From Broadband Communications and Home Networking
10.6 Noise and the Low-Noise Amplifier
The LNA is critical to the design of a satellite communications system. The system is improved directly for each improvement in the LNA. The LNA is the main element that sets the noise for the receiver. Unless there are large losses after the LNA, or the bandwidth becomes larger for some reason, which both of these factors may affect the overall noise figure of the system, the LNA is the determining element for the noise figure. Therefore, except for minor adjustments, the LNA sets the noise figure for the receiver. The complete noise factor equation is
F t=total noise factor,
F 1= noise factor of the LNA,
F 2=noise factor of the next amplifier stage,
F 3=noise factor of the third amplifier in the receiver,
G 1=gain of the LNA,
G 2=gain of the next amplifier after the LNA,
Losses=losses between the different stages.
The noise figure is calculated using the noise factor as follows:
The noise figure is the LNA (and all additional losses from the following stages according to the Equation 10.2) since the losses in the preceding stages between the antenna and the LNA are included in the budget. If there is loss between the antenna and the LNA, the signal level will be affected. The noise remains the same unless there is a change in temperature from the input of the loss to the output, which is generally not the case. Once...
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