raypsi wrote:
Hey OM:

I beg to differ there are frequencies below ZERO hertz you are just
not aware of them.

Because the "frequencies below zero" will come out of a single mixer
as equal frequencies above zero. Only the phase is different. So
when you use two mixers and quadrature signals, it is possible to
select a sideband. But with a single mixer and synchronous detection,
this is not possible.