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 circuit is from a specialized documentation in 1992 FETs. The L1 coil has 0.07 uH with...
The LM1468L can also be used. The circuit is from Motorola's Linear Interface ICs and the...
This low frequency doubler requires a 50 mV signal at the input. The circuit is from an old...