dsp.SpectrumAnalyzer

Display frequency spectrum of time-domain signals

Description

The Spectrum Analyzer System object™ displays the frequency spectrum of time-domain signals. This scope supports variable-size input, which allows the input frame size to change. Frame size is the first dimension of the input vector. The number of input channels must remain constant.

To display the spectra of signals in the Spectrum Analyzer:

1. Create the dsp.SpectrumAnalyzer object and set its properties.

2. Call the object with arguments, as if it were a function.

To learn more about how System objects work, see What Are System Objects?

Creation

Syntax

scope = dsp.SpectrumAnalyzer

scope = dsp.SpectrumAnalyzer(ports)

scope = dsp.SpectrumAnalyzer(Name,Value)

Description

scope = dsp.SpectrumAnalyzer creates a Spectrum Analyzer System object. This object displays the frequency spectrum of real- and complex-valued floating- and fixed-point signals.

scope = dsp.SpectrumAnalyzer(ports) creates a Spectrum Analyzer object and sets the NumInputPorts property to the value of ports.

scope = dsp.SpectrumAnalyzer(Name,Value) sets properties using one or more name-value pairs. Enclose each property name in single quotes.

Properties

Unless otherwise indicated, properties are nontunable, which means you cannot change their values after calling the object. Objects lock when you call them, and the release function unlocks them.

If a property is tunable, you can change its value at any time.

For more information on changing property values, see System Design in MATLAB Using System Objects.

Frequently Used

NumInputPorts — Number of input ports

1 (default) | integer between [1, 96]

Number of input ports, specified as a positive integer. Each signal coming through a separate input becomes a separate channel in the scope. You must invoke the scope with the same number of inputs as the value of this property.

InputDomain — Domain of the input signal

"Time" (default) | "Frequency"

The domain of the input signal you want to visualize. If you visualize time-domain signals, the signal is transformed to the frequency spectrum based on the algorithm specified by the Method parameter.

Scope Window Use

Open the Spectrum Settings. In the Main options section, set Input Domain.

Data Types: char | string

SpectrumType — Type of spectrum to show

"Power" (default) | "Power density" | "RMS"

Specify the spectrum type to display.

"Power" — Power spectrum

"Power density" — Power spectral density. The power spectral density is the magnitude squared of the spectrum normalized to a bandwidth of 1 hertz.

"RMS" — Root mean square. The root-mean-square shows the square root of the mean square. This option is useful when viewing the frequency of voltage or current signals.

Tunable: Yes

Scope Window Use

Open the Spectrum Settings. In the Main options section, set Type.

Data Types: char | string

ViewType — Viewer type

"Spectrum" (default) | "Spectrogram" | "Spectrum and spectrogram"

Specify the spectrum type as one of "Spectrum", "Spectrogram", or "Spectrum and spectrogram".

• "Spectrum" — shows the power spectrum.

• "Spectrogram" — shows frequency content over time. Each line of the spectrogram is one periodogram. Time scrolls from the bottom to the top of the display. The most recent spectrogram update is at the bottom of the display.

• "Spectrum and Spectrogram" — shows a dual view of a spectrum and spectrogram.

Tunable: Yes

Scope Window Use

Open the Spectrum Settings. In the Main options section, set View.

Data Types: char | string

SampleRate — Sample rate of input

10000 (default) | finite scalar

Specify the sample rate, in hertz, of the input signals as a finite numeric scalar.

Scope Window Use

Open the Spectrum Settings. In the Main options section, set Sample rate (Hz).

Method — Spectrum estimation method

"Welch" (default) | "Filter Bank"

Specify the spectrum estimation method as Welch or Filter bank.

Dependency

To enable this property, set InputDomain to "Time".

Scope Window Use

Open the Spectrum Settings. In the Main options section, set Method.

Data Types: char | string

PlotAsTwoSidedSpectrum — Two-sided spectrum flag

true (default) | false

• true — Compute and plot two-sided spectral estimates. When the input signal is complex-valued, you must set this property to true.

• false — Compute and plot one-sided spectral estimates. If you set this property to false, then the input signal must be real-valued.

  When this property is false, Spectrum Analyzer uses power-folding. The y-axis values are twice the amplitude that they would be if this property were set to true, 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 of the Nyquist rate. For more information, see pwelch.

Scope Window Use

Open the Spectrum Settings. In the Trace options section, select Two-sided spectrum.

Data Types: logical

FrequencyScale — Frequency scale

"Linear" (default) | "Log"

• "Log" — displays the frequencies on the x-axis on a logarithmic scale. To use the "Log" setting, you must also set the PlotAsTwoSidedSpectrum property to false.

• "Linear" — displays the frequencies on the x-axis on a linear scale. To use the "Linear" setting, you must also set the PlotAsTwoSidedSpectrum property to true.

Tunable: Yes

Scope Window Use

Open the Spectrum Settings. In the Trace options section, set Scale.

Data Types: char | string

Advanced

FrequencySpan — Frequency span mode

"Full" (default) | "Span and center frequency" | "Start and stop frequencies"

• "Full" - The Spectrum Analyzer computes and plots the spectrum over the entire Nyquist frequency interval.

• "Span and center frequency" - The Spectrum Analyzer computes and plots the spectrum over the interval specified by the Span and CenterFrequency properties.

• "Start and stop frequencies" - The Spectrum Analyzer computes and plots the spectrum over the interval specified by the StartFrequency and StopFrequency properties.

Tunable: Yes

Scope Window Use

Open the Spectrum Settings. In the Main options section, select Full frequency span for "Full". Otherwise, clear the Full frequency span check box and choose between Span or FStart.

Data Types: char | string

Span — Frequency span to compute spectrum

10e3 (default) | real positive scalar

Specify the frequency span, in hertz, over which the Spectrum Analyzer computes and plots the spectrum. The overall span, defined by this property and the CenterFrequency property, must fall within the Nyquist frequency interval.

Tunable: Yes

Dependency

To enable this property, set FrequencySpan to "Span and center frequency".

Scope Window Use

Open the Spectrum Settings. In the Main options section, clear the Full frequency span check box and set Span.

StartFrequency — Start frequency to compute spectrum

-5e3 (default) | real scalar

Start of the frequency interval over which spectrum is computed, specified in hertz as a real scalar. The overall span, which is defined by this property and StopFrequency, must fall within the Nyquist frequency interval.

Tunable: Yes

Dependency

To enable this property, set FrequencySpan to "Start and stop frequencies".

Scope Window Use

Open the Spectrum Settings. In the Main options section, clear the Full frequency span and change Span to FStart. Set FStart (Hz).

StopFrequency — Stop frequency to compute spectrum

5e3 (default) | real scalar

End of the frequency interval over which spectrum is computed, specified in hertz as a real scalar. The overall span, which is defined by this property and the StartFrequency property, must fall within the Nyquist frequency interval.

Tunable: Yes

Dependency

To enable this property, set FrequencySpan to "Start and stop frequencies".

Scope Window Use

Open the Spectrum Settings. In the Main options section, clear the Full frequency span and change Span to FStart. Set FStop (Hz).

CenterFrequency — Center of frequency span

0 (default) | real scalar

Specify in hertz the center frequency of the span over which the Spectrum Analyzer computes and plots the spectrum. The overall frequency span, defined by the Span and this property, must fall within the Nyquist frequency interval.

Tunable: Yes

Dependency

To enable this property, set FrequencySpan to "Span and center frequency".

Scope Window Use

Open the Spectrum Settings. In the Main, clear Full frequency span and set CF (Hz).

FrequencyResolutionMethod — Frequency resolution method

"RBW" (default) | "WindowLength" | "NumFrequencyBands"

Specify the frequency resolution method of the Spectrum Analyzer.

• "RBW" - the RBWSource and RBW properties control the frequency resolution (in Hz) of the analyzer. The FFT length is the window length that results from achieving the specified RBW value or 1024, whichever is larger.

