Advanced Member level 3
Currently i use trial and error method, values around 1 kOhm seems good in balanced mixer. Is there any rule of thumb for choosing such resistor?
In any mixer, diode (s) operate best at the point where the I/V curve has the "knee", or the sharpest bend. This operation point is often adjusted by LO input power, and also by DC loop resistance. In DC-biased mixers, the external DC bias is used to adjust this optimum operation point, so LO input power can be one tenth of that above.When i put low valued resistor, say 50 Ohms, performance degrades it becomes better up to 1000 Ohm, and then i do not see significant change around 1000-2000Ohm... I does not understand tendency here. As it is DC return, i guess U=R*I can be used somehow to understand it.
I tried rat-race topology for receiving mixer. But still not satisfied with results. I do not have VNA to check it ports, but i suspect that isolation between ports is not good, because directing antenna to electrical fan raises IF output amplitude, as i understand it is doppler from fan blades, and LO leaks through receiving antenna. Or mayve computer fan someow radiates harmonics. Still thinking about better idea to test mixer isolation.