7.2 Cascaded Noise
Components rarely work on a stand-alone basis. You will need to connect the base station to the DAS via cables. The passive DAS consists of many passive cables and many passive components, adding up the losses from the base station to the antenna and vice versa. Using active DAS you still need to use a passive cable to connect to the main unit, and the active DAS consists of amplifiers and other active components. Hybrid DAS is a mixture of a large portion of passive components and cables, with active amplifiers and distribution units.
When you chain amplifiers and passive components or cables in a system, it is called a ‘cascaded’ system. Most RF systems will be cascaded systems. The amplifier will amplify the signal and the noise of the preceding amplifier or passive component or cable, and therefore the NF builds up.
7.2.1 The Friis Formula
We can calculate the noise factor (F) of any number of chained amplifiers or passive components (stages) using the cascaded noise formula known as the Friis formula. The Friis formula can be used for any number of cascaded components, from 1 to n. is the noise factor and gain of stage 1, is the noise factor and gain of stage 2, etc.
The values for gain and NF have to be input as linear values (factors).
Linear Gain (G)