He is implementing a narrow bandwidth filter in the DSP to remove artifacts of the square wave.
It is the same procedure used by the FA-VA5 and EU-1KY antenna analyzers. Otherwise the square wave would be a big problem.
But using SA612 mixers could be a problem if they are driven into nonlinearity by all the harmonics involved.
For this reason, the FA-VA5 eschewed the SA612 in favor of a more linear mixer.
Michael Knitter has an English-language presentation where he describes, in detail, how the harmonics are handled internally by the FA-VA5.
