Spectrum Analyzer
Display frequency spectrum
- Library:
DSP System Toolbox / Sinks
DSP System Toolbox HDL Support / Sinks
Description
The Spectrum Analyzer block, referred to here as the scope, displays the frequency spectra of signals.
You can use the Spectrum Analyzer block in models running in Normal or Accelerator simulation modes. You can also use the Spectrum Analyzer block in models running in Rapid Accelerator or External simulation modes, with some limitations.
You can use the Spectrum Analyzer block inside all subsystems and conditional subsystems. Conditional subsystems include enabled subsystems, triggered subsystems, enabled and triggered subsystems, and function-call subsystems. See Conditionally Executed Subsystems Overview (Simulink) for more information.
Measurements
Cursors — Measure signal values using vertical and horizontal cursors.
Peak Finder — Find maxima, showing the x-axis values at which they occur.
Channel Measurements — Measure the occupied bandwidth or adjacent channel power ratio (ACPR).
Distortion Measurements — Measure harmonic distortion and intermodulation distortion.
CCDF Measurements — Measure the complimentary cumulative distribution function. CCDF measurements show the probability of a signal’s instantaneous power being a specified level above the signal’s average power.
Spectral Masks — Visualize spectrum limits and compare spectrum values to specification values.
Programmatic Control
You can configure and display Spectrum Analyzer settings from the command line
with the spbscopes.SpectrumAnalyzerConfiguration
object.
Ports
Input
Parameters
Spectrum Settings
The Spectrum Settings pane appears at the right side of the
Spectrum Analyzer window. These settings control how the spectrum is calculated. To
show the Spectrum Settings, in the Spectrum Analyzer menu, select View > Spectrum Settings or use the 
button in the toolbar.
Main options
Input domain — 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.
Programmatic Use
See InputDomain.
Type — Type of spectrum to display
Power (default) | Power density | RMS
Power — Spectrum Analyzer shows the power spectrum.
Power density — Spectrum Analyzer shows the power spectral density.
The power spectral density is the magnitude of the spectrum normalized to a bandwidth of
1 hertz.
RMS — Spectrum Analyzer shows the root mean squared
spectrum.
Tunable: Yes
Dependency
To use this parameter, set Input domain to Time.
Programmatic Use
See SpectrumType.
View — Spectrum view
Spectrum (default) | Spectrogram | Spectrum and spectrogram
Spectrum — Spectrum Analyzer shows the
spectrum.
Spectrogram — Spectrum Analyzer shows the
spectrogram, which displays frequency content over time. The most recent
spectrogram update is at the bottom of the display, and time scrolls
from the bottom to the top of the display.
Spectrum and spectrogram — Spectrum Analyzer
shows both the spectrum and spectrogram.
Tunable: Yes
Programmatic Use
See ViewType.
Sample rate — Sample rate of the input signal in hertz
Inherited (default) | positive scalar
Sample rate of the input signal in hertz, specified as either
Inheritedto use the same sample rate as the input signal.Positive scalar. The specified sample rate must be at least twice the input signal sample rate. Otherwise, you might see unexpected behavior in your signal visualization due to aliasing.
Programmatic Use
See SampleRate.
Method — Spectrum estimation method
Welch (default) | Filter Bank
Select Welch or Filter
Bank as the spectrum estimation method. For more
details about the two spectrum estimation algorithms, see Algorithms.
Tunable: No
Dependency
To use this parameter, set Input domain
to Time.
Programmatic Use
See Method.
Full frequency span — Use entire Nyquist frequency interval
on (default) | off
Select this check box to compute and plot the spectrum over the entire Nyquist frequency interval.
Tunable: Yes
Dependency
To use this parameter, set Input domain
to Time.
Programmatic Use
See FrequencySpan.
Span (Hz) — Frequency span in hertz
10e3 (default) | real positive scalar
Specify the frequency span in hertz. Use this parameter with the
CF
(Hz) parameter to define the frequency span around a
center frequency. This parameter defines the range of values shown on
the Frequency axis in the Spectrum Analyzer window.
