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 circuit, to control air conditioning systems, was found in July 1984 Radio Electronics...
Using bipolar, FETs and unijunction this long-lasting timer can reach 10 hours with good quality...
We found this circuit in an old American documentation (80's). The circuit generates sine signals...