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Discrete-Time Scatter Plot Scope - Display in-phase and quadrature components of modulated signal constellation

Library

Comm Sinks

Description

The Discrete-Time Scatter Plot Scope block displays scatter plots of a modulated signal, to reveal the modulation characteristics, such as pulse shaping or channel distortions of the signal.

The Discrete-Time Scatter Plot Scope block has one input port. This block accepts a complex scalar-valued or column vector input signal. The block accepts a signal with the following data types: double, single, base integer, and fixed-point for input, but will cast it as double.

See the reference page for the Discrete-Time Signal Trajectory Scope block to compare the preceding scatter plot with the trajectory of the same signal. The Discrete-Time Signal Trajectory Scope block connects the points displayed by the Discrete-Time Scatter Plot Scope block to display the signal trajectory.

Setting Samples per symbol to 8, increasing Points displayed to 100, and running the model for 100 seconds produces the following scatter plot.

Markers and Color

The Markers and Color parameters, on the Rendering Properties panel, specify the style and color of markers in the scatter plot. For details on the options for these parameters, see the reference page for the Discrete-Time Eye Diagram Scope block.

Recommended Settings

The following table summarizes the recommended parameter settings for the Discrete-Time Scatter Plot Scope.

Parameter	Recommended Setting
Samples per symbol	Same as the Samples per symbol setting in the modulator block, or the Interpolation factor setting in the interpolation block
Points displayed	10 times the alphabet size of the modulator
New points per display	Same as Points displayed for greater speed A small positive integer for best animation
Line style	Solid dash (`-`)
Line color	Blue (`b`)
Color fading	Check Color fading for animation that resembles an oscilloscope. Clear for greater speed and animation that resembles a plot.
High quality rendering	Check High quality rendering for higher quality rendering. Clear for greater speed.
Open at start of simulation	Check Open at start of simulation to view the signal at the start of simulation. Clear to view the signal after convergence to steady state and for greater initial speed.
X-axis minimum	Approximately 10% less than the expected minimum value of the signal
X-axis maximum	Approximately 10% greater than the expected maximum value of the signal

For Rapid Accelerator or External mode, set the scope up for single rate mode. To guarantee the satisfactory behavior of single rate mode, the subsystem below the block mask for this block must operate as a single-rate entity, which means the following conditions are true:

where:

sps = Samples per symbol
nppd = New points per display
Sf = Input frame size, in samples

This equation guarantees that the subsystem below the mask for this block operates as a single rate entity.

Warning If you want to use Rapid Accelerator or External mode, set this block up to run as a single rate entity because multi-rate does not support these modes.

Behavior in Enabled Subsystems

You can use the Discrete-Time Scatter Plot Scope block inside an enabled subsystem. However, you cannot use the scope block inside an enabled subsystem when the model is in a multirate multitasking environment.

When you use the scope in a multirate singletasking environment, it may generate unexpected results inside enabled subsystems. To workaround this issue, configure the scope for single-rate mode. See Recommended Settings for the parameter settings that enable single-rate mode.

Dialog Box

Samples per symbol: Number of samples per symbol.
Offset (samples): Nonnegative integer less than the number of samples per symbol, specifying the number of samples to skip before plotting points.
Points displayed: Total number of points plotted.
New points per display: Number of new points that appear in each display.

Markers: Line markers used in the scatter plot. Tunable.
Line color: The line color used in the scatter plot. Tunable.
Color fading: When selected, the points in the scatter plot fade as the interval of time after they are first plotted increases. Tunable.
High quality rendering: When selected, the block renders a slow, higher-quality picture with overwrite raster operations. When cleared, the block renders a fast, lower-quality picture with XOR raster operations. Tunable.
Show grid: Toggles the scope grid on and off. Tunable.

X-axis minimum: Minimum value the scope displays on the x-axis. Tunable.
X-axis maximum: Maximum value the scope displays on the x-axis. Tunable.
Y-axis minimum: Minimum signal value the scope displays on the y-axis. Tunable.
Y-axis maximum: Maximum signal value the scope displays on the y-axis. Tunable.
In-phase X-axis label: Label for x-axis. Tunable.
Quadrature Y-axis label: Label for y-axis. Tunable.

Open at start of simulation: When selected, the scope opens at the start of simulation. When cleared, you must double-click the block after the start of simulation to open the scope.
Point number: Displays the number of the current point in the input sequence. Tunable.
Scope position: A four-element vector of the form [left bottom width height] specifying the position of the scope window. (0,0) is the lower left corner of the display. Tunable.
Title: Title of scatter plot. Tunable.

Examples

For documentation examples that use this block, see Example: Viewing a Sinusoid and Example: Viewing a Modulated Signal.

The following demos in Communications Blockset software illustrate how to use the Discrete-Time Scatter Plot Scope block:

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