Tunable: Yes
Dependencies
To use this parameter, you must:
Set Input domain to
Time.Clear the Full frequency span check box.
Set the Span (Hz)/Fstart (Hz) drop-down to
Span (Hz).
Programmatic Use
See FrequencySpan and Span.
CF (Hz) — Center frequency in hertz
0 (default) | scalar
Specify the center frequency, in hertz. Use this parameter with the Span (Hz) parameter to define the frequency span around a center frequency. This parameter defines the value shown at the middle point of the Frequency axis on the Spectrum Analyzer window.
Tunable: Yes
Dependencies
To use this parameter, you must:
Set Input domain to
Time.Clear the Full frequency span check box.
Set the Span (Hz)/Fstart (Hz) drop-down to Span (Hz).
Programmatic Use
See CenterFrequency.
FStart (Hz) — Start frequency in hertz
-5e3 (default) | scalar
Specify the start frequency in hertz. Use this parameter with the FStop (Hz) parameter to define the range of frequency-axis values using start frequency and stop frequency. This parameter defines the value shown at the leftmost side of the Frequency axis on the Spectrum Analyzer window.
Tunable: Yes
Dependencies
To use this parameter, you must:
Set Input domain to
Time.Clear the Full frequency span check box.
Set the Span (Hz)/FStart (Hz) drop-down to
FStart (Hz).
Programmatic Use
See StartFrequency.
FStop (Hz) — Stop frequency in hertz
5e3 (default) | scalar
Specify the stop frequency, in hertz. Use this parameter with the FStart (Hz) parameter to define the range of Frequency axis values. This parameter defines the value shown at the rightmost side of the Frequency axis on the Spectrum Analyzer window.
Tunable: Yes
Dependencies
To use this parameter, you must:
Set Input domain to
Time.Clear the Full frequency span check box.
Set the Span (Hz)/FStart (Hz) drop-down to FStart (Hz).
Programmatic Use
See StopFrequency.
Frequency (Hz) — Frequency vector
Auto (default) | Input port | monotonically increasing vector
Set the frequency vector which determines the x-axis of the display.
Auto— The frequency vector is calculated from the length of the input. See Frequency Vector.Input port— When selected, an input port appears on the block for the frequency vector input.Custom vector — Enter a custom vector as the frequency vector. The length of the custom vector must be equal to the frame size of the input signal.
Tunable: No
Dependency
To use this parameter, set Input domain
to Frequency.
Programmatic Use
See FrequencyVector.
RBW (Hz) — Resolution bandwidth
Auto (default) | Input port | positive scalar
The resolution bandwidth in hertz. This parameter defines the smallest
positive frequency that can be resolved. By default, this parameter is
set to Auto. In this case, the Spectrum
Analyzer determines the appropriate value to ensure that there are 1024
RBW intervals over the specified frequency span.
If you set this parameter to a numeric value, the value must allow at least two RBW intervals over the specified frequency span. In other words, the ratio of the overall frequency span to RBW must be greater than two:
For frequency input only, you can use an input port to set the RBW value.
Tunable: Yes
Dependency
To use this parameter, set either:
Input domain to
Timeand the RBW (Hz)/Window length/Number of frequency bands drop-down toRBW (Hz).Input domain to
Frequency.
Programmatic Use
See RBW.
Input units — Units of frequency input
Auto (default) | dBm | 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.
Tunable: No
Dependency
This option is only available when Input domain
is set to Frequency.
Programmatic Use
See InputUnits.
Window length — Length of window in samples
1024 (default) | integer greater than 2
The length of the window, in samples. The window length used to control the frequency resolution and compute the spectral estimates. The window length must be an integer greater than 2.
Dependencies
To use this parameter, set:
Method to
WelchSet the RBW (Hz)/Window length/Number of frequency bands drop-down to
Window Length
Dependency
To use this parameter, set Input domain
to Time.
