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CNF decreasing and CGAIN increasing with mismatch?

Question asked by davidfc on Jan 25, 2005
Latest reply on Jan 28, 2005 by davidfc
In a simple chain that has only nonreflective switches with high isolation (60dB) as alternative paths, I've seen that with an active mixer as the final stage with reference impedance 200 ohm instead of 50, the CNF improves about 6dB and the CGAIN increases by 6dB with respect to the situation where the mixer has a reference impedance of 50 ohm. The output port's impedance is changed with the mixer too...

Could any of you explain this?, I'd expect that CNF improves with mismatch as only the forward noise power is considered, but I don't understand the CGAIN improvment at the same time.

The mixer is working as a direct conversion mixer (frequency difference is 0 Hz).