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Display frequency spectrum of time-domain signals
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:
Create the dsp.SpectrumAnalyzer
object and set its properties.
Call the object with arguments, as if it were a function.
To learn more about how System objects work, see What Are System Objects? (MATLAB).
scope = dsp.SpectrumAnalyzer
scope = dsp.SpectrumAnalyzer(ports)
scope = dsp.SpectrumAnalyzer(Name,Value)
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.
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 (MATLAB).
NumInputPorts
— Number of input ports1
(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 signalTime
(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.
Open the Spectrum Settings. In the Main options section, set Input Domain.
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 sum of the squared
means. This option is useful when viewing the frequency of voltage or current signals.
Tunable: Yes
Open the Spectrum Settings. In the Main options section, set Type.
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
Open the Spectrum Settings. In the Main options section, set View.
SampleRate
— Sample rate of input10000
(default) | finite scalarSpecify the sample rate, in hertz, of the input signals as a finite numeric scalar.
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
.
To enable this property, set InputDomain to
'Time'
.
Open the Spectrum Settings. In the Main options section, set Method.
PlotAsTwoSidedSpectrum
— Two-sided spectrum flagtrue
(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
.
Open the Spectrum Settings. In the Trace options section, select Two-sided spectrum.
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
Open the Spectrum Settings. In the Trace options section, set Scale.
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
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
.
Span
— Frequency span to compute spectrum10e3
(default) | real positive scalarSpecify 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
To enable this property, set FrequencySpan to 'Span and center
frequency'
.
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 scalarStart 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
To enable this property, set FrequencySpan to 'Start and stop
frequencies'
.
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 spectrum5e3
(default) | real scalarEnd 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
To enable this property, set FrequencySpan to 'Start and stop
frequencies'
.
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 span0
(default) | real scalarSpecify 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
To enable this property, set FrequencySpan to 'Span and center
frequency'
.
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
To enable this property, set InputDomain to
'Time'
.
Open the Spectrum Settings. In the Main options section, set the frequency resolution method by selecting the RBW (Hz) dropdown.
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
To enable this property, set either:
InputDomain to
'Time'
and FrequencyResolutionMethod to
'RBW'
.
InputDomain
to 'Frequency'
.
Open the Spectrum Settings. In the Main options section, set RBW (Hz).
RBW
— Resolution bandwidth9.76
(default) | real positive scalarRBW 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:
You can specify the overall span in different ways based on how you set the FrequencySpan property.
To enable, set:
RBWSource to
'Property'
Open the Spectrum Settings. In the Main options section, set RBW (Hz).
WindowLength
— Window length1024
(default) | integer greater than 2Control 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
To enable this property, set:
FrequencyResolutionMethod to
'WindowLength'
, which controls the frequency resolution based on your window length
setting
Method to
'Welch'
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
To enable this property, set FrequencyResolutionMethod to
'WindowLength'
.
Open the Spectrum Settings. In the Main options section, next to the RBW (Hz) option, enter
a number or select Auto
.
FFTLength
— Length of FFT1024
(default) | positive integerSpecify 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
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'
.
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 band12
(default) | positive even scalarSpecify 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.
To enable this property, set Method to 'Filter Bank'
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.
To enable this property, set InputDomain to
'Frequency'
.
Open the Spectrum Settings. In the Frequency input options section, set Frequency (Hz).
FrequencyVector
— Custom frequency vector[-5000 5000]
(default) | monotonically increasing vectorSet 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.
To enable this property, set FrequencyVectorSource
to 'Property'
.
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 percentage0
(default) | real, scalar valueThe 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
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
Open the Spectrum Settings. In the Window options section, set Window.
CustomWindow
— Custom window function'hann'
(default) | character array | stringSpecify 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
Define and use a custom window function.
function w = my_hann(L) w = hann(L, 'periodic') end scope.Window = 'Custom'; scope.CustomWindow = 'my_hann'
To use this property, set Window to 'Custom'
.
Open the Spectrum Settings. In the Window options section, in the Window option box, enter a custom window function name.
SidelobeAttenuation
— Sidelobe attenuation of window60
(default) | real positive scalarThe window sidelobe attenuation, in decibels (dB). The value must be greater than or equal to
45
.
Tunable: Yes
To enable this property, set Window to 'Chebyshev'
or
'Kaiser'
.
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.
This option is only available when InputDomain is set to
Frequency
.
Open the Spectrum Settings. In the Frequency input options section, set Input units.
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
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' , |
Open the Spectrum Settings. In the Trace options section, set Units.
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.
To enable this property, set SpectrumUnits to
'dBFS'
.