Programmatic Use
See WindowLength.
Number of frequency bands — FFT length
Auto (default) | positive integer
Specify the fast Fourier transform (FFT) length to control the number
of frequency bands. If the value is Auto, the
Spectrum Analyzer uses the entire frame size to estimate the spectrum.
If you specify the number of frequency bands, you set the input buffer
size.
Dependencies
To use this parameter, set:
Method to
Filter BankSet the RBW (Hz)/Window length/Number of frequency bands drop-down to
Number of frequency bands
Programmatic Use
See FFTLength
Taps per band — Number of filter taps
12 (default) | positive even integer
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 Taps Per Band + FFT Length.
Dependency
To use this parameter, you must set the RBW (Hz)/Window length/Number of frequency bands drop-down to Number of frequency bands.
Programmatic Use
See NumTapsPerBand.
NFFT — Number of FFT points
Auto (default) | positive integer
Specify the length of the FFT that Spectrum Analyzer uses to compute
spectral estimates. Acceptable options are
Auto or a positive integer.
The NFFT value must be greater than or equal to
the value of the Window length parameter. By
default, when NFFT is set to
Auto, the Spectrum Analyzer sets
NFFT equal to the value of Window
length. When in RBW mode, the specified RBW value is used
to calculate an FFT length that equals the window length.
When this parameter is set to a positive integer, this parameter is
equivalent to the n parameter of the fft function.
Dependencies
To use this parameter, you must set the RBW (Hz)/Window length/Number of frequency bands drop-down to Window length.
Programmatic Use
See FFTLength.
Samples/update — Required number of input samples
positive scalar
This property is read-only.
The number of input samples required to compute one spectral update. You cannot modify this parameter; it is shown in the spectrum analyzer for informational purposes only. This parameter is directly related to RBW (Hz)/Window length/Number of frequency bands. For more details, see Algorithms.
If the input does not have enough samples to achieve the resolution bandwidth that you specify, Spectrum Analyzer produces a message on the display.
Spectrogram Settings
Channel — Spectrogram channel
channel name
Select the signal channel for which the spectrogram settings apply.
Dependencies
To use this option, set View
to Spectrogram or Spectrum
and spectrogram.
Programmatic Use
See SpectrogramChannel.
Time res. (s) — Time resolution in seconds
Auto (default) | positive number
Time resolution is the amount of data, in seconds, used to compute a spectrogram line. The minimum attainable resolution is the amount of time it takes to compute a single spectral estimate. The tooltip displays the minimum attainable resolution given the current settings.
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 use this option, set View
to Spectrogram or Spectrum
and spectrogram.
Programmatic Use
See TimeResolution.
Time span — Time span in seconds
Auto (default) | positive scalar
The time span over which the Spectrum Analyzer displays the
spectrogram specified in seconds. The time span is the product of the
desired number of spectral lines and the time resolution. The tooltip
displays the minimum allowable time span, given the current settings. If
the time span is set to Auto, 100 spectral
lines are used.
Tunable: Yes
Dependency
To use this option, set View
to Spectrogram or Spectrum
and spectrogram.
Programmatic Use
See TimeSpan.
Window Options
Overlap (%) — Segment overlap percentage
0 (default) | scalar between 0 and 100
This parameter defines the amount of overlap between the previous and current buffered data segments. 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
Programmatic Use
See OverlapPercent.
Window — Windowing method
Hann (default) | Rectangular | Blackman-Harris | Chebyshev | Flat Top | Hamming | Kaiser | custom window function name
The windowing method to apply to the spectrum. Windowing is used to control the effect of sidelobes in spectral estimation. The window you specify affects the window length required to achieve a resolution bandwidth and the required number of samples per update. For more information about windowing, see Windows (Signal Processing Toolbox).
Tunable: Yes
Programmatic Use
See Window.
