One of the most common designs for a SSB modulator (phase shift method) involves an additive mixer as the very last stage. Here is a block diagram for reference:
This additive mixer must provide isolation between the two inputs. I am only aware of one method for doing this, which is to "couple" the signals onto each other using well-chosen filters. Here is an example in the case of a multiplicative mixer:
However, this method only works when the two input signals are vastly different in frequency. How would I isolate two input signals when they occupy the same frequency band?
Given that I am dealing with RF frequencies, op-amp mixers are out of the question. Surely there is an active circuit design which provides sufficient isolation, with low distortion and able to run on a single ended supply.