Also known as: DC offset, DC spike, center spike
A DC offset is a constant (zero-frequency) component in the IQ stream that shows up as a spike in the exact centre of the spectrum and waterfall.1 It is an artefact of zero-IF/baseband receivers — local-oscillator leakage and converter bias — not a real signal on the air.
How it works
In a direct-conversion receiver the local oscillator runs at the same frequency as the wanted signal. Some of that LO energy inevitably leaks back to the mixer input and mixes with itself — self-mixing — producing a constant output that sits at 0 Hz, exactly the tuned centre. Bias in the ADC and the analog signal path adds more constant offset. Because these are steady, they appear at DC: a persistent narrow line dead centre in the spectrum that stays put no matter how you tune, distinguishing it from any real carrier, which moves as you retune.
The DC term carries no information — it is the average of the IQ stream — so removing it costs nothing but must be done carefully, since an aggressive DC blocker can also attenuate genuine signal energy that happens to sit near 0 Hz.
In practice
- Tune slightly off-centre. The simplest fix is to offset the tuner so the channel of interest does not sit under the spike, then shift it back digitally. Many SDR applications do this automatically as an “offset tuning” or “LO offset” mode.
- DC-blocking filter. A DC blocker — a high-pass IIR filter with a notch at 0 Hz, or subtracting a running mean — removes the constant component in software.
- Distinguish from IQ imbalance. DC offset is a centre spike; a separate zero-IF artefact, IQ imbalance, instead produces mirror images of real signals reflected across 0 Hz. They have different causes and different corrections, and both are worth recognising when reading a waterfall.
Relevance to SDR
The DC spike is a common source of “phantom carrier” confusion for newcomers reading a spectrum display, who mistake the fixed centre line for a real transmission. It is characteristic of the low-cost zero-IF front ends used in RTL-SDR-class dongles and many SDR tuners. GopherTrunk tunes each channel with its own digital down-converter, so a channel is generally moved off the raw centre before demodulation, keeping the wanted signal clear of the hardware DC artefact.