of the harmonic distortion meter published in the 1977 Summer Circuits issue. In place of transistors J-FET op-amps are used, whilst the circuit offers the choice of four switched spot frequencies as opposed to the single frequency available with the earlier version. The basic principle and operation of the circuit is the same, i.e. by applying bootstrapping to a twin-T network the Q of the filter is increased to the point where attenuation of the harmonics is eliminated. The filter thus rejects only the fundamental of the input (sinewave) signal, al lowing the harmonic distortion products to be measured or examined on an oscilloscope.