• "WindowLength" - applies only when the Method property is set to "Welch". The WindowLength property controls the frequency resolution. You can control the number of FFT points only when the FrequencyResolutionMethod property is "WindowLength".

• "NumFrequencyBands" - applies only when the Method property is set to "Filter Bank". The FFTLengthSource and FFTLength properties control the frequency resolution.

Tunable: Yes

Dependency

To enable this property, set InputDomain to "Time".

Scope Window Use

Open the Spectrum Settings. In the Main options section, set the frequency resolution method by selecting the RBW (Hz) dropdown.

Data Types: char | string

RBWSource — Source of resolution bandwidth value

"Auto" (default) | "Property"

Specify the source of the resolution bandwidth (RBW) as either "Auto" or "Property".

• "Auto" — The Spectrum Analyzer adjusts the spectral estimation resolution to ensure that there are 1024 RBW intervals over the defined frequency span.

• "Property" — Specify the resolution bandwidth directly using the RBW property.

Tunable: Yes

Dependency

To enable this property, set either:

• InputDomain to "Time" and FrequencyResolutionMethod to "RBW".

• InputDomain to "Frequency".

Scope Window Use

Open the Spectrum Settings. In the Main options section, set RBW (Hz).

Data Types: char | string

RBW — Resolution bandwidth

9.76 (default) | real positive scalar

RBW controls the spectral resolution of Spectrum Analyzer. Specify the resolution bandwidth in hertz as a real positive scalar. You must specify a value to ensure that there are at least two RBW intervals over the specified frequency span. Thus, the ratio of the overall span to RBW must be greater than two:

$$\frac{span}{RBW}>2$$

You can specify the overall span in different ways based on how you set the FrequencySpan property.

Dependency

To enable, set:

• RBWSource to "Property"

Scope Window Use

Open the Spectrum Settings. In the Main options section, set RBW (Hz).

WindowLength — Window length

1024 (default) | integer greater than 2

Control the frequency resolution by specifying the window length, in samples used to compute the spectral estimates. The window length must be an integer scalar greater than 2.

Tunable: Yes

Dependencies

To enable this property, set:

• FrequencyResolutionMethod to "WindowLength", which controls the frequency resolution based on your window length setting

• Method to "Welch"

Scope Window Use

Open the Spectrum Settings. Change the RBW (Hz) dropdown to Window length.

FFTLengthSource — Source of the FFT length

"Auto" (default) | "Property"

• "Auto" - sets the FFT length to the window length specified in the WindowLength property or 1024, whichever is larger.

• "Property" - number of FFT points using the FFTLength property. FFTLength must be greater than WindowLength.

Tunable: Yes

Dependency

To enable this property, set FrequencyResolutionMethod to "WindowLength".

Scope Window Use

Open the Spectrum Settings. In the Main options section, next to the RBW (Hz) option, enter a number or select Auto.

Data Types: char | string

FFTLength — Length of FFT

1024 (default) | positive integer

Specify the length of the FFT that the Spectrum Analyzer uses to compute spectral estimates.

If FrequencyResolutionMethod is "RBW", the FFT length is set as the window length required to achieve the specified resolution bandwidth value or 1024, whichever is larger.

Tunable: Yes

Dependencies

To use this property, the following must be true:

• FrequencyResolutionMethod is set to "WindowLength" or "NumFrequencyBands"

• FFTLength is greater than or equal to the WindowLength.

• FFTLengthSource is set to "Property".

Scope Window Use

Open the Spectrum Settings. In the Main options section, next to the RBW (Hz) option, enter a number or select Auto.

NumTapsPerBand — Number of filter taps per frequency band

12 (default) | positive even scalar

Specify the number of filter taps or coefficients for each frequency band. This number must be a positive even integer. This value corresponds to the number of filter coefficients per polyphase branch. The total number of filter coefficients is equal to NumTapsPerBand + FFTLength.

Dependency

To enable this property, set Method to "Filter Bank"

Scope Window Use

Open the Spectrum Settings. In the Main options section, set Taps per band.

FrequencyVectorSource — Source of frequency vector

"Auto" (default) | "Property"

• "Auto" — The frequency vector is calculated from the length of the input. See Frequency Vector.

• "Property" — Enter a custom vector as the frequency vector.

Dependency

To enable this property, set InputDomain to "Frequency".

Scope Window Use

Open the Spectrum Settings. In the Frequency input options section, set Frequency (Hz).

Data Types: char | string

FrequencyVector — Custom frequency vector

[-5000 5000] (default) | monotonically increasing vector

Set the frequency vector that determines the x-axis of the display. The vector must be monotonically increasing and of the same size as the input frame size.

Dependency

To enable this property, set FrequencyVectorSource to "Property".

Scope Window Use

Open the Spectrum Settings. In the Frequency input options section, set Frequency (Hz).

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

OverlapPercent — Overlap percentage

0 (default) | real, scalar value

The percentage overlap between the previous and current buffered data segments, specified as a real, scalar value. The overlap creates a window segment that is used to compute a spectral estimate. The value must be greater than or equal to zero and less than 100.

Tunable: Yes

Scope Window Use

Open the Spectrum Settings. In the Window options section, set Overlap (%).

Window — Window function

"Hann" (default) | "Rectangular" | "Chebyshev" | "Flat Top" | "Hamming" | "Kaiser" | "Blackman-Harris" | "Custom"

Specify a window function for the spectral estimator. The following table shows preset windows. For more information, follow the link to the corresponding function reference in the Signal Processing Toolbox™ documentation.

Window Option	Corresponding Signal Processing Toolbox Function
"Rectangular"	rectwin
"Chebyshev"	chebwin
"Flat Top"	flattopwin
"Hamming"	hamming
"Hann"	hann
"Kaiser"	kaiser
"Blackman-Harris"	blackmanharris

To set your own spectral estimation window, set this property to "Custom" and specify a custom window function in the CustomWindow property.

Tunable: Yes

Scope Window Use

Open the Spectrum Settings. In the Window options section, set Window.

Data Types: char | string

CustomWindow — Custom window function

"hann" (default) | character array | string scalar

Specify a custom window function as a character array or string. The custom window function name must be on the MATLAB path. This property is useful if you want to customize the window using additional properties available with the Signal Processing Toolbox version of the window function.

Tunable: Yes

Example

Define and use a custom window function.

function w = my_hann(L)
    w = hann(L, 'periodic')
end

scope.Window = 'Custom';
scope.CustomWindow = 'my_hann'

Dependency

To use this property, set Window to "Custom".

Scope Window Use

Open the Spectrum Settings. In the Window options section, in the Window option box, enter a custom window function name.

Data Types: char | string

SidelobeAttenuation — Sidelobe attenuation of window

60 (default) | real positive scalar

The window sidelobe attenuation, in decibels (dB). The value must be greater than or equal to 45.

Tunable: Yes

Dependency

To enable this property, set Window to "Chebyshev" or "Kaiser".

Scope Window Use

Open the Spectrum Settings. In the Window options section, set Attenuation (dB).

InputUnits — Units of frequency input

"dBm" (default) | "dBV" | "dBW" | "Vrms" | "Watts"

Select the units of the frequency-domain input. This property allows the Spectrum Analyzer to scale frequency data if you choose a different display unit with the Units property.

Dependency

This option is only available when InputDomain is set to Frequency.

Scope Window Use

Open the Spectrum Settings. In the Frequency input options section, set Input units.

Data Types: char | string

SpectrumUnits — Units of the spectrum

"Auto" (default) | "dBm" | "dBFS" | "dBV" | "dBW" | "Vrms" | "Watts"

Specify the units in which the Spectrum Analyzer displays power values.

Tunable: Yes

Dependency

The available spectrum units depend on the value of SpectrumType.

