User Manual DDS200 Digital Demodulation System 070-9952-00

Manual OperationConstellation Diagram. Using the CONSTELL DIAGRAM… key, the DDS200 isswitched to the constellation analyzer mode. The constellation diagram is anextremely useful tool for assessing the quality of the transmitted signal. In thismode, no BER measurement (via the IEC/IEEE bus) can be performed, and allMPEG outputs are disabled. Before the other keys for configuring the diagramwill be described, the basic diagrams are explained.Figure 2–39 shows a constellation diagram with no input signal present; only thenoise is displayed. Noticeable is the gradually decreasing density of measuredvalues towards the outside. Figure 2–40 shows a QAM signal with unsynchronizedsymbol clock. The characteristic here is the clear spatial limitation ofmeasured values. For a correct synchronization of this signal, the order of QAMand/or the symbol rate should be checked (refer to SYMBOL RATE onpage 2–30).Figure 2–39: Noise at input, no QAM signal2–50 DDS200 Digital Demodulation System User Manual