Open the Spectrum Settings. In the Trace options section, set Full
scale to Auto
or enter a number.
FullScale
— Full scale1
(default) | positive scalarSpecify a real positive scalar for the dBFS
full scale.
Tunable: Yes
To enable this option set:
SpectrumUnits to
'dBFS'
FullScaleSource to
'Property'
Open the Spectrum Settings. In the Trace options section, set
Full scale to Auto
or enter a number.
SpectralAverages
— Number of spectral averages1
(default) | positive integerThe 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
To enable this property, set ViewType to 'Spectrum'
.
Open the Spectrum Settings. In the Trace options section, set Averages.
ReferenceLoad
— Reference load1
(default) | real positive scalarThe load the scope uses as a reference to compute power levels.
Tunable: Yes
Open the Spectrum Settings. In the Trace options section, set Reference load.
FrequencyOffset
— Frequency offset0
(default) | scalar | vectorScalar — 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
Open the Spectrum Settings. In the Trace options section, set Offset (Hz).
SpectrogramChannel
— Channel for which spectrogram is plotted1
(default) | positive scalar integerSpecify 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
To enable this property, set ViewType to
'Spectrogram'
or 'Spectrum and spectrogram'
.
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
To enable this property, set ViewType to
'Spectrogram'
or 'Spectrum and spectrogram'
.
Open the Spectrum Settings. In the Spectrogram options section, set Time res (s).
TimeResolution
— Time resolution0.001
(default) | positive scalarSpecify 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
To enable this property, set:
ViewType to 'Spectrogram'
or 'Spectrum and spectrogram'
TimeResolutionSource to
'Property
.
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
To enable this property, set ViewType to
'Spectrogram'
or 'Spectrum and spectrogram'
.
Open the Spectrum Settings. In the Spectrogram options section, set Time span (s).
TimeSpan
— Time span0.1
(default) | positive scalarSpecify 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
To enable this property, set:
ViewType to 'Spectrogram'
or 'Spectrum and spectrogram'
.
TimeSpanSource to
'Property'
.
Open the Spectrum Settings. In the Spectrogram options section, in the Time span (s) box, enter a number.
MeasurementChannel
— Channel for which measurements are obtained1
(default) | positive integerChannel 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
Click on Tools > Measurements and open the Trace Selection settings.
Data Types: double
SpectralMask
— Spectral mask linesSpectralMaskSpecification
objectSpecify
whether to display upper and lower spectral mask lines on a spectrum plot. This property uses
SpectralMaskSpecification
properties to enable and configure the spectral masks. The
SpectralMaskSpecification
properties are:
EnabledMasks
— Masks to enable, specified as a character vector or string. Valid values
are 'None'
, 'Upper'
, 'Lower'
, or 'Upper and
lower'
.
Default: 'None'
UpperMask
— Upper limit spectral mask, specified as a
scalar or two-column matrix. If UpperMask
is a scalar, the upper
limit mask uses the power value of the scalar for all frequency values applicable to
the Spectrum Analyzer. If UpperMask
is a matrix, the first column
contains the frequency values (Hz), which correspond to the
x-axis values. The second column contains the power values, which
correspond to the associated y-axis values. To apply offsets to
the power and frequency values, use the ReferenceLevel
and
MaskFrequencyOffset
property values, respectively.
Default: Inf
LowerMask
— Lower limit spectral mask, specified as a
scalar or two-column matrix. If LowerMask
is a scalar, the lower
limit mask uses the power value of the scalar for all frequency values applicable to
the Spectrum Analyzer. If LowerMask
is a matrix, the first column
contains the frequency values (Hz), which correspond to the
x-axis values. The second column contains the power values, which
correspond to the associated y-axis values. To apply offsets to
the power and frequency values, use the ReferenceLevel
and
MaskFrequencyOffset
property values, respectively.
Default: -Inf
ReferenceLevel
— Reference level for mask power values,
specified as either 'Custom'
or 'Spectrum
peak'
. When ReferenceLevel
is
'Custom'
, the CustomReferenceLevel
property value is used as the reference to the power values, in dBr, in the
UpperMask
and LowerMask
properties. When
ReferenceLevel
is 'Spectrum peak'
, the
peak value of the current spectrum of the SelectedChannel
is
used.
Default: 'Custom'
CustomReferenceLevel
— Custom reference level, specified
as a real value, in the same units as the power units. The reference level is the
value to which the power values in the UpperMask
and
LowerMask
properties are referenced. This property applies
when ReferenceLevel
is set to 'Custom'
. This
property uses the same units as the PowerUnits
property of the
Spectrum Analyzer.
