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Spectrum Analyzer

Display frequency spectrum

  • Spectrum Analyzer block

Libraries:
Simscape / Utilities

Description

Note

The Spectrum Analyzer block in Simscape™ provides a subset of the functionality provided by the DSP System Toolbox™ block with the same name. This page describes the block configuration and functionality available with a Simscape license. If you also have a DSP System Toolbox license, then the Spectrum Analyzer block in the Simscape > Utilities library is identical to the block in the DSP System Toolbox > Sinks library. For more information, see Spectrum Analyzer (DSP System Toolbox).

The Spectrum Analyzer block displays frequency spectra of input signals with discrete sample times.

In a Simscape model, to use a Spectrum Analyzer block instead of a regular scope:

  1. Add a Spectrum Analyzer block to your block diagram.

  2. If your model uses a variable-step solver, also add a Rate Transition block and connect it to the input port of the Spectrum Analyzer block. Set the Output port sample time parameter to the sample time you want the Spectrum Analyzer to use.

    If your model uses a local solver, then it outputs physical signals with discrete sample times and you do not need to add a Rate Transition block. However, if you need to downsample from the solver fixed step size, you can use a Rate Transition block. For more information on using local solvers, see Making Optimal Solver Choices for Physical Simulation.

  3. Use a PS-Simulink Converter block to connect the physical output signal of interest to the input port of the Spectrum Analyzer block (or to the input of the Rate Transition block, if you are using one). For more information, see Connecting Simscape Diagrams to Simulink Sources and Scopes. You can also use additional signal processing blocks between the PS-Simulink Converter and the Spectrum Analyzer blocks to enhance signal quality.

  4. Run the simulation. Once the simulation is complete, the Spectrum Analyzer, referred to here as the scope, opens and displays the frequency spectrum of the signal.

Note

To prevent the scope from opening when you run your model, right-click the scope icon and select Comment Out. If the scope is already open, and you comment it out in the model, the scope displays this message: No data can be shown because this scope is commented out. Select Uncomment to turn the scope on again.

Programmatic Control

You can configure and display the Spectrum Analyzer settings from the command line with the SpectrumAnalyzerBlockConfiguration object.

Ports

Input

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Connect the signals you want to visualize. You can have up to 96 input ports. Input signals must have these characteristics:

The Spectrum Analyzer block supports variable-size input signals, that is, the frame size of the signals can change during simulation. When the signal frame size changes, the sample rate the scope uses changes accordingly, which in turn updates the frequency span of the spectrum display.

The Spectrum Analyzer block accepts fixed-point input, but converts it to double for display.

This port is unnamed until you enable the RBW port. (since R2024a)

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | fixed point
Complex Number Support: Yes

Specify the resolution bandwidth in Hz through this port. RBW defines the smallest positive frequency that can be resolved by the scope. You can also specify the RBW value using the RBW (Hz) parameter on the Estimation tab.

Dependency

To enable this port, set the RBW (Hz) parameter on the Estimation tab to Input port.

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | Boolean | fixed point

Parameters

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This section lists the block parameters for the Spectrum Analyzer block in Simscape and RF Blockset™. For a complete list of parameters, see Spectrum Analyzer (DSP System Toolbox).

Scope Tab

Views

Set the type of spectrum to display as one of these values:

  • Power — Spectrum Analyzer shows the power spectrum.

  • Power Density — Spectrum Analyzer shows the power spectral density. The power spectral density is the squared magnitude of the spectrum normalized to a bandwidth of 1 Hz.

  • RMS — Spectrum Analyzer shows the root mean squared spectrum. Use this option to view the frequency of voltage or current signals.

Tunable: Yes

Programmatic Use

Block Parameter: SpectrumType
Type: character vector or string scalar

Data Types: char | string

Bandwidth

Specify the sample rate the scope uses in Hz as one of the following:

  • Inherited –– Use this option to specify the same sample rate as the input signal.

  • Positive scalar –– The sample rate you specify must be at least twice the sample rate of the input signal. Otherwise, you might see unexpected behavior when visualizing your signal in the scope due to aliasing.

When the signal frame size changes, the sample rate the scopes uses changes accordingly which in turn updates the frequency span of the spectrum display.

To display the sample rate on the status bar, click the kebab icon in the right most corner of the status bar icon in the status bar and select Sample Rate.

Tunable: Yes

Programmatic Use

Block Parameter: SampleRate, SampleRateSource
Type: double

Specify the offset to apply to the frequency axis (x-axis) in units of Hz as one of the following:

  • Scalar — Apply the same frequency offset to all channels.

