In English, the word complex is synonymous with difficult, but here it has a very simple and precise meaning.
When one speaks of a complex signal, simply means.
“The signal is described not by one but by two signals.”
Why is one signal sometimes not enough?
For a classic signal representation, one quantity that varies over time (e.g. voltage or current) is usually sufficient.
However, this has a significant disadvantage:
- The phase of the signal is then not clearly known
- There is no fixed phase reference
As soon as phase information becomes important, a single signal is insufficient.
In that case, it is necessary to describe the signal with two mutually perpendicular components.
These two signals represent a single complex signal. They are nothing more than the projection onto the X-axis (0°) and the projection onto the Y-axis (90°) of a single rotating arrow (vector) that represents the signal.
The word complex originates from mathematics, where numbers consisting of a real part (X-axis) and an imaginary part (Y-axis) are also called complex numbers.
The numbers on the X-axis (0°) are referred to as the real part or real signal. This is represented as the I signal (in phase).
The numbers on the Y-axis (90°) are referred to as the quadrature or imaginary signal. This is represented as the Q signal (quadrature or 90° shifted).
Digital complex signals
When referring to a digital IQ signal, it means that each point in time is represented by two samples:
- one I-sample
- one Q-sample
Together, these two samples contain all the information about:
- The amplitude
- The instantaneous phase of the signal
This makes it possible to correctly process signals where phase information is important (such as with DAB/DAB+) digitally.
“I and Q are not two signals, but two descriptions of the same signal.”