What is Noise Figure?

The Noise figure of an RF component such as a low noise amplifier or a mixer is a measure of how much it increases the noise in a system that it’s a part of. In the picture below, both signal and noise into an LNA are amplified. However the total noise level increases more than the signal level. This is due to the added internal noise from the LNA and it results in a degradation in signal-to-noise ratio.

Noise Figure of an LNA

Noise figure is more of a concern in receiver systems than it is in transmitter systems. Within the context of receivers, the lower its noise figure, the more sensitive it will be and the better its ability to detect weak signals.

Here is a comparison of the noise figure of different Software-defined radios as measured by the Libre Space Foundation team and published in their detailed report on SDRs.

Airspy Mini12 dB
SDR Play 5 dB
Pluto SDR5 dB
Lime SDR Mini15 dB
Noise Figure Measurements at 1280 MHz for various SDRs

Clearly the best performers are the SDR Play and Pluto SDR – by a significant 10 dB relative to the worst performer. How does this difference impact performance? At 900 MHz, the SDR Play or Pluto SDR will have nearly twice the listening range relative to the Lime SDR Mini. That’s a significant advantage!

How easy is it to reduce receiver noise figure?

In a receiver chain, the component at the front-end contributes most to noise figure. This is why the first component in a receiver chain is typically a low noise amplifier. In the case of the RTL-SDR, Airspy or Lime SDR, a low noise amplifier module can easily be connected to the RF input. This will improve performance. It’s straightforward to get significant gains in noise figure performance this way. Practically you can get between 0.5 dB to 1 dB of Noise figure with relative ease using off-the-shelf LNAs. Any improvements to get below 0.5 dB are very challenging and expensive to achieve.

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