  • Vector — Apply a specific frequency offset for each channel. The vector length must be equal to the number of input channels.

    The overall span must fall within the Nyquist Frequency Interval. You can control the overall span in different ways based on how you set the Span (Hz) parameter.

Tunable: Yes

Programmatic Use

Block Parameter: FrequencyOffset
Type: double

Configuration > Spectrum Analyzer Settings (Gear button)

The number of input ports to the block, specified as an integer between 1 and 96. To change the number of input ports, drag a new input signal line to the block and the block automatically creates new ports.

Programmatic Use

Block Parameter: NumInputPorts
Type: character vector or string scalar
Values: scalar between 1 and 96

Select this parameter to automatically open the Spectrum Analyzer window when you run the simulation.

Programmatic Use

Block Parameter: OpenAtSimulationStart
Type: logical

Specify the font size of labels in the display as Small, Medium, Large, and Extra Large.

Tunable: Yes

Programmatic Use

Block Parameter: -
Type: character vector or string scalar

Specify whether to display a Line or Stem plot in the Spectrum display.

Tunable: Yes

Programmatic Use

Block Parameter: PlotType
Type: character vector or string scalar

Specify the y-axis label in the Spectrum display as a character vector or a string scalar. To display signal units, add (%<SignalUnits>) to the label. When simulation starts, Simulink replaces (%SignalUnits) with the units associated with the signals. For example, for a velocity signal with units of m/s enter:

Velocity (%<SignalUnits>)

Tunable: Yes

Programmatic Use

Block Parameter: YLabel
Type: character vector or string scalar

Specify the y-axis limits in the Spectrum Analyzer display as a two-element numeric vector of the form [ymin ymax]. The units of the y-axis limits depend on the Spectrum Unit in the Spectrum tab.

Tunable: Yes

Programmatic Use

Block Parameter: YLimits
Type: double

Specify the display title. Enter %<SignalLabel> to use the signal labels in the Simulink model as the axes titles.

Tunable: Yes

Programmatic Use

Block Parameter: Title
Type: character vector or string

Select this check box to show the grid in the Spectrum Analyzer display.

Tunable: Yes

Programmatic Use

Block Parameter: ShowGrid
Type: logical

When you select this parameter, the scope preserves the colors when you copy the display to the clipboard using the Copy Display and the Print options in the Scope tab > Share section.

Tunable: Yes

Specify the background color in the scope figure.

Tunable: Yes

Specify the background color of the axes.

Tunable: Yes

Specify the color of the labels, grid, and the channel names in the legend.

Tunable: Yes

Specify the channel for which you want to modify the visibility, line color, style, width, and marker properties.

Tunable: Yes

Select this check box to display the channel you have selected. If you clear this check box, the selected channel is no longer visible. You can also click the signal name in the legend to control its visibility. For more details, see Legend.

Tunable: Yes

Specify the line style for the selected channel.

Tunable: Yes

Specify the line width for the selected channel.

Tunable: Yes

Specify a data point marker for the selected channel. This parameter is similar to the Marker property for plots. You can choose any of the marker symbols from the drop-down list.

Tunable: Yes

Specify the line color for the selected channel.

Tunable: Yes

Configuration

Click the Legend button to enable the Spectrum Analyzer to display. The legend displays the signal names from the model. For signals with multiple channels, the scope appends a channel index after the signal name. Continuous signals have straight lines before their names and discrete signals have step-shaped lines.

You can control which signals are visible using the legend. To hide a signal in the scope legend, click the signal name. To display the signal, click the signal name again. Alternatively, you can control which signal is visible using the Visible parameter in the Spectrum Analyzer Settings (Gear button).

To display only one signal and hide all other signals, right-click the name of the signal you want the scope to display. To show all signals, press Esc.

Note

The legend displays only the first 20 signals. You cannot view or control any additional signals from the legend.

Tunable: Yes

Programmatic Use

Block Parameter: ShowLegend
Type: logical

Share

Click this button to copy the scope display to the clipboard. You can preserve the color in the display by selecting the Preserve colors for copy to clipboard parameter.

Click this button to save the scope display as an image or a PDF or to print the display.

Estimation Tab

Frequency Resolution

Specify the resolution bandwidth in Hz. This parameter defines the smallest positive frequency that can be resolved by the scope. By default, this parameter is set to Auto. In this case, the spectrum analyzer determines the appropriate value to ensure that there are 1024 RBW intervals over the specified frequency span.

If you set this parameter to Input port, you can specify the RBW value through an input port on the block.

