Also known as: baseband, complex baseband
Baseband is a signal centred at (or near) zero frequency, after the carrier has been mixed away.1 An SDR delivers complex baseband IQ samples — the channel shifted down to 0 Hz — which is the natural form for software to filter and demodulate. The term contrasts with passband, where the same information rides on a carrier somewhere up the spectrum.
How it works
A receiver mixes the wanted channel down until its centre sits at 0 Hz. A real baseband signal (one number per sample) would fold the spectrum onto itself — a component just above 0 Hz would be indistinguishable from one just below. Radios avoid this by using complex baseband: two sampled streams, in-phase (I) and quadrature (Q), that together represent both positive and negative frequencies around the centre.2 With I and Q, a signal 10 kHz above the tuned frequency and one 10 kHz below occupy genuinely different parts of the complex spectrum, so no information is lost. This is why an SDR’s native output is a stream of complex samples rather than a single audio-like waveform.
Working at baseband is a matter of convenience and rate. Once a channel is at 0 Hz, only its own bandwidth needs to be represented, so it can be decimated to a low sample rate and processed cheaply. All the receiver’s remaining filtering and demodulation happen here.
In practice
Producing baseband directly in the analog front end — mixing RF straight to 0 Hz — is direct conversion (zero-IF). It is simple and cheap, but it puts the sensitive part of the spectrum right where two hardware artefacts live: a DC offset spike from LO leakage at the exact centre, and IQ imbalance — gain or phase mismatch between the I and Q paths that leaks a mirror image of each signal across 0 Hz. Both are corrected in software or sidestepped by tuning slightly off-centre. Architectures that mix to a small low-IF instead keep the wanted channel just off 0 Hz to dodge the DC spike, then shift it the last step digitally.
Relevance to SDR
Baseband is where nearly all SDR software actually operates. GopherTrunk’s digital down-converter translates each control or voice channel to complex baseband, filters and decimates it to the per-protocol channel rate, and hands that stream to the demodulator. Reading a signal as IQ at baseband — with its symmetric positive and negative frequency axes — is the foundation every later stage builds on.
Sources
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Baseband — Wikipedia, on signals centred near zero frequency and the passband contrast. ↩
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In-phase and quadrature components — Wikipedia, on the I/Q representation that lets complex baseband carry positive and negative frequencies. ↩