InputDomain	SpectrumType	Allowed SpectrumUnits
Time	Power or Power density	"dBFS", "dBm", "dBW", "Watts"
	RMS	"Vrms", "dBV"
Frequency	―	"dBm", "dBV", "dBW", "Vrms", "Watts",

Scope Window Use

Open the Spectrum Settings. In the Trace options section, set Units.

Data Types: char | string

FullScaleSource — Source of full scale

"Auto" (default) | "Property"

Specify the source of the dBFS scaling factor as either "Auto" or "Property".

• "Auto" - The Spectrum Analyzer adjusts the scaling factor based on the input data.

• "Property" - Specify the full-scale scaling factor using the FullScale property.

Dependency

To enable this property, set SpectrumUnits to "dBFS".

Scope Window Use

Open the Spectrum Settings. In the Trace options section, set Full scale to Auto or enter a number.

Data Types: char | string

FullScale — Full scale

1 (default) | positive scalar

Specify a real positive scalar for the dBFS full scale.

Tunable: Yes

Dependency

To enable this option set:

• SpectrumUnits to "dBFS"

• FullScaleSource to "Property"

Scope Window Use

Open the Spectrum Settings. In the Trace options section, set Full scale to Auto or enter a number.

AveragingMethod — Smoothing method

"Running" (default) | "Exponential"

Specify the smoothing method as:

• Running — Running average of the last n samples. Use the SpectralAverages property to specify n.

• Exponential — Weighted average of samples. Use the ForgettingFactor property to specify the weighted forgetting factor.

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

Dependency

To enable this property, set ViewType to "Spectrum" or "Spectrum and spectrogram".

Scope Window Use

Open the Spectrum Settings. In the Trace options section, set Averaging method.

Data Types: char | string

SpectralAverages — Number of spectral averages

1 (default) | positive integer

The Spectrum Analyzer computes the current power spectrum estimate by computing a running average of the last N power spectrum estimates. This property defines N.

Tunable: Yes

Dependency

To enable this property, set ViewType to "Spectrum".

Dependency

This property applies only when the AveragingMethod is "Running".

Scope Window Use

Open the Spectrum Settings. In the Trace options section, set Averages.

ForgettingFactor — Weighting forgetting factor

0.9 (default) | scalar in the range (0,1]

Specify the exponential weighting as a scalar value greater than 0 and less than or equal to 1.

Dependency

This property applies only when the AveragingMethod is "Exponential".

Scope Window Use

Open the Spectrum Settings. In the Trace options section, set Forgetting factor.

ReferenceLoad — Reference load

1 (default) | real positive scalar

The load the scope uses as a reference to compute power levels.

Tunable: Yes

Scope Window Use

Open the Spectrum Settings. In the Trace options section, set Reference load.

FrequencyOffset — Frequency offset

0 (default) | scalar | vector

• Scalar — Apply the same frequency offset to all channels, specified in hertz as a character vector.

• Vector — apply a specific frequency offset for each channel, specify a vector of frequencies. The vector length must be equal to number of input channels.

  The frequency-axis values are offset by the values specified in this property. 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 FrequencySpan property.

Tunable: Yes

Scope Window Use

Open the Spectrum Settings. In the Trace options section, set Offset (Hz).

Spectrogram

SpectrogramChannel — Channel for which spectrogram is plotted

1 (default) | positive scalar integer

Specify the channel for which the spectrogram is plotted, as a real, positive scalar integer in the range [1 N], where N is the number of input channels.

Tunable: Yes

Dependency

To enable this property, set ViewType to "Spectrogram" or "Spectrum and spectrogram".

Scope Window Use

Open the Spectrum Settings. In the Spectrogram options section, select a Channel.

TimeResolutionSource — Source of the time resolution value

"Auto" (default) | "Property"

Specify the source for the time resolution of each spectrogram line as either "Auto" or "Property". The TimeResolution property shows the time resolution for the different frequency resolution methods and time resolution properties.

Tunable: Yes

Dependency

To enable this property, set ViewType to "Spectrogram" or "Spectrum and spectrogram".

Scope Window Use

Open the Spectrum Settings. In the Spectrogram options section, set Time res (s).

Data Types: char | string

TimeResolution — Time resolution

0.001 (default) | positive scalar

Specify the time resolution of each spectrogram line as a positive scalar, expressed in seconds.

The time resolution value is determined based on frequency resolution method, the RBW setting, and the time resolution setting.

Method	Frequency Resolution Method	Frequency Resolution Setting	Time Resolution Setting	Resulting Time Resolution in Seconds
Welch or Filter Bank	RBW (Hz)	Auto	Auto	1/RBW
Welch or Filter Bank	RBW (Hz)	Auto	Manually entered	Time Resolution
Welch or Filter Bank	RBW (Hz)	Manually entered	Auto	1/RBW
Welch or Filter Bank	RBW (Hz)	Manually entered	Manually entered	Must be equal to or greater than the minimum attainable time resolution, 1/RBW. Several spectral estimates are combined into one spectrogram line to obtain the desired time resolution. Interpolation is used to obtain time resolution values that are not integer multiples of 1/RBW.
Welch	Window length	—	Auto	1/RBW
Welch	Window length	—	Manually entered	Must be equal to or greater than the minimum attainable time resolution. Several spectral estimates are combined into one spectrogram line to obtain the desired time resolution. Interpolation is used to obtain time resolution values that are not integer multiples of 1/RBW.
Filter Bank	Number of frequency bands	—	Auto	1/RBW
Filter Bank	Number of frequency bands	—	Manually entered	Must be equal to or greater than the minimum attainable time resolution, 1/RBW.

Tunable: Yes

Dependency

To enable this property, set:

• ViewType to "Spectrogram" or "Spectrum and spectrogram"

• TimeResolutionSource to "Property.

Scope Window Use

Open the Spectrum Settings. In the Spectrogram options section, in the Time res (s) box, enter a number.

TimeSpanSource — Source of time span value

"Auto" (default) | "Property"

Specify the source for the time span of the spectrogram as either "Auto" or "Property". If you set this property to "Auto", the spectrogram displays 100 spectrogram lines at any given time. If you set this property to "Property", the spectrogram uses the time duration you specify in seconds in the TimeSpan property.

Tunable: Yes

Dependency

To enable this property, set ViewType to "Spectrogram" or "Spectrum and spectrogram".

Scope Window Use

Open the Spectrum Settings. In the Spectrogram options section, set Time span (s).

Data Types: char | string

TimeSpan — Time span

0.1 (default) | positive scalar

Specify the time span of the spectrogram display in seconds. You must set the time span to be at least twice as large as the duration of the number of samples required for a spectral update.

Tunable: Yes

Dependency

To enable this property, set:

• ViewType to "Spectrogram" or "Spectrum and spectrogram".

• TimeSpanSource to "Property".

Scope Window Use

Open the Spectrum Settings. In the Spectrogram options section, in the Time span (s) box, enter a number.

Measurements

MeasurementChannel — Channel for which measurements are obtained

1 (default) | positive integer

Channel for which the measurements are obtained, specified as a real, positive integer greater than 0 and less than or equal to 100. The maximum number you can specify is the number of channels (columns) in the input signal.

Tunable: Yes

Scope Window Use

Click on Tools > Measurements and open the Trace Selection settings.

Data Types: double

SpectralMask — Spectral mask lines

SpectralMaskSpecification object

Specify whether to display upper and lower spectral mask lines on a spectrum plot. This property uses properties from a SpectralMaskSpecification object to enable and configure the spectral masks.

Tunable: Yes

Scope Window Use

Open the Spectral Mask pane and modify the Settings options.

PeakFinder — Peak finder measurement

PeakFinderSpecification object

Enable peak finder to compute and display the largest calculated peak values. The PeakFinder property uses the PeakFinderSpecification properties.