If you set this parameter to a numeric value, the value must allow at least two RBW intervals over the specified frequency span. In other words, the ratio of the overall frequency span to RBW must be greater than two:

spanRBW>2

To display this property on the status bar, click the kebab icon in the right most corner of the status bar icon in the status bar and select RBW.

Tunable: Yes

Programmatic Use

Block Parameter: RBWSource, RBW
Type: character vector, string scalar, double

Averaging

Specify the smoothing method as one of the following:

  • Exponential — The block computes the average over samples weighted by an exponentially decaying forgetting factor. Use the Forgetting Factor parameter to specify the weighted forgetting factor.

  • VBW — The block uses a lowpass filter to smooth the trace and decrease the noise. Use the VBW (Hz) parameter to specify the video bandwidth (VBW) value.

For more information on the averaging methods, see Averaging Method.

Tunable: Yes

Programmatic Use

Block Parameter: AveragingMethod
Type: character vector or string scalar

Specify the video bandwidth as one of the following:

  • Auto –– The Spectrum Analyzer adjusts the VBW such that the equivalent forgetting factor is 0.9.

  • Input port –– An input port appears on the scope and you can specify the VBW value through this port.

  • Positive scalar –– You specify a positive scalar. The Spectrum Analyzer adjusts the VBW using this value. The value you specify must be less than or equal to Sample Rate (Hz)/2.

For more details on the video bandwidth method, see Averaging Method.

The Spectrum Analyzer shows the VBW value in the status bar at the bottom of the display. To display the VBW value, click the kebab icon in the right most corner of the status bar icon in the status bar and select VBW.

Tunable: Yes

Dependency

To enable this parameter, set Averaging Method to VBW.

Programmatic Use

Block Parameter: VBWSource, VBW
Type: double

Specify the forgetting factor of the exponential weighted averaging method as a scalar in the range [0,1].

Tunable: Yes

Dependency

To enable this parameter, set Averaging Method to Exponential.

Programmatic Use

Block Parameter: ForgettingFactor
Type: double

Window Options

Specify the windowing method to apply to the spectrum. Windowing is used to control the effect of sidelobes in spectral estimation. The window you specify affects the window length required to achieve a resolution bandwidth and the required number of samples per update. For more information about windowing, see Windows (Signal Processing Toolbox).

You can access additional windowing algorithms if you install DSP System Toolbox.

Tunable: Yes

Programmatic Use

Block Parameter: Window
Type: character vector or string scalar

Specify the percentage of overlap between the previous and the current buffered data segments as a scalar in the range [0 100). The overlap creates a window segment that the scope uses to compute a spectral estimate. The value must be greater than or equal to zero and less than 100.

Tunable: Yes

Programmatic Use

Block Parameter: OverlapPercent
Type: double

Measurements Tab

Channel

The channel for which you need to obtain measurements, specified as a positive integer in the range [1 N], where N is the number of input channels.

Tunable: Yes

Dependency

To enable this parameter, pass some data through the scope.

Programmatic Use

See MeasurementChannel.

Cursors

Click the Data Cursors button to enable data cursor measurements. Each cursor tracks a vertical line along the signal. The scope displays the difference between x- and y-values of the signal at the two cursors in the box between the cursors.

Tunable: Yes

Programmatic Use

See Enabled.

Select this parameter to position the cursors on the signal data points.

Tunable: Yes

Programmatic Use

See SnapToData.

Select this parameter to lock the frequency difference between the two cursors.

Tunable: Yes

Programmatic Use

See LockSpacing.

Peaks

Click the Peak Finder button to enable peak finder measurements. An arrow appears on the plot at each maxima and a Peaks panel appears at the bottom of the scope window.

Tunable: Yes

Programmatic Use

See Enabled.

Specify the maximum number of peaks to show as a positive integer less than 100.

Tunable: Yes

Programmatic Use

See NumPeaks.

Specify the level above which the scope detects peaks as a real scalar.

Tunable: Yes

Programmatic Use

See MinHeight.

Specify the minimum number of samples between adjacent peaks as a positive integer.

Tunable: Yes

Programmatic Use

See MinDistance.

Specify the minimum difference between the height of the peak and its neighboring samples as a nonnegative scalar.

Tunable: Yes

Programmatic Use

See Threshold.

Click the Label Peaks button to label the peaks. The scope displays the labels (P1, P2, …) above the arrows in the plot.

Tunable: Yes

Programmatic Use

See LabelPeaks.

Distortion

Click the Distortion button to enable distortion measurements. A Distortion panel appears at the bottom of the scope window when you click this button.

