I am making a Tx chain using IQ mixer, so the signal first passes through Hybrid-90 and then IQ mixer and they are combined (as shown in Figure below). The combiner at the output doesn't have any phase difference (i.e., both paths have 0 degree of phase). I am getting +3dB in my signal at the output of combiner (which makes sense) but noise floor remains the same. why? Can someone please tell me how the noise floor adds up? In my opinion the noise should also be increased by +3dB because in my opinion the two path's noise are coherent because they are coming from same source.