Cascaded Noise Figure Question

Thread Starter

imzack

Joined Nov 3, 2010
72
I am having trouble having a intuitive understanding of how the noise figure equation works.

I understand how a attenuator, for example, has a direct impact on the noise figure; by measuring the impact of signal to noise in vs SNR Out

But when we go into a cascaded network, I dont see how placing the attenuator in a different location in the chain would effect the noise figure.

I would figure a 3dB attenutor would effect the NF the same way, no matter where its placed (ie before or after a amplifier)

Does anyone have a diagram or any knifty analogy, to help get my mind wrapped around why putting a attenuator after a amp, has a different effect then putting it before an amp? ( I can clearly see this per the Friis formula, but don't have a nice way to understand it otherwise)

Thanks
noise_Fig.jpg
 
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