Tunable: Yes

Programmatic Use

See Enabled.

Specify the type of measurement data to display as Harmonic or Intermodulation. For more details, see Distortion Measurements.

Tunable: Yes

Programmatic Use

See Type.

Specify the number of harmonics to measure as a positive integer less than or equal to 99.

Tunable: Yes

Dependency

To enable this parameter, set Distortion Type to Harmonic.

Programmatic Use

See NumHarmonics.

When you select this parameter, the spectrum analyzer adds numerical labels to harmonics in the spectrum display.

Tunable: Yes

Programmatic Use

See LabelValues.

When you select this parameter, the spectrum analyzer adds numerical labels to the first-order intermodulation product and third-order frequencies in the spectrum analyzer display.

Tunable: Yes

Programmatic Use

See LabelValues.

Spectrum Tab

Trace Options

Select this check box to enable a two-sided spectrum view. In this view, the spectrum analyzer shows both negative and positive frequencies. When the input signal is complex-valued, you must select this parameter. If you clear this check box, the spectrum analyzer shows a one-sided spectrum with positive frequencies only. In this case, the input signal data must be real valued.

When you clear this check box, the spectrum analyzer uses power folding. The y-axis values are twice the amplitude that they would be if you were to select this parameter, except at 0 and the Nyquist frequency. A one-sided power spectral density (PSD) contains the total power of the signal in the frequency interval from DC to half the Nyquist rate. For more information, see pwelch (Signal Processing Toolbox).

Tunable: Yes

Programmatic Use

Block Parameter: PlotAsTwoSidedSpectrum
Type: logical

Scale

Specify the scale to display frequencies as Linear or Log. When the frequency span contains negative frequency values, you cannot choose the logarithmic option.

Tunable: Yes

Dependency

To set the Frequency Scale to Log, clear the Two-Sided Spectrum check box in the Trace Options section in the Spectrum tab. If you select the Two-Sided Spectrum check box, then the Frequency Scale parameter is set to Linear.

Programmatic Use

Block Parameter: FrequencyScale
Type: character vector or string scalar

Specify the reference load in ohms that the Spectrum Analyzer uses as a reference to compute the power values.

Tunable: Yes

Dependency

To enable this parameter, set:

  • Spectrum type to Power or Power Density.

Programmatic Use

Block Parameter: ReferenceLoad
Type: double

Specify the units in which the Spectrum Analyzer displays the power values as one of the following:

  • dBm

  • dBV

  • dBuV (since R2023b)

  • Vrms

  • dBm/Hz

Tunable: Yes

Dependency

The units available depend on the value you choose for the Spectrum parameter in the Scope tab.

Scope tab > Spectrum optionAvailable Units
PowerdBm
Power DensitydBm/Hz
RMSdBV, dBuV (since R2023b), Vrms

Programmatic Use

Block Parameter: SpectrumUnits
Type: character vector or string scalar

Property Inspector Only

Input channel names, specified as a character vector, string, or array. The names appear in the legend, Settings, and Measurements panels. If you do not specify the names, the scope labels the channels as Channel 1, Channel 2, etc.

Example: ["A","B"]

Dependency

To see channel names, select Legend in the Scope tab.

Programmatic Use

Block Parameter: ChannelNames
Type: cell array of character vectors or string array

  • Auto — If you have not specified Title and Y-Label, the scope maximizes all plots.

  • On — The scope maximizes all plots and hides all values in Title and Y-label.

  • Off — The scope does not maximize plots.

Hover over the Spectrum Analyzer to see the maximize axes button .

Tunable: Yes

Programmatic Use

Block Parameter: MaximizeAxes
Type: character vector or string scalar

  • OnceAtStop — Scale y-axis after simulation completes.

  • Manual — Manually scale y-axis range with the Scale Y-axis Limits toolbar button.

  • Auto — Scale y-axis range during and after simulation.

  • Updates — Scale y-axis after the number of time steps specified in the Number of Updates text box (100 by default). Scaling occurs only once during each run.

Tunable: Yes

Programmatic Use

Block Parameter: AxesScaling
Type: character vector or string scalar

Set this property to delay auto scaling the y-axis.

Tunable: Yes

Dependency

To enable this property, set Axes Scaling to Updates.

Programmatic Use

Block Parameter: AxesScalingNumUpdates
Type: character vector or string scalar
Values: scalar

More About

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Algorithms

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Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.

Version History

Introduced in R2016b

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See Also

| (DSP System Toolbox) | (DSP System Toolbox)