This circuit from a Popular Electronics from the 90's can generate signals from 0.03 Hz to 3 kHz depending on the value of the capacitor and resistor.
The circuit shown is from the Texas Instruments optoelectronics manual, applying to low power...
This circuit was found in a 1968 General Electric documentation. It has an input impedance of 2 M ohms...
This filter that rejects a frequency range determined by the double-T was found in the August 1993...