Open Filter Visualization Tool


h = fvtool(...)


fvtool(b,a) opens FVTool and displays the magnitude response of the digital filter defined with numerator, b and denominator, a. Using FVTool you can display the phase response, group delay, impulse response, step response, pole-zero plot, and coefficients of the filter. You can export the displayed response to a file with File > Export.


If the input to fvtool is single precision, the magnitude response is calculated using single-precision arithmetic.

fvtool(sos) opens FVTool and displays the magnitude response of the digital filter defined by the L-by-6 matrix of second order sections


The rows of sos contain the numerator and denominator coefficients bik and aik of the cascade of second-order sections of H(z):


The number of sections, L, must be greater than or equal to 2. If the number of sections is less than 2, fvtool considers the input to be a numerator vector.

fvtool(d) opens FVTool and displays the magnitude response of a digital filter, d. Use designfilt to generate d based on frequency-response specifications.

fvtool(b1,a1,b2,a2,...,bN,aN) opens FVTool and displays the magnitude responses of multiple filters defined with numerators, b1, …, bN, and denominators, a1, ..., aN.

fvtool(sos1,sos2,...,sosN) opens FVTool and displays the magnitude responses of multiple filters defined with second order section matrices, sos1, sos2, ..., sosN.

fvtool(Hd) opens FVTool and displays the magnitude responses for the dfilt filter object, Hd, or the array of dfilt filter objects.

fvtool(Hd1,Hd2,...,HdN) opens FVTool and displays the magnitude responses of the filters in the dfilt objects Hd1, Hd2, ...HdN.

If you have the DSP System Toolbox™ product installed, you can also use fvtool(H) and fvtool(H1,H2,...) to analyze:

dsp.DigitalDownConverter and dsp.DigitalUpConverter System objects support FVTool. You must pass the 'Arithmetic' input to the FVTool when you call FVTool on these System objects.

When the input filter is a dfilt object, FVTool performs fixed-point analysis if the arithmetic property of the filter object is set to 'fixed'. However, for filter System objects, fvtool(H,'Arithmetic',ARITH,...) analyzes H, based on the arithmetic specified in the ARITH input.

ARITH can be one of 'double', 'single', or 'fixed'. The 'Arithmetic' input is only relevant for the analysis of filter System objects. The arithmetic setting ARITH, applies to all the filter System objects that you input to FVTool. When you specify 'double' or 'single', the function performs double- or single-precision analysis. When you specify 'fixed' , the arithmetic changes depending on the setting of the CoefficientDataType property and whether the System object is locked or unlocked.

 Details for Fixed-Point Arithmetic

If you do not specify the arithmetic for non-CIC structures, and the System object is in an unlocked state, the function uses double-precision arithmetic. If the System object is locked, the function performs analysis based on the locked input data type. CIC structures only support fixed-point arithmetic.

Analysis methods noisepsd and freqrespest have behavior restrictions in fvtool. To see the rules, click the links to these methods.

h = fvtool(...) returns a figure handle h. You can use this handle to interact with FVTool from the command line. See Controlling FVTool from the MATLAB Command Line.

FVTool has two toolbars.

  • An extended version of the MATLAB® plot editing toolbar. The following table shows the toolbar icons specific to FVTool.



    Restore default view. This view displays buffer regions around the data and shows only significant data. To see the response using standard MATLAB plotting, which shows all data values, use View > Full View.

    Toggle legend

    Toggle grid

    Link to Filter Designer (appears only if FVTool was started from Filter Designer)

    Toggle Add mode/Replace mode (appears only if FVTool was launched from Filter Designer)

  • Analysis toolbar with the following icons

    Magnitude response of the current filter. See freqz and zerophase for more information.

    To see the zero-phase response, right-click the y-axis label of the Magnitude plot and select Zero-phase from the context menu.

    Phase response of the current filter. See phasez for more information.

    Superimposes the magnitude response and the phase response of the current filter. See freqz for more information.

    Shows the group delay of the current filter. Group delay is the average delay of the filter as a function of frequency. See grpdelay for more information.

    Shows the phase delay of the current filter. Phase delay is the time delay the filter imposes on each component of the input signal. See phasedelay for more information.

    Impulse response of the current filter. The impulse response is the response of the filter to a impulse input. See impz for more information.

    Step response of the current filter. The step response is the response of the filter to a step input. See stepz for more information.

    Pole-zero plot, which shows the pole and zero locations of the current filter on the z-plane. See zplane for more information.

    Filter coefficients of the current filter, which depend on the filter structure (e.g., direct-form, lattice, etc.) in a text box. For SOS filters, each section is displayed as a separate filter.

    Detailed filter information.

Linking to Filter Designer

In the Filter Designer app, selecting View > Filter Visualization Tool or the Full View Analysis toolbar button when an analysis is displayed starts FVTool for the current filter. You can synchronize Filter Designer and FVTool with the toolbar button. Any changes made to the filter in Filter Designer are immediately reflected in FVTool.

