What is an SDR?

A Software Defined Radio moves into software what a classic radio does with dedicated circuits. The hardware shrinks to the bare minimum; everything else — filtering, demodulating, decoding — becomes code.

 antenna                                            software
   │    ┌─────┐   ┌───────────┐   ┌──────┐   ┌─────┐   ┌──────────────┐
   └───→│ LNA │──→│   mixer   │──→│filter│──→│ ADC │──→│ FFT · demod  │
        └─────┘   └───────────┘   └──────┘   └─────┘   │ decoding     │
       amplifies   shifts the      anti-     samples   └──────────────┘
       w/o noise   band to 0       alias     as I/Q      your CPU
                        ↑
                  local oscillator
                  (= the tuning knob)

The stages, one by one

LNA (Low-Noise Amplifier): amplifies the whisper caught by the antenna while adding as little noise as possible. Every dB of noise added here is lost forever.
Mixer + local oscillator: multiplies the signal by a reference frequency to shift the band of interest down to zero. "Tuning" the radio = changing that oscillator. When a mission tunes the radio, this is the setting it sends.
Anti-aliasing filter: cuts whatever spills outside the window before digitisation (see Échantillonnage, Nyquist et aliasing).
ADC: the analogue-to-digital converter samples as I/Q. From here on, everything is numbers.

Two architectures to know

Direct conversion (HackRF, RTL-SDR…): a single mix, straight to zero. Simple and compact, but it leaves a signature: the famous centre spike ("DC spike") visible exactly at the tuning frequency — an artefact, not a signal. Reflex: offset by a few hundred kHz when studying a precise signal.
Superheterodyne: one or more intermediate frequencies before baseband. More components, but better image rejection — the architecture of high-end communications receivers.

The huge advantage… and the trade-off

One box, a thousand uses. The same chip listens to FM, aviation, IoT, drones — just change the software. The trade-off: quality depends on the hardware (noise, linearity, ADC bits) and on the CPU available for DSP.

The common families

SDR	Price	Coverage	Max bandwidth	Bits	Best for
RTL-SDR v3/v4	~$35	24–1766 MHz	2.4 MHz	8	starting out, ADS-B, FM, ISM
HackRF One	~$330	1 MHz–6 GHz	20 MHz	8	all-terrain, 2.4 GHz, wideband
Airspy Mini	~$130	24–1700 MHz	6 MHz	12	VHF/UHF listening quality
SDRplay RSP1B	~$130	1 kHz–2 GHz	10 MHz	14	HF + VHF finesse
ADALM-Pluto	~$270	325 MHz–3.8 GHz	20 MHz	12	experimentation, transmit (licence!)

Yours is the Le HackRF One — and the reasoned buying guide lives in Quel matériel pour débuter ?.

Your turn

You see a peak pinned exactly at centre, whatever the tuning. Signal or artefact? (DC spike — a direct-conversion artefact.)
Why is the LNA the most critical component in the chain? (The noise it adds is amplified by every following stage — it sets the sensitivity.)