Noise figure and noise temperature

Noise figure (NF) compares a component’s output signal-to-noise ratio with its input SNR, assuming the input noise comes from a source at the standard reference temperature of 290 K. A perfect, noiseless amplifier has NF = 0 dB. Noise temperature says the same thing in absolute terms: the temperature of a resistor at the input that would produce the same added noise.

Datasheets for LNAs and SDRs quote noise figure. Radio astronomy works in kelvin, because the system temperature is a sum of the receiver, the sky and the ground, and you can only add things that are in the same units. Convert before you add.

Conversion table

Notice how non-linear it is. Going from 1.0 to 0.5 dB looks like a small change in dB and halves the noise temperature. At the low end, every tenth of a dB is worth several kelvin, which is a meaningful fraction of a good system’s total.

Loss is noise

A passive lossy component (cable, connector, filter, a splitter) at physical temperature T adds (L − 1) × T of noise, where L is the loss as a linear ratio. At room temperature its noise figure is simply its loss in dB. So 0.5 dB of connectors and adapters in front of your LNA is 35 K added before amplification, as much as a good LNA. Outside on a cold night the same loss adds a little less, since T is lower.

That’s why the LNA goes at the feed, and it’s the core of LAB-011. To combine several stages, use the Friis formula, or let the receive-chain noise calculator do the conversion and the cascade together.