Discussion
Using Friis with \( F_1 = 2 \), \( G_1 = 10 \) and \( F_2 = 4 \), the overall noise factor is
Answer: D
Applying the cascade formula:
\[ \begin{aligned} F &= F_1 + \frac{F_2-1}{G_1} \\ &= 2 + \frac{3}{10} = 2.3 \end{aligned} \]
so 2.3, about 3.6 dB. Adding the factors directly gives 6.0 and ignores the first stage's gain entirely. 2.0 ignores the second stage. And 4.2 follows from no correct substitution. The formula's structure is the important part: the second stage's contribution is divided by the gain ahead of it. Checking the units of each quantity catches a wrong formula before the arithmetic begins.
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