Field Guide · term

Also known as: IQ data, IQ samples

IQ data is the stream of paired numbers an SDR produces — I (in-phase) and Q (quadrature, 90° apart). Together each pair captures both the amplitude and phase of the signal at that instant, so a single complex sample fully describes the wave rather than just its height.1

IQ phase amplitude
Each IQ sample is a point on the complex plane: distance from the origin is amplitude, angle is phase.

How it works

The receiver generates two copies of a local-oscillator tone 90° apart — a cosine and a sine — and multiplies the incoming signal by each. The cosine product is I, the sine product is Q. Written as a complex number I + jQ, each sample is a point on the complex plane: its distance from the origin is amplitude and its angle is phase, which is exactly what a constellation diagram plots. This representation is a sampled analytic signal — a signal with no negative-frequency mirror — which is the mathematical reason a single real channel cannot do the same job.2

The decisive property IQ buys you is the ability to tell apart frequencies above and below the tuned centre. A real-only sample cannot: a tone 10 kHz above the local oscillator and a tone 10 kHz below both beat at 10 kHz and are indistinguishable. With two quadrature channels the direction of rotation on the IQ plane is preserved, so positive and negative offsets are separated. That is why an SDR can present a symmetric band around its tuned frequency and why the captured bandwidth can straddle zero.

Variants

  • Baseband (zero-IF) IQ — the signal is centred at 0 Hz. This is what most direct-conversion SDRs emit; the trade-off is a DC spike and any IQ imbalance landing right in the middle of the band.
  • Low-IF IQ — the band is offset slightly so the DC artefact sits outside the channel of interest.
  • Real (interleaved) samples from direct sampling — a single ADC stream that DSP later converts to analytic IQ with a Hilbert transform.

In practice

On disk and on the wire, IQ is just interleaved I, Q, I, Q values — 8-bit unsigned for RTL-SDR, 16-bit signed or 32-bit float for higher-end radios (the .cfile GopherTrunk replays is complex float32). The sample rate sets how many pairs per second arrive; the bit depth sets the dynamic range. A persistent DC offset or gain mismatch between the I and Q paths shows as a fixed point or a tilted, elliptical constellation, which downstream equalisers must correct.

Relevance to SDR

Everything GopherTrunk does begins with the IQ stream from the radio: tuning, filtering, demodulation, the constellation and spectrum scopes, and the offline replay path all operate on it. The demodulator recovers symbols by tracking the phase and amplitude of successive IQ samples.

Sources

  1. In-phase and quadrature components — Wikipedia, on representing a signal as paired I and Q components. ↩

  2. Analytic signal — Wikipedia, on the complex-valued signal with no negative-frequency content that IQ sampling approximates. ↩

See also

Learn it step by step

From the blog