Found in a Popular Electronics from the 90s, this circuit is intended to measure the harmonic distortion of an audio signal. The circuit can be tuned between 970 and 1030 Hz.
This type of circuit and with this integrated circuit appears several times on this website. The...
Using transistors of the time, this circuit was published in Radio Electronics magazine in...
The integrated circuit used in this 1982 project is not very common today. It can be adapted to...