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How to choose right DC return resistor for schottky diode mixer?

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Terminator3

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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?
 

vfone

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Is not mandatory to use a resistor. For higher currents (greater than few mA) you can use a choke.
 

Terminator3

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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.
 

jiripolivka

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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.

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.
Where diode pairs are used in mixers, diodes should be selected to have the "knee" at the same voltage or current, to achieve balancing and by that the good LO/RF isolation.
 
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