Default: 0
SelectedChannel
— Input channel with peak spectrum to use as the mask reference level,
specified as an integer. This property applies when ReferenceLevel
is set to 'Spectrum
peak'
.
Default: 1
MaskFrequencyOffset
— Frequency offset, specified as a finite, numeric scalar. Frequency
offset is the amount of offset to apply to frequency values in the UpperMask
and
LowerMask
properties.
Default: 0
All SpectralMaskSpecification
properties are tunable.
Masks are overlaid on the spectrum. If the mask is green, the signal is passing the mask limitations. If the mask is red, the signal is failing the mask limits.
You can check the status of the spectral mask using any of these methods:
To modify the spectral mask and see the spectral mask status, in the scope
toolbar, select the spectral mask button, . In the Spectral Mask pane that
opens, you can modify the masks and see details about what percentage of the
time the mask is succeeding, which mask is failing, how many times the mask
failed, and which channels are causing the failure.
To get the current status of the spectral masks, call the function getSpectralMaskStatus
.
To perform an action every time the mask fails, use the
MaskTestFailed
event. To trigger a function when the
mask fails, create a listener to the MaskTestFailed
event
and define a callback function to trigger. For more details about using
events, see Events (MATLAB).
Tunable: Yes
Open the Spectral Mask pane and modify the Settings options.
PeakFinder
— Peak finder measurementPeakFinderSpecification
objectEnable 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
Open the Peak Finder pane () and modify the Settings
options.
CursorMeasurements
— Cursor measurementsCursorMeasurementsSpecification
objectEnable 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.
Open the Cursor Measurements pane () and modify the Settings
options.
ChannelMeasurements
— Channel measurementsChannelMeasurementsSpecification
objectEnable 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.
Open the Channel Measurements pane () and modify the Measurement
and Channel Settings options.
DistortionMeasurements
— Distortion measurementsDistortionMeasurementsSpecification
objectEnable 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.
Open the Distortion Measurements pane () and modify the Distortion
and Harmonics options.
CCDFMeasurements
— CCDF measurementsCCDFMeasurementsSpecification
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
–– Show a reference CCDF curve of additive white Gaussian noise. Set this property to true
to plot a reference CCDF curve.
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.
Open the CCDF Measurements pane () and enable the Plot Gaussian
reference option.
Name
— Window name'Spectrum Analyzer'
(default) | character vector | stringTitle of the scope window.
Tunable: Yes
Position
— Window position[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
To enable this property, set:
ViewType to
'Spectrum'
or 'Spectrum and spectrogram'
PlotNormalTrace to
true
Open the Style properties and set Plot type.
PlotNormalTrace
— Normal trace flagtrue
(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
To enable this property, set ViewType to
'Spectrum'
or 'Spectrum and spectrogram'
.
Open the Spectrum Settings. In the Trace options section, select Normal trace.
PlotMaxHoldTrace
— Max-hold trace flagfalse
(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
To enable this property, set ViewType to
'Spectrum'
or 'Spectrum and spectrogram'
.
Open the Spectrum Settings. In the Trace options section, select Max-hold trace.
PlotMinHoldTrace
— Min-hold trace flagfalse
(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
To enable this property, set ViewType to
'Spectrum'
or 'Spectrum and spectrogram'
.
Open the Spectrum Settings. In the Trace options section, select Min-hold trace.
ReducePlotRate
— Improve performance with reduced plot ratetrue
(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
Select Playback > Reduce plot rate to improve performance.
Title
— Display title''
(default) | character vector | stringSpecify the display title as a character vector or string. To use the input name as the display title, enter
'%<SignalLabel>'
.
Tunable: Yes
Open the Configuration Properties. On the Display tab, set Title.
Data Types: char
| string
YLabel
— Y-axis label''
(default) | character vector | stringSpecify 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
To enable this property, set ViewType to
'Spectrum'
or 'Spectrum and spectrogram'
.
Open the Configuration Properties. Set Y-label.
ShowLegend
— Show legendfalse
(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.
Tunable: Yes
Open the Configuration Properties. On the Display tab, select Show legend.
ChannelNames
— Channel namesSpecify 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
To see channel names, set ShowLegend
to true
.
On the legend, double-click the channel name.
ShowGrid
— Grid visibilityfalse
(default) | true
Set this property to true
to show gridlines on the plot.
Tunable: Yes
Open the Configuration Properties. On the Display tab, set Show grid.
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
To enable this property, set the ViewType property to
'Spectrum'
or 'Spectrum and spectrogram'
.
The units directly depend upon the SpectrumUnits property.
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]
To enable this property, set the ViewType property to
'Spectrogram'
or 'Spectrum and spectrogram'
.