Attenuation — Sidelobe attenuation
60 (default) | scalar greater than or equal to 45
The sidelobe attenuation in decibels (dB). The value must be greater
than or equal to 45.
Dependency
This parameter applies only when you set the
Window parameter to
Chebyshev or
Kaiser.
Programmatic Use
See SidelobeAttenuation.
NENBW — Normalized effective noise bandwidth
scalar
This property is read-only.
The normalized effective noise bandwidth of the window. You cannot modify this parameter; it is shown for informational purposes only. This parameter is a measure of the noise performance of the window. The value is the width of a rectangular filter that accumulates the same noise power with the same peak power gain.
The rectangular window has the smallest NENBW, with a value of 1. All other windows have a larger NENBW value. For example, the Hann window has an NENBW value of approximately 1.5.
Trace Options
Units — Spectrum units
dBm (default) | dBW | Watts | Vrms | dBV | dBFS
The units of the spectrum. The available values depend on the value of the Type parameter.
Tunable: Yes
Programmatic Use
See SpectrumUnits.
Full scale — Full scale for dBFS units
Auto (default) | positive real scalar
The full scale used for the decibel full scale (dBFS) units. By default, the Spectrum Analyzer uses the entire spectrum scale. Specify a positive real scalar for the dBFS full scale.
Tunable: Yes
Dependencies
To enable this parameter, set:
Input domain to
TimeUnits to
dBFS
Programmatic Use
See FullScale.
Averaging method — Smoothing method
Running (default) | Exponential
Specify the smoothing method as:
Running— Running average of the last n samples. Use theAveragesproperty to specify n.Exponential— Weighted average of samples. Use theForgetting factorproperty to specify the weighted forgetting factor.
For more information about the averaging methods, see Averaging Method.
Programmatic Use
See AveragingMethod.
Averages — Number of spectral averages
1 (default) | positive integer
Specify the number of spectral averages as a positive integer. The spectrum analyzer computes the current power spectrum estimate by computing a running average of the last N power spectrum estimates. This parameter defines the number of spectral averages, N.
Dependencies
This parameter applies only when:
View is
SpectrumorSpectrum and spectrogram.Averaging method is
Running.
Programmatic Use
See SpectralAverages.
Forgetting factor — 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 parameter applies only when the Averaging method is
Exponential.
Programmatic Use
See ForgettingFactor.
Reference load — Reference load
1 (default) | positive real scalar
The reference load in ohms that the Spectrum Analyzer uses as a reference to compute power values.
Dependency
To use this parameter, set Input domain to Time.
Programmatic Use
See ReferenceLoad.
Scale — Scale of frequency axis
Linear (default) | Logarithmic
Choose a linear or logarithm scale for the frequency axis. When the frequency span contains negative frequency values, you cannot choose the logarithmic option.
Programmatic Use
See FrequencyScale.
Offset — Constant frequency offset
0 (default) | scalar
The constant frequency offset to apply to the entire spectrum, or a vector of frequencies to apply to each spectrum for multiple inputs. The offset parameter is added to the values on the Frequency axis in the Spectrum Analyzer window. This parameter is not used in any spectral computations. You must take the parameter into consideration when you set the Span (Hz) and CF (Hz) parameters to ensure that the frequency span is within the Nyquist frequency interval.
Dependency
To use this parameter, set Input domain to Time.
Programmatic Use
See FrequencyOffset.
Normal trace — Normal trace view
on (default) | off
When this check box is selected, the Spectrum Analyzer calculates and plots the power spectrum or power spectrum density. Spectrum Analyzer performs a smoothing operation by averaging several spectral estimates.
Dependencies
To clear this check box, you must first select either the Max hold trace
or the Min hold trace
parameter. This parameter applies only when View
is Spectrum or Spectrum and
spectrogram.
Programmatic Use
See PlotNormalTrace.
Max hold trace — Maximum hold trace view
off (default) | on
Select this check box to enable Spectrum Analyzer to plot the maximum spectral values of all the estimates obtained.