The PeakFinderSpecification properties are:

• MinHeight –– Level above which peaks are detected, specified as a scalar value.

  Default: -Inf

• NumPeaks –– Maximum number of peaks to show, specified as a positive integer scalar less than 100.

  Default: 3

• MinDistance –– Minimum number of samples between adjacent peaks, specified as a positive real scalar.

  Default: 1

• Threshold –– Minimum height difference between peak and its neighboring samples, specified as a nonnegative real scalar.

  Default: 0

• LabelFormat –– Coordinates to display next to the calculated peak value, specified as a character vector or a string scalar. Valid values are "X", "Y", or "X + Y".

  Default: "X + Y"

• Enable –– Set this property to true to enable peak finder measurements. Valid values are true or false.

  Default: false

All PeakFinderSpecification properties are tunable.

Tunable: Yes

Scope Window Use

Open the Peak Finder pane () and modify the Settings options.

CursorMeasurements — Cursor measurements

CursorMeasurementsSpecification object

Enable cursor measurements to display screen or waveform cursors. The CursorMeasurements property uses the CursorMeasurementsSpecification properties.

The CursorMeasurementsSpecification properties are:

• Type –– Type of the display cursors, specified as either "Screen cursors" or "Waveform cursors".

  Default: "Waveform cursors"

• ShowHorizontal –– Set this property to true to show the horizontal screen cursors. This property applies when you set the Type property to "Screen cursors".

  Default: true

• ShowVertical –– Set this property to true to show the vertical screen cursors. This property applies when you set the Type property to "Screen cursors".

  Default: true

• Cursor1TraceSource –– Specify the waveform cursor 1 source as a positive real scalar. This property applies when you set the Type property to "Waveform cursors".

  Default: 1

• Cursor2TraceSource –– Specify the waveform cursor 2 source as a positive real scalar. This property applies when you set the Type property to "Waveform cursors".

  Default: 1

• LockSpacing –– Lock spacing between cursors, specified as a logical scalar.

  Default: false

• SnapToData –– Snap cursors to data, specified as a logical scalar.

  Default: true

• XLocation –– x-coordinates of the cursors, specified as a real vector of length equal to 2.

  Default: [-2500 2500]

• YLocation –– y-coordinates of the cursors, specified as a real vector of length equal to 2. This property applies when you set the Type property to "Screen cursors".

  Default: [-55 5]

• Enable –– Set this property to true to enable cursor measurements. Valid values are true or false.

  Default: false

All CursorMeasurementsSpecification properties are tunable.

Scope Window Use

Open the Cursor Measurements pane () and modify the Settings options.

ChannelMeasurements — Channel measurements

ChannelMeasurementsSpecification object

Enable channel measurements to compute and display the occupied bandwidth or adjacent channel power ratio. The ChannelMeasurements property uses the ChannelMeasurementsSpecification properties.

The ChannelMeasurementsSpecification properties are:

• Algorithm –– Type of measurement data to display, specified as either "Occupied BW" or "ACPR".

  Default: "Occupied BW"

• FrequencySpan –– Frequency span mode, specified as either "Span and center frequency" or "Start and stop frequencies"

  Default: "Span and center frequency"

• Span –– Frequency span over which the channel measurements are computed, specified as a real, positive scalar in Hz. This property applies when you set the FrequencySpan property to "Span and center frequency".

  Default: 2000 Hz

• CenterFrequency –– Center frequency of the span over which the channel measurements are computed, specified as a real scalar in Hz. This property applies when you set the FrequencySpan property to "Span and center frequency".

  Default: 0 Hz

• StartFrequency –– Start frequency over which the channel measurements are computed, specified as a real scalar in Hz. This property applies when you set the FrequencySpan property to "Start and stop frequencies".

  Default: -1000 Hz

• StopFrequency –– Stop frequency over which the channel measurements are computed, specified as a real scalar in Hz. This property applies when you set the FrequencySpan property to "Start and stop frequencies".

  Default: 1000 Hz

• PercentOccupiedBW –– Percent of power over which to compute the occupied bandwidth, specified as a positive real scalar. This property applies when you set the Algorithm property to "Occupied BW".

  Default: 99

• NumOffsets –– Number of adjacent channel pairs, specified as a real, positive integer. This property applies when you set the Algorithm property to "ACPR".

  Default: 2

• AdjacentBW –– Adjacent channel bandwidth, specified as a real, positive scalar. This property applies when you set the Algorithm property to "ACPR".

  Default: 1000

• FilterShape –– Filter shape for both main and adjacent channels, specified as "None", "Gaussian", or "RRC". This property applies when you set the Algorithm property to "ACPR".

  Default: "None"

• FilterCoeff –– Channel filter coefficient, specified as a real scalar between 0 and 1. This property applies when you set the Algorithm property to "ACPR" and the FilterShape property to either "Gaussian" or "RRC".

  Default: 0.5

• ACPROffsets –– Frequency of the adjacent channel relative to the center frequency of the main channel, specified as a real vector of length equal to the number of offset pairs specified in NumOffsets. This property applies when you set the Algorithm property to "ACPR".

  Default: [2000 3500]

• Enable –– Set this property to true to enable channel measurements. Valid values are true or false.

  Default: false

All ChannelMeasurementsSpecification properties are tunable.

Scope Window Use

Open the Channel Measurements pane () and modify the Measurement and Channel Settings options.

DistortionMeasurements — Distortion measurements

DistortionMeasurementsSpecification object

Enable distortion measurements to compute and display the harmonic distortion and intermodulation distortion. The DistortionMeasurements property uses the DistortionMeasurementsSpecification properties.

The DistortionMeasurementsSpecification properties are:

• Algorithm –– Type of measurement data to display, specified as either "Harmonic" or "Intermodulation".

  Default: "Harmonic"

• NumHarmonics –– Number of harmonics to measure, specified as a real, positive integer. This property applies when you set the Algorithm to "Harmonic".

  Default: 6

• Enable –– Set this property to true to enable distortion measurements.

  Default: false

All DistortionMeasurementsSpecification properties are tunable.

Scope Window Use

Open the Distortion Measurements pane () and modify the Distortion and Harmonics options.

CCDFMeasurements — CCDF measurements

CCDFMeasurementsSpecification object

Enable CCDF measurements to display the probability of the input signal's instantaneous power being a certain amount of dB above the signal's average power. The CCDFMeasurements property uses the CCDFMeasurementsSpecification properties.

The CCDFMeasurementsSpecification properties are:

• PlotGaussianReference –– Set this property to true to plot a reference CCDF curve. The reference CCDF curve represents the power of a complex white Gaussian noise, calculated as a chi-squared distribution.

  Default: false

• Enable –– Set this property to true to enable CCDF measurements. Valid values are true or false.

  Default: false

All CCDFMeasurementsSpecification properties are tunable.

Scope Window Use

Open the CCDF Measurements pane () and enable the Plot Gaussian reference option.

Visualization

Name — Window name

"Spectrum Analyzer" (default) | character vector | string scalar

Title of the scope window.

Tunable: Yes

Data Types: char | string

Position — Window position

screen center (default) | [left bottom width height]

Spectrum Analyzer window position in pixels, specified by the size and location of the scope window as a four-element double vector of the form [left bottom width height]. You can place the scope window in a specific position on your screen by modifying the values to this property.

By default, the window appears in the center of your screen with a width of 800 pixels and height of 450 pixels. The exact center coordinates depend on your screen resolution.

Tunable: Yes

PlotType — Plot type for normal traces

"Line" (default) | "Stem"

Specify the type of plot to use for displaying normal traces as either "Line" or "Stem". Normal traces are traces that display free-running spectral estimates.

Tunable: Yes

Dependencies

To enable this property, set:

• ViewType to "Spectrum" or "Spectrum and spectrogram"

• PlotNormalTrace to true

Scope Window Use

Open the Style properties and set Plot type.

