This circuit was found in a 1924 Gernsback publication. The variable capacitors are dual. Listening is through a high-impedance phone.
The circuit shown was found in the 1994 National Semiconductor's Linear Application Handbook. The...
Phase and amplitude can be controlled in this circuit found in the May 1988 Radio Electronics...
This circuit was obtained from a 1968 documentation and can be adapted to use current equivalent...