I've heard that high side mixing is preferred to low side
mixing because the LO harmonics will be higher and less
likely to cause problems.

It also seems to me that (f1 + f2) is typically discarded
and what's used is just the (f1 - f2).

Any truth to either of these impressions? If so, what's
the rational?

Depending on the availability of off the shelf VCO parts
one might what to use (f1 + f2) or low side mixing and I
was just curious what (if any) additional problems I should
look out for.

-- John