Data Types: char | string

PlotNormalTrace — Normal trace flag

true (default) | false

Set this property to false to remove the display of the normal traces. These traces display the free-running spectral estimates. Even when the traces are removed from the display, the Spectrum Analyzer continues its spectral computations.

Tunable: Yes

Dependency

To enable this property, set ViewType to "Spectrum" or "Spectrum and spectrogram".

Scope Window Use

Open the Spectrum Settings. In the Trace options section, select Normal trace.

Data Types: logical

PlotMaxHoldTrace — Max-hold trace flag

false (default) | true

To compute and plot the maximum-hold spectrum of each input channel, set this property to true. The maximum-hold spectrum at each frequency bin is computed by keeping the maximum value of all the power spectrum estimates. When you toggle this property, the Spectrum Analyzer resets its maximum-hold computations.

Tunable: Yes

Dependency

To enable this property, set ViewType to "Spectrum" or "Spectrum and spectrogram".

Scope Window Use

Open the Spectrum Settings. In the Trace options section, select Max-hold trace.

Data Types: logical

PlotMinHoldTrace — Min-hold trace flag

false (default) | true

To compute and plot the minimum-hold spectrum of each input channel, set this property to true. The minimum-hold spectrum at each frequency bin is computed by keeping the minimum value of all the power spectrum estimates. When you toggle this property, the Spectrum Analyzer resets its minimum-hold computations.

Tunable: Yes

Dependency

To enable this property, set ViewType to "Spectrum" or "Spectrum and spectrogram".

Scope Window Use

Open the Spectrum Settings. In the Trace options section, select Min-hold trace.

Data Types: logical

ReducePlotRate — Improve performance with reduced plot rate

true (default) | false

The simulation speed is faster when this property is set to true.

• true — the scope logs data for later use and updates the display at fixed intervals of time. Data occurring between these fixed intervals might not be plotted.

• false — the scope updates every time it computes the power spectrum. Use the false setting when you do not want to miss any spectral updates at the expense of slower simulation speed.

Tunable: Yes

Scope Window Use

Select Playback > Reduce plot rate to improve performance.

Title — Display title

'' (default) | character vector | string scalar

Specify the display title as a character vector or string.

Tunable: Yes

Scope Window Use

Open the Configuration Properties. Set Title.

Data Types: char | string

YLabel — Y-axis label

'' (default) | character vector | string scalar

Specify the text for the scope to display to the left of the y-axis.

Regardless of this property, Spectrum Analyzer always displays power units as one of the SpectrumUnits values.

Tunable: Yes

Dependency

To enable this property, set ViewType to "Spectrum" or "Spectrum and spectrogram".

Scope Window Use

Open the Configuration Properties. Set Y-label.

Data Types: char | string

ShowLegend — Show legend

false (default) | true

To show a legend with the input names, set this property to true.

From the legend, you can control which signals are visible. This control is equivalent to changing the visibility in the Style dialog box. In the scope legend, click a signal name to hide the signal in the scope. To show the signal, click the signal name again. To show only one signal, right-click the signal name. To show all signals, press Esc.

Note

The legend only shows the first 20 signals. Any additional signals cannot be viewed or controlled from the legend.

Tunable: Yes

Scope Window Use

Open the Configuration Properties. On the Display tab, select Show legend.

Data Types: logical

ChannelNames — Channel names

empty cell (default) | cell array of character vectors

Specify the input channel names as a cell array of character vectors. The names appear in the legend, Style dialog box, and Measurements panels. If you do not specify names, the channels are labeled as Channel 1, Channel 2, etc.

Tunable: Yes

Dependency

To see channel names, set ShowLegend to true.

Scope Window Use

On the legend, double-click the channel name.

Data Types: char

ShowGrid — Grid visibility

true (default) | false

Set this property to true to show gridlines on the plot.

Tunable: Yes

Scope Window Use

Open the Configuration Properties. On the Display tab, set Show grid.

Data Types: logical

YLimits — Y-axis limits

[-80, 20] (default) | [ymin ymax]

Specify the y-axis limits as a two-element numeric vector, [ymin ymax].

Example: scope.YLimits = [-10,20]

Tunable: Yes

Dependencies

• To enable this property, set the ViewType property to "Spectrum" or "Spectrum and spectrogram".

• The units directly depend upon the SpectrumUnits property.

Scope Window Use

Open the Configuration Properties. Set Y-limits (maximum) and Y-limits (minimum).

ColorLimits — Scale spectrogram color limits

[-80, 20] (default) | [colorMin colorMax]

Control the color limits of the spectrogram using a two-element numeric vector, [colorMin colorMax].

Example: scope.ColorLimits = [-10,20]

Dependencies

• To enable this property, set the ViewType property to "Spectrogram" or "Spectrum and spectrogram".

• The units directly depend upon the SpectrumUnits property.

Scope Window Use

Open the Configuration Properties. Set Color-limits (minimum) and Color-limits (maximum).

AxesScaling — Axes scaling mode

"Auto" (default) | "Manual" | "OnceAtStop" | "Updates"

Specify when the scope automatically scales the axes. Valid values are:

• "Auto" — The scope scales the axes as-needed to fit the data, both during and after simulation.

• "Manual" — The scope does not scale the axes automatically.

• "OnceAtStop" — The scope scales the axes when the simulation stops.

• "Updates" — The scope scales the axes once after 10 updates.

Scope Window Use

Select Tools > Axes Scaling.

Data Types: char | string

AxesLayout — Orientation of the spectrum and spectrogram

"Vertical" (default) | "Horizontal"

Specify the layout type as "Horizontal" or "Vertical". A vertical layout stacks the spectrum above the spectrogram. A horizontal layout puts the two views side-by-side.

Tunable: Yes

Dependency

To enable this property, set ViewType to "Spectrum and spectrogram".

Scope Window Use

Open the Spectrum Settings. Set Axes layout.

Data Types: char | string

Usage

Syntax

scope(signal)

scope(signal1,signal2,...,signalN)

Description

scope(signal) updates the spectrum of the signal in the spectrum analyzer.

scope(signal1,signal2,...,signalN) displays multiple signals in the spectrum analyzer. The signals must have the same frame length, but can vary in number of channels. You must set the NumInputPorts property to enable multiple input signals.

Input Arguments

signal — Input signal or signals to visualize

scalar | vector | matrix

Specify one or more input signals to visualize in the dsp.SpectrumAnalyzer. Signals can have a different number of channels, but must have the same frame length.

Example: scope(signal1, signal2)

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | fi

Object Functions

To use an object function, specify the System object as the first input argument. For example, to release system resources of a System object named obj, use this syntax:

release(obj)

Specific to dsp.SpectrumAnalyzer

generateScript	Generate MATLAB script to create scope with current settings
getMeasurementsData	Get the current measurement data displayed on the spectrum analyzer
getSpectralMaskStatus	Get test results of current spectral mask
getSpectrumData	Save spectrum data shown in spectrum analyzer
isNewDataReady	Check spectrum analyzer for new data

Specific to Scopes

show	Display scope window
hide	Hide scope window
isVisible	Determine visibility of scope

Common to All System Objects

step	Run System object algorithm
release	Release resources and allow changes to System object property values and input characteristics
reset	Reset internal states of System object

If you want to restart the simulation from the beginning, call reset to clear the scope window displays. Do not call reset after calling release.

Examples

Spectrum Analyzer for One-Sided Power Spectrum

View a one-sided power spectrum made from the sum of fixed real sine waves with different amplitudes and frequencies.

