Alternative Fundamental Squarer
 

The classic fundamental squarer is the use of the 74AC86. However, I have had equal results, with a simpler circuit.

A potential divider, consisting of two series 15k resistors from the +ve to ground, with the centre of the resistors , connected to the input of a 74AC04 hex inverter, and the sine wave applied to the same point, via a 1 nF capacitor .