Subtract Blocks in GNURadio

[user11422]

Hi, I'm trying to make a mixer by subtracting the transmitted signal from the received signal to analyze the changes in the frequency which has occurred. but the subtract block does not seem to be working. I have tried to use this block while using 2 signal source blocks and it works fine with them but when I try to use them with the osmocom source blocks and subtract the received signal it doesn't work. it just shows the peak at its spot with and without the subtract block in place. I even tried to subtract two osmocom source signals (receivers) from each other which theoretically should give me zero output (like 2-2=0) but the output is the same as I would receive with 1 receiver block in place (meaning it did not subtract the signal). please if someone could tell me what am I doing wrong and if someone has any GRC flowgraph of a mixer that would be phenomenal. as I need to get the change in frequency I really need a mixer flowgraph regards