Fs = 100e6;  % Sampling frequency
fSz = 5000;  % Frame size

sin1 = dsp.SineWave(1e0,  5e6,0,'SamplesPerFrame',fSz,'SampleRate',Fs);
sin2 = dsp.SineWave(1e-1,15e6,0,'SamplesPerFrame',fSz,'SampleRate',Fs);
sin3 = dsp.SineWave(1e-2,25e6,0,'SamplesPerFrame',fSz,'SampleRate',Fs);
sin4 = dsp.SineWave(1e-3,35e6,0,'SamplesPerFrame',fSz,'SampleRate',Fs);
sin5 = dsp.SineWave(1e-4,45e6,0,'SamplesPerFrame',fSz,'SampleRate',Fs);

scope = dsp.SpectrumAnalyzer;
scope.SampleRate = Fs;
scope.SpectralAverages = 1;
scope.PlotAsTwoSidedSpectrum = false;
scope.RBWSource = 'Auto';
scope.PowerUnits = 'dBW';
for idx = 1:1e2 
     y1 = sin1();
     y2 = sin2();
     y3 = sin3();
     y4 = sin4();
     y5 = sin5();
     scope(y1+y2+y3+y4+y5+0.0001*randn(fSz,1));
end

Run the release method to let property values and input characteristics change. The scope automatically scales the axes.

release(scope)

Run the clear function to close the Spectrum Analyzer window.

clear('scope');

Spectrogram of Chirp Signal

This example shows the spectrogram for a chirp signal with added random noise.

Fs = 233e3;
frameSize = 20e3;
chirp = dsp.Chirp('SampleRate',Fs,...
  'SamplesPerFrame',frameSize,...
  'InitialFrequency',11e3,...
  'TargetFrequency',11e3+55e3);

scope = dsp.SpectrumAnalyzer('SampleRate',Fs);
scope.ViewType = 'Spectrogram';
scope.RBWSource = 'Property';
scope.RBW = 500;
scope.TimeSpanSource = 'Property';
scope.TimeSpan = 2;
scope.PlotAsTwoSidedSpectrum = false;

for idx = 1:50
  y = chirp()+ 0.05*randn(frameSize,1);
  scope(y);
end

release(scope)

Spectrum Analyzer For Two-Sided Power Spectrum

View a two-sided power spectrum of a sine wave with noise on the Spectrum Analyzer.

sin = dsp.SineWave('Frequency',100,'SampleRate',1000);
sin.SamplesPerFrame = 1000;
scope = dsp.SpectrumAnalyzer('SampleRate',sin.SampleRate);
for ii = 1:250
  x = sin() + 0.05*randn(1000,1);
  scope(x);
end

Run the release method to change property values and input characteristics. The scope automatically scales the axes. It updates the display one more time if any data is in the internal buffer.

release(scope);

Run the MATLAB clear function to close the Spectrum Analyzer window.

clear('scope');

Display Frequency Input from Spectral Estimation

Use the Spectrum Analyzer to display frequency input from spectral estimates of sinusoids embedded in white Gaussian noise.

Initialization

Initialize two dsp.SpectrumEstimator objects to display. Set one object to use the Welch-based spectral estimation technique with a Hann window, set the other object use a filter bank estimation. Specify a noisy sine wave input signal with four sinusoids at 0.16, 0.2, 0.205, and 0.25 cycles/sample. View the spectral estimate using a third object, a spectrum analyzer, set to process frequency input.

FrameSize = 420;
Fs = 1;
Frequency = [0.16 0.2 0.205 0.25];
sinegen = dsp.SineWave('SampleRate',Fs,'SamplesPerFrame',FrameSize,...
    'Frequency',Frequency,'Amplitude',[2e-5 1  0.05  0.5]);
NoiseVar = 1e-10;
numAvgs = 8;

hannEstimator = dsp.SpectrumEstimator('PowerUnits','dBm',...
    'Window','Hann','FrequencyRange','onesided',...
    'SpectralAverages',numAvgs,'SampleRate',Fs);

filterBankEstimator = dsp.SpectrumEstimator('PowerUnits','dBm',...
    'Method','Filter bank','FrequencyRange','onesided',...
    'SpectralAverages',numAvgs,'SampleRate',Fs);

spectrumPlotter = dsp.SpectrumAnalyzer('InputDomain','Frequency',...
    'SampleRate',Fs,...
    'SpectrumUnits','dBm','YLimits',[-120,40],...
    'PlotAsTwoSidedSpectrum',false,...
    'ChannelNames',{'Hann window','Filter bank'},'ShowLegend',true);

Streaming

Stream the input. Compare the spectral estimates in the spectrum analyzer.

for i = 1:1000
    x = sum(sinegen(),2) + sqrt(NoiseVar)*randn(FrameSize,1);
    Pse_hann = hannEstimator(x);
    Pfb = filterBankEstimator(x);
    spectrumPlotter([Pse_hann,Pfb])
end

Obtain Measurement Data Programmatically for dsp.SpectrumAnalyzer System object

Compute and display the power spectrum of a noisy sinusoidal input signal using the dsp.SpectrumAnalyzer System object™. Measure the peaks, cursor placements, adjacent channel power ratio, distortion, and CCDF values in the spectrum by enabling the following properties:

• PeakFinder

• CursorMeasurements

• ChannelMeasurements

• DistortionMeasurements

• CCDFMeasurements

Initialization

The input sine wave has two frequencies: 1000 Hz and 5000 Hz. Create two dsp.SineWave System objects to generate these two frequencies. Create a dsp.SpectrumAnalyzer System object to compute and display the power spectrum.

Fs = 44100;
Sineobject1 = dsp.SineWave('SamplesPerFrame',1024,'PhaseOffset',10,...
    'SampleRate',Fs,'Frequency',1000);
Sineobject2 = dsp.SineWave('SamplesPerFrame',1024,...
    'SampleRate',Fs,'Frequency',5000);
SA = dsp.SpectrumAnalyzer('SampleRate',Fs,'Method','Filter bank',...
    'SpectrumType','Power','PlotAsTwoSidedSpectrum',false,...
    'ChannelNames',{'Power spectrum of the input'},'YLimits',[-120 40],'ShowLegend',true);

Enable Measurements Data

To obtain the measurements, set the Enable property of the measurements to true.

SA.CursorMeasurements.Enable = true;
SA.ChannelMeasurements.Enable = true;
SA.PeakFinder.Enable = true;
SA.DistortionMeasurements.Enable = true;

Use getMeasurementsData

Stream in the noisy sine wave input signal and estimate the power spectrum of the signal using the spectrum analyzer. Measure the characteristics of the spectrum. Use the getMeasurementsData function to obtain these measurements programmatically. The isNewDataReady function indicates when there is new spectrum data. The measured data is stored in the variable data.

data = [];
for Iter = 1:1000
    Sinewave1 = Sineobject1();
    Sinewave2 = Sineobject2();
    Input = Sinewave1 + Sinewave2;
    NoisyInput = Input + 0.001*randn(1024,1);
    SA(NoisyInput);
     if SA.isNewDataReady
        data = [data;getMeasurementsData(SA)];
     end
end

The right side of the spectrum analyzer shows the enabled measurement panes. The values shown in these panes match with the values shown in the last time step of the data variable. You can access the individual fields of data to obtain the various measurements programmatically.

Compare Peak Values

Peak values are obtained by the PeakFinder property. Verify that the peak values obtained in the last time step of data match the values shown on the spectrum analyzer plot.

peakvalues = data.PeakFinder(end).Value 

peakvalues = 3×1

   26.9850
   24.1735
  -52.3506

frequencieskHz = data.PeakFinder(end).Frequency/1000

frequencieskHz = 3×1

    4.9957
    0.9905
    7.8166

Tips

• To close the scope window and clear its associated data, use the MATLAB^® clear function.

• To hide or show the scope window, use the hide and show functions.

• Use the MATLAB mcc function to compile code containing a Spectrum Analyzer.

  You cannot open Spectrum Analyzer configuration dialog boxes if you have more than one compiled component in your application.

Algorithms

Spectrum Estimation — Welch's Method

When you choose the Welch method, the power spectrum estimate is averaged modified periodograms.

