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

 

This is because of fundamental properties of thermodynamics. An attenuator absorbs energy and gives off head. However, due to thermodynamics, an attenuator at any temperate above 0 degrees Kelvin (absolute zero) also converts heat to energy, which appears as noise on its output. So, although the attenuator attenuates input signal and noise by the same amount, it also ADDS noise. So, the net signal-to-noise on its output is degraded. For an attenuator at standard temperate (290 degrees Kelvin, if I recall correctly) the noise figure of the attenuator is exactly equal to its attenuation. Since the noise figure of any component affects (not effects) the overall RF chain Signal-to-Noise Ratio is less if it it after amplification,

 

Since the noise figure of any component affects (not effects) the overall RF chain Signal-to-Noise Ratio is less if it it after amplification, the attenuator’s noise matter less if the overall chain noise has been increased by an amplifier before it. An important parameter is “Noise Measure”, which takes into account both the gain and noise figure of a linear device. This is defined so that, in a chain of linear RF components, to minimize the noise figure of the chain, the device with the lowest noise figure should be first, with the second lowest noise figure second, …, and with the highest noise figure last. If the gain and noise figure of a device as power ratios (not dB) are G and F, the noise measure is defined as
= (F-1)/(1-1/G).

 

Whoops; In my last comment, should be:
An important parameter is “Noise Measure”, which takes into account both the gain and noise figure of a linear device. This is defined so that, in a chain of linear RF components, to minimize the noise figure of the chain, the device with the lowest noise measure should be first, with the second lowest noise measure second, …, and with the highest noise measure last. If the gain and noise figure of a device as power ratios (not dB) are G and F, the noise measure is defined as
= (F-1)/(1-1/G)

 

Hello there after looking at this document I believe it was written specifically for you. It's a good read. Use only as reference please.