Constellation diagram

January 22, 2014.

The constellation diagram is a graphic representation (called I-Q) of the digital symbols received over a period of time.

There are different types of constellation diagrams for the different modulation modes. With the field strength meter RANGERNeo 2 it is possible to display constellations for DVB-T/T2, DVB-C/C2, and DVB-S/S2 signals.

Constellation diagram for a DVB-T2 channel (high definition digital terrestrial television) in the RANGER Neo 2 field strength meter
Constellation diagram for a DVB-T2 channel (high definition digital terrestrial television)
in the RANGERNeo 2 field strength meter

In the case of an ideal transmission channel, free of noise and interferences, all symbols are recognised by the demodulator without mistakes. In this case, they are represented in the constellation diagram as well defined points hitting in the same area and forming a clear dot.

Noise and impairments cause the demodulator to not always read the symbols correctly. In this case the hits disperse and create different shapes that at the end will allow to determine at a glance the type of noise in the signal.

64QAM constellation diagram
64QAM constellation diagram
for DVB-C system
in the field strength meter screen
8PSK constellation diagram for DVB-S2
8PSK constellation diagram
for DVB-S2 (high definition satellite)
in the field strength meter screen

Every modulation type is represented differently. A DVB-C 16QAM signal is represented on the screen by a total of 16 different zones, and a DVB-C 64QAM is represented on the screen by a total of 64 different zones and so on.

The constellation shows in different colours the density of hits and includes zooming and scrolling possibilities and also a clear button to clean the picture.