This PLL with operational amplifiers was found in 1969 documentation. The configuration can be adapted to use modern components. The circuit has a gain of 15 V / V and can operate with 10 kHz signals.
This filter that rejects determines frequency (notch) was found in a Popular Electronics of 1993....
This circuit operates with frequencies from 0.5 to 400 MHz. We find it in a 90’s NASA...
This circuit for adjusting sources simulating a load was found in a publication from 1993. The...