Dependency
This parameter applies only when View
is Spectrum or Spectrum and
spectrogram.
Programmatic Use
See PlotMaxHoldTrace.
Min hold trace — Minimum hold trace view
off (default) | on
Select this check box to enable Spectrum Analyzer to plot the minimum spectral values of all the estimates obtained.
Dependency
This parameter applies only when View
is Spectrum or Spectrum and
spectrogram.
Programmatic Use
See PlotMinHoldTrace.
Two-sided spectrum — Enable two-sided spectrum view
off (default) | on
Select this check box to enable a two-sided spectrum view. In this view, both negative and positive frequencies are shown. If you clear this check box, Spectrum Analyzer shows a one-sided spectrum with only positive frequencies. Spectrum Analyzer requires that this parameter is selected when the input signal is complex-valued.
Programmatic Use
Configuration Properties
The Configuration Properties dialog box controls visual
aspects of the Spectrum Analyzer. To open the Configuration Properties, in the
Spectrum Analyzer menu, select View > Configuration Properties or select the 
button in the toolbar drop-down.
Title — Display title
character vector | string
Specify the display title. Enter %<SignalLabel> to use the signal labels in the Simulink model as the axes titles.
Tunable: Yes
Programmatic Use
See Title.
Show legend — Display signal legend
off (default) | on
Show signal legend. The names listed in the legend are the signal names from the model. For signals with multiple channels, a channel index is appended after the signal name. Continuous signals have straight lines before their names and discrete signals have step-shaped lines.
From the legend, you can control which signals are visible. This control is equivalent to changing the visibility in the Style parameters. 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, which hides all other signals. 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.
Dependency
To enable this parameter, set View to Spectrum or Spectrum and spectrogram.
Programmatic Use
See ShowLegend.
Show grid — Show internal grid lines
on (default) | off
Y-limits (minimum) — Y-axis minimum
-80 (default) | scalar
Y-limits (maximum) — Y-axis maximum
20 (default) | scalar
Y-label — Y-axis label
character vector | string
To display signal units, add (%<SignalUnits>) to the label. At the beginning of a simulation, Simulink replaces (%SignalUnits) with the units associated with the signals. For example, if you have a signal for velocity with units of m/s enter
Velocity (%<SignalUnits>)
Programmatic Use
See YLabel.
Color map — Spectrogram colormap
jet(256) (default) | hot(256) | bone(256) | cool(256) | copper(256) | gray(256) | parula(256) | 3-column matrix
Color-limits (minimum) — Spectrogram minimum
-80 (default) | scalar
Specify the signal power for the minimum color value of the spectrogram.
Tunable: Yes
Dependency
To use this parameter, set View
to Spectrogram or Spectrum
and spectrogram.
Programmatic Use
See ColorLimits.
Color-limits (maximum) — Spectrogram maximum
20 (default) | scalar
Specify the signal power for the maximum color value of the spectrogram.
Tunable: Yes
Dependency
To use this parameter, set View
to Spectrogram or Spectrum
and spectrogram.
Programmatic Use
See ColorLimits.
Style
The Style dialog box controls how to Spectrum Analyzer appears. To open
the Style properties, in the Spectrum Analyzer
menu, select View > Style or select the 
button in the toolbar
drop-down.
Figure color — Window background
gray (default) | color picker
Specify the color that you want to apply to the background of the scope figure.
Plot type — Plot type
Line (default) | Stem
Axes colors — Axes background color
black (default) | color picker
Specify the color that you want to apply to the background of the axes.
Properties for line — Channel for visual property settings
channel names
Specify the channel for which you want to modify the visibility, line properties, and marker properties.
Visible — Channel visibility
on (default) | off
Specify whether the selected channel is visible. If you clear this check box, the line disappears. You can also change signal visibility using the scope legend.
Line — Line style
line, 0.5, yellow (default)
Specify the line style, line width, and line color for the selected channel.
