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Model Noise in an RF System
In the two preceding equations, ZS is the nominal impedance, which is 50 ohms, and z+ is
the Hermitian conjugation of z.
The blockset obtains the noise figure, NF, from the noise factor:
NF = 10 log( F)
Calculate System Noise Figure
The blockset uses a recursive process to calculate system noise figure. The noise
correlation matrices for the first two elements of the cascade are combined into a single
matrix, and the process is repeated.
The following figure shows a cascaded network consisting of two 2-port networks, each
represented by its ABCD-parameters.
First, the blockset calculates noise correlation matrices CA' and CA” for the two networks.
Then, the blockset combines CA′ and CA″ into a single correlation matrix CA using the
equation
 A′ B′  ″  A′
CA = C′A + 
 CA 
 C′ D′ 
 C′
B′ 

D′
The ABCD-parameter matrices in the cascade combine according to matrix
multiplication:
A

C
B  A′
 =
D  C′
B′   A″

D′  C″
B″ 

D″ 
If there is another element in the cascade, the same calculations will be performed using
these ABCD-parameters as well as the ABCD-parameters corresponding to the following
element. The recursion will terminate with a noise correlation matrix pertaining to the
entire system. The blockset then calculates the system noise figure from this matrix.
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