This circuit is from 1964 documentation but can be easily implemented based on current components. Input waveforms and detection are shown next to the diagram.
We found this circuit in the 1993 Motorola Linear Interface ICs. The operational is unusual today, but...
This digital mixer uses CMOS integrated circuits. The circuit was published by us in a magazine...
Using an operational amplifier, this circuit was found in the March 1996 issue of Electronics Now...