dog1
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Hello,
It's been quite a while since I last did a RF design, so I may be wrong.
I don't think the port resistance gives you the problem. refering to the attached picture, the NF of the system is related to the ratio of Vn+In* (input resistance of the system) and R1, where Vn and In are the input referred noise contributed BY THE SYSTEM, and is intrinsic to the SYSTEM and not affected by R2 or port 2. R2 may indeed affect the (input resistance of the system), thus affecting the NF. But if your system is unilateral to a good extent (as most RF system of this kind can do), then R2 shouldn't affect (input resistance of the system) by a factor of 2.
An explanation that is more vague but is closer to the definition of NF is that when the port 2 is changed, (the gain of the signal) and (the noise from the system and the input port) scales altogether, and the output S/N stays more or less the same when the input S/N is not changed.
some ideas of where the 3dB comes from: 1. check the gain and NF (singnle-differentail?) of the balun. 2. check if the overall noise is SSB or DSB (double side band).
Hope you can find out where it is going wrong.
BR
CHEN
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