What Is Noise Figure? Cascade Noise, the Friis Formula and Where to Put the LNA
Noise figure is how many decibels of noise a stage adds on top of a perfect receiver, and in a chain those figures do not add. The Friis cascade formula divides every later stage by the gain ahead of it, which is why the first stage sets the system noise figure and why a lossy feeder run in front of the amplifier is the most expensive mistake on the tower. This guide covers noise figure, noise factor and noise temperature, the cascade formula, why passive loss equals noise figure decibel for decibel, a worked four stage chain that gains 2.9 dB purely by moving the LNA, how much gain is enough, the dynamic range you pay for it, Y factor measurement, and when chasing a lower noise figure buys you nothing at all.
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