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Cascaded Noise factor of RF system

For components like op-amps, we specify noise in terms of equivalent voltages and current. But at system level its difficult to account for noise contributions of each component in each sub-system. Instead we use another term widely know as Noise Factor (F) of a sub-system.
Lets discuss the calculations required to obtain Total Noise Factor (Ftotal) at input of the cascaded RF system.
Block Diagram - Noise contributions
Block 1
Gain=G1
Block 3
Gain=G3
Block 2
Gain=G2
(F1-1)kTB
(F2-1)kTB
(F3-1)kTB
Block Diagram: N access noise contributions at the input of system.
Rest of our calculations are based on the equivalent noise Factor of the system with cascaded components. In order to calculate total noise factor, we will need to refer the noise back at the input of the system. Block diagram below shows the Total Noise contributions.
Block 3
Gain=G3
Block 2
Gain=G2
Block 1
Gain=G1
(F2-1)kTB/G1
(F3-1)kTB/(G1G2)
(F1 -1)kTB
Total noise factor at the input of the system is calculated with formula below:
Total Noise Factor of cascaded system with 3 blocks is below

Ftotal = F1+(F2-1)/G1+ (F3-1)/(G1G2)
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SNR:- Signal to Noise Ratio

It is the ratio of Signal power to Noise power.

Noise Factor:

Noise Factor is ratio of input SNR to the output SNR of a receiver.

Noise Figure:

Noise Figure is a measure of degradation of SNR when a signal passes through a system.

If there is no noise (ideal) in a system then we will have following:-

F = 1

NF = 0

So, NF is always greater than or equal to 1

SNR = Signal Power / Noise Power
F = SNRin / SNRout
NF = 10 log10 F
Associated Theory
In above diagram, we have noise contributions by each sub-block as (Fn-1)kTB.
Sub block has gains G1, G2 and G3.
Sub-block has individual noise contribution as F1, F2 and F3.
k is Boltzman constant,
T is temperature (degrees Kelvin) and
B is frequency bandwidth (in Hz).