Noise floor, SNR and sensitivity

Plug a receiver into nothing: you still see a low, restless line. That's the noise floor — the random energy that is always there. No signal can be read below this floor.

Where does the noise come from?

Thermal: electron agitation in every conductor. Incompressible — it's physics (−174 dBm/Hz at room temperature, the "floor of floors").
The receiver itself: every stage adds its own; this degradation is summarised by the noise figure — a few dB for a good LNA.
The environment: switch-mode supplies, screens, USB cables, routers, LEDs… In town, this man-made noise vastly dominates thermal noise on most bands. That's the one you can fight.

SNR: the only measure that matters

A signal is only useful if it rises above the floor. The gap between the two is the SNR (Signal-to-Noise Ratio), in dB:

SNR = signal_power_dB − floor_dB

SNR	What you get
< 6 dB	barely detectable presence, nothing usable
6–12 dB	reliable detection, robust modes decode (ADS-B, FT8)
12–20 dB	decent FM listening, comfortable decoding
20–35 dB	clean local station, crisp audio
> 35 dB	very close transmitter — watch for saturation

A nearby FM station can peak +40 dB above the floor; a distant sensor barely +6 dB. This is the number the console's "Signals heard" panel shows you live.

Two levers (and one trap)

Raise the signal: a better antenna, better placed (Régler son antenne, pas à pas), or get closer.
Lower the noise: move the SDR away from interference sources (shielded USB extension), switch off suspect power supplies, filter.
The trap: raising the gain creates no SNR. Gain amplifies signal and noise; its only job is to lift both above the receiver's own noise. Too much gain → saturation → SNR goes down.

Sensitivity and dynamic range

A receiver's sensitivity is the smallest signal it can pull out of the noise. Its dynamic range is the gap between the weakest and the strongest it can handle at the same time without saturating — crucial when a powerful transmitter sits next to a weak one (8 bits ≈ 48 dB, see Échantillonnage, Nyquist et aliasing).

This site's detector applies exactly that logic: it estimates the floor (≈ 20th percentile of the bins) then keeps only what exceeds floor + threshold.

Your turn

Floor at −85 dB, FM peak at −52 dB: SNR? (33 dB — crisp listening.)
Unplugging the external monitor drops your floor by 6 dB. What did you gain? (+6 dB of SNR on every signal — the equivalent of a 4× better antenna, for free.)
Gain maxed out: floor rises 20 dB and the peak rises 20 dB too. SNR gained? (Zero — and you're courting saturation.)

👉 Measure a station's SNR: Catch an FM station