This filter was found in an 80's English publication. However, it can be implemented with modern operating systems. The circuit can tune frequencies in a range from 10,000 to 1.
This Philips circuit of the 70's makes use of components of the time. The DZD40 is an uncommon...
This circuit with two operating modes was found in the September 1989 issue of Radio Electronics...
This circuit is from an old documentation making use of a 3,579,645 MHz analog color television...