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14 March 2013

Check Constellation Diagrams for Digital Data Integrity

Data-communication systems that rely on quadrature signal generation to phase-encode data can run into a number of signal-corrupting snags. These can include things like Gaussian noise, non-coherent single-frequency interference, phase noise, and attenuation in the channel and/or receiver, to name a few. But did you know that you can use your digital oscilloscope to diagnose problems like these?

The solution is to use constellation diagrams, which are commonly used to analyze the performance of quadrature signal generation schemes. The constellation diagram is an X-Y display which shows the data states of phase- or phase amplitude-encoded data. The input is baseband data consisting of the in-phase (I) and quadrature (Q) components. The figure shows a constellation diagram for a 16-state circular 16 QAM signal. There are 16 data states associated with each transmitted symbol encoded using a unique magnitude and phase. These are shown on the X-Y display, with persistence turned on, while the other traces are the X-T and Y-T components.

This data is externally clocked using the symbol clock so only the data from the data states is plotted. We exclude the data transitions between states. External Clock is available on Teledyne LeCroy’s WaveRunner 6 Zi, HRO, and HDO6000 series oscilloscopes. The figure shows the external clock setup. The oscilloscope will accept ECL, TTL, or zero-crossing levels. The External Sampling clock is applied through the Ext input on the front panel.

Note that the timing of the symbol clock may need to be adjusted to assure that the signal waveforms are sampled at the correct time. This is usually accomplished outside of the oscilloscope. The Horizontal scale (Time/Division) knob is used to set the record length. Record lengths from 50S to 10MS can be selected in multiples of 1, 2, or 5.

Figure 1: The creation of a constellation diagram for a circular 16 QAM signal requires the I and Q components along with the symbol clock as the external sampling input.