This sophisticated remote field strength meter circuit was found in a 73 magazine for radio amateurs from the1990s. The circuit measures signals in the 1650 kHz range.
A 100 nF capacitor and 100 uH inductor form this circuit suggested by General Electric in a 1964...
This 1967 Raytheon circuit uses an optical coupler of the era and has a range of 800 to 200 Hz....
We found this circuit in a 1990 documentation. It works with the power of the digital circuit...