Given the signal input, x, the Spectrum Analyzer does the following:

1. Multiplies x by the given window and scales the result by the window power. The Spectrum Analyzer uses the RBW or the Window Length setting in the Spectrum Settings pane to determine the data window length.

2. Computes the FFT of the signal, Y, and takes the square magnitude using Z = Y.*conj(Y).

3. Computes the current power spectrum estimate by taking the moving average of the last N number of Z's, and scales the answer by the sample rate. For details on the moving average methods, see Averaging Method.

Spectrum Analyzer requires that a minimum number of samples to compute a spectral estimate. This number of input samples required to compute one spectral update is shown as Samples/update in the Main options pane. This value is directly related to resolution bandwidth, RBW, by the following equation, or to the window length, by the equation shown in step 2.

$$N_{samples}=\frac{\left(1-\frac{O_p}{100}\right)\times NENBW\times F_s}{RBW}$$

The normalized effective noise bandwidth, NENBW, is a factor that depends on the windowing method. Spectrum Analyzer shows the value of NENBW in the Window Options pane of the Spectrum Settings pane. Overlap percentage, O_p, is the value of the Overlap % parameter in the Window Options pane of the Spectrum Settings pane. F_s is the sample rate of the input signal. Spectrum Analyzer shows sample rate in the Main Options pane of the Spectrum Settings pane.

1. When in RBW (Hz) mode, the window length required to compute one spectral update, N_window, is directly related to the resolution bandwidth and normalized effective noise bandwidth:

   $$N_{window}=\frac{NENBW\times F_s}{RBW}$$

   When in Window Length mode, the window length is used as specified.

2. The number of input samples required to compute one spectral update, N_samples, is directly related to the window length and the amount of overlap by the following equation.

   $$N_{samples}=\left(1-\frac{O_p}{100}\right)N_{window}$$

   When you increase the overlap percentage, fewer new input samples are needed to compute a new spectral update. For example, if the window length is 100, then the number of input samples required to compute one spectral update is given as shown in the following table.

   Op	Nsamples
   0%	100
   50%	50
   80%	20

3. The normalized effective noise bandwidth, NENBW, is a window parameter determined by the window length, N_window, and the type of window used. If w(n) denotes the vector of N_window window coefficients, then NENBW is given by the following equation.

   $$NENBW=N_{window}\times \frac{{\displaystyle \sum _{n=1}^{N_{window}}{w}^2(n)}}{{\left[{\displaystyle \sum _{n=1}^{N_{window}}w(n)}\right]}^2}$$

4. When in RBW (Hz) mode, you can set the resolution bandwidth using the value of the RBW (Hz) parameter on the Main options pane of the Spectrum Settings pane. You must specify a value to ensure that there are at least two RBW intervals over the specified frequency span. The ratio of the overall span to RBW must be greater than two:

   $$\frac{span}{RBW}>2$$

   By default, the RBW (Hz) parameter on the Main options pane 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. When you set RBW (Hz) to Auto, RBW is calculated as:

   $$RB{W}_{auto}=\frac{span}{1024}$$

5. When in Window Length mode, you specify N_window and the resulting RBW is:

   $$\frac{NENBW\times F_s}{N_{window}}$$

Sometimes, the number of input samples provided are not sufficient to achieve the resolution bandwidth that you specify. When this situation occurs, Spectrum Analyzer displays a message:

Spectrum Analyzer removes this message and displays a spectral estimate when enough data has been input.

Note

The number of FFT points (N_fft) is independent of the window length (N_window). You can set them to different values if N_fft is greater than or equal to N_window.

Spectrum Estimation — Filter Bank

When you choose the Filter Bank method, the Spectrum Analyzer uses an analysis filter bank to estimate the power spectrum.

The filter bank splits the broadband input signal, x(n), of sample rate fs, into multiple narrow band signals, y₀(m), y₁(m), … , y_M-1(m), of sample rate fs/M.

The variable M represents the number of frequency bands in the filter bank. When the frequency resolution method is set to NumFrequencyBands, M is equal to the value you specify for the number of frequency bands. When the frequency resolution method is set to RBW, M is equal to the number of data points that are needed to achieve the specified RBW value or 1024, whichever is larger. The number of taps per frequency band specifies the number of filter coefficients for each frequency band of the filter bank. The total number of filter coefficients is equal to number of taps per band times the number of frequency bands, M. For more information on the analysis filter bank and how it is implemented, see the More About and the Algorithm sections in dsp.Channelizer.

After the broadband input signal is split into multiple narrow bands, the Spectrum Analyzer computes the power in each narrow band using the following equation. Each Z_i value becomes the estimate of the power over that narrow frequency band.

$$Z_i=\frac{1}{L}{\displaystyle \sum _{m=0}^{L-1}{\left|y_i[m]\right|}^2}$$

L is length of the narrow band signal, y_i(m), and i = 1, 2, …, M−1.

The power values in all the narrow bands (denoted by the Z_i) form the Z vector.

$$Z=[Z_0,Z_1,Z_2,\cdots ,Z_{M-1}]$$

The current Z vector is averaged with the previous Z vectors using one of the two moving average methods: Running or Exponential weighting. The output of the averaging operation forms the spectral estimate vector. For details on the two averaging methods, see Averaging Method.

The Spectrum Analyzer uses the RBW (Hz) or the Number of frequency band property in the Spectrum Settings pane to determine the input frame length.

Spectrum Analyzer requires a minimum number of samples to compute a spectral estimate. This number of input samples required to compute one spectral update is shown as Samples/update in the Main options pane. This value is directly related to resolution bandwidth, RBW, by the following equation.

$$N_{samples}=\frac{F_s}{RBW}$$

F_s is the sample rate of the input signal. Spectrum Analyzer shows sample rate in the Main Options pane of the Spectrum Settings pane.

1. When in RBW (Hz) mode, you can set the resolution bandwidth using the value of the RBW (Hz) parameter on the Main options pane of the Spectrum Settings pane. You must specify a value to ensure that there are at least two RBW intervals over the specified frequency span. The ratio of the overall span to RBW must be greater than two:

   $$\frac{span}{RBW}>2$$

   By default, the RBW parameter on the Main options pane 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. Thus, when you set RBW to Auto, it is calculated by the following equation.$$RB{W}_{auto}=\frac{span}{1024}$$

2. When in Number of frequency bands mode, you specify the input frame size. When the number of frequency bands is Auto, the resulting RBW is:

   $$RBW=\frac{F_s}{\text{Input Frame Size}}$$

   When the number of frequency bands is manually specified, the resulting RBW is:

   $$RBW=\frac{F_s}{FFTLength}$$

Sometimes, the number of input samples provided are not sufficient to achieve the resolution bandwidth that you specify. When this situation occurs, Spectrum Analyzer displays a message:

Spectrum Analyzer removes this message and displays a spectral estimate when enough data has been input.

Nyquist frequency interval

When the PlotAsTwoSidedSpectrum property is set to true, the interval is $$\left[-\frac{SampleRate}{2},\frac{SampleRate}{2}\right]+FrequencyOffset$$ hertz.

When the PlotAsTwoSidedSpectrum property is set to false, the interval is $$\left[0,\frac{SampleRate}{2}\right]+FrequencyOffset$$ hertz.

Periodogram and Spectrogram

Spectrum Analyzer calculates and plots the power spectrum, power spectrum density, and RMS computed by the modified Periodogram estimator. For more information about the Periodogram method, see periodogram.

Power Spectral Density — The power spectral density (PSD) is given by the following equation.

$$\mathrm{PSD}\left(f\right)=\frac{1}{P}{\displaystyle \sum _{p=1}^P\frac{{\left|{\displaystyle \sum _{n=1}^{N_{FFT}}{x}^p\left[n\right]e^{-j2\pi f(n-1)T}}\right|}^2}{F_s\times {\displaystyle \sum _{n=1}^{N_{window}}{w}^2\left[n\right]}}}$$

In this equation, x[n] is the discrete input signal. On every input signal frame, Spectrum Analyzer generates as many overlapping windows as possible, with each window denoted as x^(p)[n], and computes their periodograms. Spectrum Analyzer displays a running average of the P most current periodograms.

