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CNF vs NF noise fgure calculations

Question asked by jdixon on Mar 21, 2005
Latest reply on Mar 21, 2005 by JERRY_MARKETOS
I have been attempting to calculate the noise figure of a feedforward amplifier using the CNF parameter. The results have not always made sense.

In reviewing my calculation I took apart the system to make the calculation simpler and found an anomoly with the model for the single directional coupler (Coupler). It seems that if I simply cascade a coupler on the front of an amplifier, the calculated noise using the CNF parameter does not always make sense. Replacing the Coupler with an attenuator of equivalent loss, yields a noise figure that seems correct.

Any ideas what I am missing or have done wrong?