dBFS (decibels relative to full scale) measures level inside the digital domain
of an SDR. Here 0 dBFS is the ceiling — the largest value the
ADC can represent — and real samples sit
below it as negative numbers.1
dBFS is the digital scale inside the SDR; 0 dBFS is the ADC's ceiling, so real samples sit below it.
How it works
If a signal reaches 0 dBFS the converter clips, flattening peaks and spraying
distortion across the spectrum. Leaving headroom below 0 dBFS keeps the signal clean.
Relevance to SDR
Gain should be set so the strongest signal peaks
comfortably under 0 dBFS. dBm describes the world outside the radio;
dBFS describes the world inside it.
Sources
dBFS — Wikipedia, decibels relative to digital full scale. ↩
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