The units directly depend upon the SpectrumUnits
property.
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.
Select Tools > Axes Scaling.
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
To enable this property, set ViewType to 'Spectrum and
spectrogram'
.
Open the Spectrum Settings. Set Axes layout.
For versions earlier than R2016b, use the step
function to run the System object algorithm. The arguments to
step
are the object you created, followed by
the arguments shown in this section.
For example, y = step(obj,x)
and y = obj(x)
perform equivalent operations.
scope(signal)
scope(signal1,signal2,...,signalN)
scope(
updates the spectrum of the signal in the spectrum
analyzer.signal
)
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.
signal
— Input signal or signals to visualizeSpecify 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
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)
getSpectralMaskStatus | Get test results of current spectral mask |
getSpectrumData | Save spectrum data shown in spectrum analyzer |
isNewDataReady | Check spectrum analyzer for new data |
getMeasurementsData | Get the current measurement data displayed on the spectrum analyzer |
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');
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)
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');
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/FrameSize,... '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
dsp.SpectrumAnalyzer
System objectCompute 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
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.
When you set the Method property to Welch
, the following algorithms
apply. The Spectrum Analyzer uses the RBW
or the Window Length
setting in the Spectrum Settings pane to determine the data window length. Then, it partitions
the input signal into a number of windowed data segments. Finally, Spectrum Analyzer uses the modified periodogram
method to compute spectral updates, averaging the windowed periodograms for each segment.
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.
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, Op, is the value of the Overlap % parameter in the Window Options pane of the Spectrum Settings pane. Fs is the sample rate of the input signal. Spectrum Analyzer shows sample rate in the Main Options pane of the Spectrum Settings pane.
When in RBW (Hz) mode, the window length required to compute one spectral update, Nwindow, is directly related to the resolution bandwidth and normalized effective noise bandwidth:
When in Window Length mode, the window length is used as specified.
The number of input samples required to compute one spectral update, Nsamples, is directly related to the window length and the amount of overlap by the following equation.
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 |
The normalized effective noise bandwidth, NENBW, is a window parameter determined by the window length, Nwindow, and the type of window used. If w(n) denotes the vector of Nwindow window coefficients, then NENBW is given by the following equation.
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:
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:
When in Window Length mode, you specify Nwindow and the resulting RBW is:
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.
The number of FFT points (Nfft) is independent of the window length (Nwindow). You can set them to different values if Nfft is greater than or equal to Nwindow.
When you set the Method property to Filter Bank
, the following
algorithms apply. 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.
Fs is the sample rate of the input signal. Spectrum Analyzer shows sample rate in the Main Options pane of the Spectrum Settings pane.
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:
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.
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:
When the number of frequency bands is manually specified, the resulting RBW is:
For more information about the filter bank algorithm, see Polyphase Implementation.
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.
When the PlotAsTwoSidedSpectrum property is
set to true
, the interval is hertz.
When the PlotAsTwoSidedSpectrum
property is set to false
, the
interval is hertz.
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.
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.
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.
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:
When the PlotAsTwoSidedSpectrum property is set to false, the frequency vector is:
The Occupied BW is calculated as follows.
Calculate the total power in the measured frequency range.
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
of the total power.
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
of the total power.
The bandwidth between the lower and upper power frequency values is the occupied bandwidth.
The frequency halfway between the lower and upper frequency values is the center frequency.
The Distortion Measurements are computed as follows.
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.
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.
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.
Once the DC, fundamental, and harmonic content is removed from the spectrum, the power of the remaining spectrum is examined for its sum (Premaining), peak value (Pmaxspur), and median value (Pestnoise).
The sum of all the removed bandwidth is computed as Wsum = W0 + W1 + W2 +...+ Wn.
The sum of powers of the second and higher-order harmonics are computed as Pharmonic = P2 + P3 + P4 +...+ Pn.
The sum of the noise power is estimated as:
Where dF is the absolute difference between frequency bins, and RBW is the resolution bandwidth of the window.
The metrics for SNR, THD, SINAD, and SFDR are then computed from the estimates.
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.
The DC component is ignored.
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.
Nth 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.
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):
TOIlower = PF1 + (PF2 - P(2F1-F2))/2
TOIupper = PF2 + (PF1 - P(2F2-F1))/2
TOI = + (TOIlower + TOIupper)/2
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).
아래 MATLAB 명령에 해당하는 링크를 클릭하셨습니다.
이 명령을 MATLAB 명령 창에 입력해 실행하십시오. 웹 브라우저에서는 MATLAB 명령을 지원하지 않습니다.
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