Assume you have a 12khz signal mixed with a 1.8mhz signal. Actual values not important, this is just for arguments sake. Your mixer output would then contain 12khz, 1.8mhz, 1.812mhz (the desired output) and 1.788mhz. Now correct me if I'm mistaken, but the inputs should also be 180 degree phase shifted (inverted). So then can I, by mixing the output with the input signals again, remove the 12khz and 1.8mhz? Furthermore, and this is the main question of this post, how do we get rid of the 1.788mhz signal?