Marker — Data point markers
none (default)
Specify marks for the selected channel to show at its data points. This parameter is similar to the 'Marker' property for plots. You can choose any of the marker symbols from the drop-down.
Axes Scaling
The Axes Scaling dialog box controls the axes limits of the Spectrum Analyzer. To open the Axes Scaling properties, in the Spectrum Analyzer menu, select Tools > Axes Scaling > Axes Scaling Properties.
Axes scaling/Color scaling — Automatic axes scaling
Auto (default) | Manual | After N Updates
Specify when the scope automatically scales the y-axis. If the
spectrogram is displayed, specify when the scope automatically scales
the color axis. By default, this parameter is set to
Auto, and the scope does not shrink the
y-axis limits when scaling the axes or color. You can select one of the
following options:
Auto— The scope scales the axes or color as needed, both during and after simulation. Selecting this option shows the Do not allow Y-axis limits to shrink or Do not allow color limits to shrink.Manual— When you select this option, the scope does not automatically scale the axes or color. You can manually scale the axes or color in any of the following ways:Select Tools > Scaling Properties.
Press one of the Scale Axis Limits toolbar buttons.
When the scope figure is the active window, press Ctrl+A.
After N Updates— Selecting this option causes the scope to scale the axes or color after a specified number of updates. This option is useful, and most efficient, when your frequency signal values quickly reach steady-state after a short period. Selecting this option shows the Number of updates edit box where you can modify the number of updates to wait before scaling.
Tunable: Yes
Programmatic Use
See AxesScaling.
Do not allow Y-axis/color limits to shrink — Axes scaling limits
on (default) | off
When you select this parameter, the y-axis is allowed to grow during axes scaling operations. If the spectrogram is displayed, selecting this parameter allows the color limits to grow during axis scaling. If you clear this check box, the y-axis or color limits can shrink during axes scaling operations.
Dependency
This parameter appears only when you select
Auto for the Axis
scaling or Color scaling
parameter. When you set the Axes scaling or
Color scaling parameter to
Manual or After N
Updates, the y-axis or color
limits can shrink.
Number of updates — Number of updates before scaling
10 (default) | positive number
The number of updates after which the axes scale, specified as a positive integer. If the spectrogram is displayed, this parameter specifies the number of updates after which the color axes scales.
Tunable: Yes
Dependency
This parameter appears only when you set Axes scaling/Color scaling to After N Updates.
Programmatic Use
Scale limits at stop — Scale axes at stop
off (default) | on
Select this check box to scale the axes when the simulation stops. If the spectrogram is displayed, select this check box to scale the color when the simulation stops. The y-axis is always scaled. The x-axis limits are only scaled if you also select the Scale X-axis limits check box.
Data range (%) — Percent of axes
100 (default) | number in the range [1,100]
Set the percentage of the axis that the scope uses to display the data when scaling the axes. If the spectrogram is displayed, set the percentage of the power values range within the colormap. Valid values are from 1 through 100. For example, if you set this parameter to 100, the scope scales the axis limits such that your data uses the entire axis range. If you then set this parameter to 30, the scope increases the y-axis or color range such that your data uses only 30% of the axis range.
Tunable: Yes
Align — Alignment along axes
Center (default) | Bottom | Top | Left | Right
Specify where the scope aligns your data along the axis when it scales the axes. If the spectrogram is displayed, specify where the scope aligns your data along the axis when it scales the color. If you are using CCDF Measurements, the x axis is also configurable.
Tunable: Yes
Model Examples
Block Characteristics
Data Types |
|
Direct Feedthrough |
|
Multidimensional Signals |
|
Variable-Size Signals |
|
Zero-Crossing Detection |
|
Algorithms
Welch's Method
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) toAuto, 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.
Note
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.
Filter Bank
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 toAuto, 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.
Harmonic Measurements
The harmonic distortion measurements use the spectrum trace shown in the display as the input to the measurements. The default
Hannwindow 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