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.

The DC spike sits exactly at the tuned centre; it's an artefact to ignore or notch, not a station.

Overview

Operators avoid it by tuning slightly off-centre so the channel of interest doesn’t sit under the spike, or by enabling a DC-blocking filter. It is a common source of “phantom carrier” confusion for newcomers reading a waterfall.

Sources

1. DC bias — Wikipedia, on a constant zero-frequency component in a signal. ↩

See also

• Baseband
• IQ data
• Local oscillator

Learn it step by step

• The FFT & reading a waterfall