Introduction

Constellation diagrams are commonly used to analyze the performance of data communications systems using quadrature signal-generation techniques to phase-encode digital data. The constellation diagram is an X-Y display that 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. Figure 1 shows a constellation diagram for a sixteen-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 while the other traces are the X-T and Y-T components.

Figure 1:

A 16 QAM signal consisting of the in-phase (I) and quadrature (Q) components plotted on an X-Y plot. The data is sampled using a synchronous external sample clock

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. Figure 1 shows the external clock setup. The scope will accept ECL, TTL or zero-crossing levels.

Specifications for the clock inputs follow:

0.1 Hz – 100 MHz
Samples on rising edge
ECL: Threshold: -1.3V, Amplitude >e;150 mVpk-pk
TTL: Threshold: +1.6V, Amplitude >e;1.8 Vpk-pk
0V: Threshold: 0V, Amplitude >e;150 mVpk-pk
Minimum pulse width: 3 ns, minimum slew rate: 100 mV/ns
Maximum jitter +/-32 clocks

Load impedance can be set to 50 Ohms or 1 MOhm. The external sampling clock is applied to the Ext(ernal) 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 scope.

In external sampling the oscilloscope time base is set by the user in terms of the total number of samples set in the Timebase annotation box. This is also shown in Figure 1. The Horizontal scale (Time/Division) knob is used to set the record length. Record lengths of from 50S to 10MS can be selected in multiples of 1, 2, or 5 (model dependent).

Constellation diagrams require that persistence be turned on for the X-Y plot. The display setup is used to turn on trace persistence as shown in Figure 2.

Figure 2:

Setting up display persistence

Cursors can be used to read the component amplitudes as well as the phase and vector magnitude of the states. This is shown in Figure 3. The absolute horizontal cursor, represented by the cross icon on both the X-Y plot and the component traces, reads the component trace amplitude in the C2 and C3 trace annotation boxes. Under the X-Y display the angle reads the phase of the selected state relative to the I component and the magnitude (radius) the vector. These cursors track on both the X-Y and X-T/Y-T displays.

Figure 3:

Using X-Y cursors to read the phase and magnitude of the selected state

Here is a collection of typical constellation diagrams:

Figure 4:

Constellation diagram of a PHP cellular phone signal

Figure 5:

Constellation diagram for a CDMA signal