Power Spectrum — The power spectrum is the product of the power spectral density and the resolution bandwidth, as given by the following equation.

$$P_{spectrum}\left(f\right)=\mathrm{PSD}\left(f\right)\times RBW=\mathrm{PSD}\left(f\right)\times \frac{F_s\times NENBW}{N_{window}}=\frac{1}{P}{\displaystyle \sum _{p=1}^P\frac{{\left|{\displaystyle \sum _{n=1}^{N_{FFT}}{x}^p\left[n\right]e^{-j2\pi f(n-1)T}}\right|}^2}{{\left[{\displaystyle \sum _{n=1}^{N_{window}}w\left[n\right]}\right]}^2}}$$

Spectrogram — You can plot any power as a spectrogram. Each line of the spectrogram is one periodogram. The time resolution of each line is 1/RBW, which is the minimum attainable resolution. Achieving the resolution you want may require combining several periodograms. You then use interpolation to calculate noninteger values of 1/RBW. In the spectrogram display, time scrolls from top to bottom, so the most recent data is shown at the top of the display. The offset shows the time value at which the center of the most current spectrogram line occurred.

Frequency Vector

When set to Auto, the frequency vector for frequency-domain input is calculated by the software.

When the PlotAsTwoSidedSpectrum property is set to true, the frequency vector is:

$$\left[-\frac{SampleRate}{2},\frac{SampleRate}{2}\right]$$

When the PlotAsTwoSidedSpectrum property is set to false, the frequency vector is:

$$\left[0,\frac{SampleRate}{2}\right]$$

Occupied BW

The Occupied BW is calculated as follows.

1. Calculate the total power in the measured frequency range.

2. Determine the lower frequency value. Starting at the lowest frequency in the range and moving upward, the power distributed in each frequency is summed until this result is

   $$\frac{100-OccupiedBW\%}{2}$$

   of the total power.

3. Determine the upper frequency value. Starting at the highest frequency in the range and moving downward, the power distributed in each frequency is summed until the result reaches

   $$\frac{100-OccupiedBW\%}{2}$$

   of the total power.

4. The bandwidth between the lower and upper power frequency values is the occupied bandwidth.

5. The frequency halfway between the lower and upper frequency values is the center frequency.

Distortion Measurements

The Distortion Measurements are computed as follows.

1. Spectral content is estimated by finding peaks in the spectrum. When the algorithm detects a peak, it records the width of the peak and clears all monotonically decreasing values. That is, the algorithm treats all these values as if they belong to the peak. Using this method, all spectral content centered at DC (0 Hz) is removed from the spectrum and the amount of bandwidth cleared (W₀) is recorded.

2. The fundamental power (P₁) is determined from the remaining maximum value of the displayed spectrum. A local estimate (Fe₁) of the fundamental frequency is made by computing the central moment of the power near the peak. The bandwidth of the fundamental power content (W₁) is recorded. Then, the power from the fundamental is removed as in step 1.

3. The power and width of the higher-order harmonics (P₂, W₂, P₃, W₃, etc.) are determined in succession by examining the frequencies closest to the appropriate multiple of the local estimate (Fe₁). Any spectral content that decreases monotonically about the harmonic frequency is removed from the spectrum first before proceeding to the next harmonic.

4. Once the DC, fundamental, and harmonic content is removed from the spectrum, the power of the remaining spectrum is examined for its sum (P_remaining), peak value (P_maxspur), and median value (P_estnoise).

5. The sum of all the removed bandwidth is computed as W_sum = W₀ + W₁ + W₂ +...+ W_n.

   The sum of powers of the second and higher-order harmonics are computed as P_harmonic = P₂ + P₃ + P₄ +...+ P_n.

6. The sum of the noise power is estimated as:

   $$P_{noise}=(P_{remaining}\cdot dF+P_{est.noise}\cdot W_{sum})/RBW$$

   Where dF is the absolute difference between frequency bins, and RBW is the resolution bandwidth of the window.

7. The metrics for SNR, THD, SINAD, and SFDR are then computed from the estimates.

   $$\begin{array}{l}THD=10\cdot {\log }_{10}\left(\frac{P_{harmonic}}{P_1}\right)\\ SINAD=10\cdot {\log }_{10}\left(\frac{P_1}{P_{harmonic}+P_{noise}}\right)\\ SNR=10\cdot {\log }_{10}\left(\frac{P_1}{P_{noise}}\right)\\ SFDR=10\cdot {\log }_{10}\left(\frac{P_1}{\max \left(P_{maxspur},\max \left(P_2,P_3,\mathrm{...},P_n\right)\right)}\right)\end{array}$$

Harmonic Measurements

1. The harmonic distortion measurements use the spectrum trace shown in the display as the input to the measurements. The default Hann window setting of the Spectrum Analyzer may exhibit leakage that can completely mask the noise floor of the measured signal.

   

   The harmonic measurements attempt to correct for leakage by ignoring all frequency content that decreases monotonically away from the maximum of harmonic peaks. If the window leakage covers more than 70% of the frequency bandwidth in your spectrum, you may see a blank reading (–) reported for SNR and SINAD. If your application can tolerate the increased equivalent noise bandwidth (ENBW), consider using a Kaiser window with a high attenuation (up to 330 dB) to minimize spectral leakage.

   

2. The DC component is ignored.

3. After windowing, the width of each harmonic component masks the noise power in the neighborhood of the fundamental frequency and harmonics. To estimate the noise power in each region, Spectrum Analyzer computes the median noise level in the nonharmonic areas of the spectrum. It then extrapolates that value into each region.

4. N^th order intermodulation products occur at A*F1 + B*F2,

   where F1 and F2 are the sinusoid input frequencies and |A| + |B| = N. A and B are integer values.

5. For intermodulation measurements, the third-order intercept (TOI) point is computed as follows, where P is power in decibels of the measured power referenced to 1 milliwatt (dBm):

   • TOI_lower = P_F1 + (P_F2 - P_(2F1-F2))/2

   • TOI_upper = P_F2 + (P_F1 - P_(2F2-F1))/2

   • TOI = + (TOI_lower + TOI_upper)/2

Averaging Method

The moving average is calculated using one of the two methods:

• Running — For each frame of input, average the last N-scaled Z vectors, which are computed by the algorithm. The variable N is the value you specify for the number of spectral averages. If the algorithm does not have enough Z vectors, the algorithm uses zeros to fill the empty elements.

  

• Exponential — The moving average algorithm using the exponential weighting method updates the weights and computes the moving average recursively for each Z vector that comes in by using the following recursive equations:

  $$\begin{array}{l}{w}_N=\lambda w_{N-1}+1\\ {\overline{z}}_N=\left(1-\frac{1}{w_N}\right){\overline{z}}_{N-1}+\left(\frac{1}{w_N}\right)z_N\end{array}$$

  • λ — Forgetting factor

  • $$w_N$$ — Weighting factor applied to the current Z vector

  • $$z_N$$ — Current Z vector

  • $${\overline{z}}_{N-1}$$ — Moving average until the previous Z vector

  • $$\left(1-\frac{1}{w_N}\right){\overline{z}}_{N-1}$$ — Effect of the previous Z vectors on the average

  • $${\overline{z}}_N$$ — Moving average including the current Z vector

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using MATLAB® Coder™.

Usage notes and limitations:

• Supports MEX code generation by treating the calls to the object as extrinsic. Does not support code generation for standalone applications.

• See System Objects in MATLAB Code Generation (MATLAB Coder).