Two link modes are provided via the toggle toolbar button  / :

  • Replace — removes the filter currently displayed in FVTool and inserts the new filter.

  • Add — retains the filter currently displayed in FVTool and adds the new filter to the display.

Modifying the Axes

You can change the x- or y-axis units by right-clicking the mouse on the axis label or by right-clicking on the plot and selecting Analysis Parameters. Available options for the axes units are as follows.

PlotX-Axis UnitsY-Axis Units


Normalized Frequency
Linear Frequency
Magnitude (dB)
Magnitude squared


Normalized Frequency
Linear Frequency
Continuous Phase

Magnitude and Phase

Normalized Frequency
Linear Frequency

(y-axis on left side)

Magnitude (dB)
Magnitude squared

(y-axis on right side)

Continuous Phase

Group Delay

Normalized Frequency
Linear Frequency

Phase Delay

Normalized Frequency
Linear Frequency

Impulse Response



Step Response




Real Part

Imaginary Part

Modifying the Plot

You can use any of the plot editing toolbar buttons to change the properties of your plot.

Analysis Parameters are parameters that apply to the displayed analyses. To display them, right-click in the plot area and select Analysis Parameters from the menu. (Note that you can access the menu only if the Edit Plot button is inactive.) The following analysis parameters are displayed. (If more than one response is displayed, parameters applicable to each plot are displayed.) Not all of these analysis fields are displayed for all types of plots:

  • Normalized Frequency — if checked, frequency is normalized between 0 and 1, or if not checked, frequency is in Hz

  • Frequency Scaley-axis scale (Linear or Log)

  • Frequency Range — range of the frequency axis or Specify freq. vector

  • Number of Points — number of samples used to compute the response

  • Frequency Vector — vector to use for plotting, if Specify freq. vector is selected in Frequency Range.

  • Magnitude Displayy-axis units (Magnitude, Magnitude (dB), Magnitude squared, or Zero-Phase)

  • Phase Unitsy-axis units (Degrees or Radians)

  • Phase Display — type of phase plot (Phase or Continuous Phase)

  • Group Delay Unitsy-axis units (Samples or Time)

  • Specify Length — length type of impulse or step response (Default or Specified)

  • Length — number of points to use for the impulse or step response

In addition to the above analysis parameters, you can change the plot type for Impulse and Step Response plots by right-clicking and selecting Line with Marker, Stem or Line from the context menu. You can change the x-axis units by right-clicking the x-axis label and selecting Samples or Time.

To save the displayed parameters as the default values to use when Filter Designer or FVTool is opened, click Save as default.

To restore the default values, click Restore original defaults.

Data tips display information about a particular point in the plot. See Interactively Explore Plotted Data (MATLAB) for information on data tips.

If you have the DSP System Toolbox software, FVTool displays a specification mask along with your designed filter on a magnitude plot.


To use View > Passband zoom, your filter must have been designed using fdesign or Filter Designer. Passband zoom is not provided for cascaded integrator-comb (CIC) filters because CICs do not have conventional passbands.

Overlaying a Response

You can overlay a second response on the plot by selecting Analysis > Overlay Analysis and selecting an available response. A second y-axis is added to the right side of the response plot. The Analysis Parameters dialog box shows parameters for the x-axis and both y-axes. See Display Analysis Parameters for a sample Analysis Parameters dialog box.

Controlling FVTool from the MATLAB Command Line

After you obtain the handle for FVTool, you can control some aspects of FVTool from the command line. In addition to the standard Handle Graphics® properties (see Handle Graphics in the MATLAB documentation), FVTool has the following properties:

  • 'Analysis' — displays the specified type of analysis plot. The following table lists all analysis types and how to invoke them. Note that the only analyses that use filter internals are magnitude response estimate and round-off noise power, which are available only with the DSP System Toolbox product.

    Analysis TypeAnalysis Option

    Magnitude plot


    Phase plot


    Magnitude and phase plot


    Group delay plot


    Phase delay plot


    Impulse response plot


    Step response plot


    Pole-zero plot


    Filter coefficients


    Filter information


    Magnitude response estimate

    (available only with the DSP System Toolbox product, see freqrespest for more information)


    Round-off noise power

    (available only with the DSP System Toolbox product, see noisepsd for more information)


  • 'Grid' — controls whether the grid is 'on' or 'off'

  • 'Legend' — controls whether the legend is 'on' or 'off'

  • 'Fs' — controls the sampling frequency of filters in FVTool. The sampling frequency vector must be of the same length as the number of filters or a scalar value. If it is a vector, each value is applied to its corresponding filter. If it is a scalar, the same value is applied to all filters.

  • SosViewSettings — (This option is available only if you have the DSP System Toolbox product.) For second-order sections filters, this controls how the filter is displayed. The SOSViewSettings property contains an object so you must use this syntax to set it: set(h.SOSViewSettings,'View',viewtype), where viewtype is one of the following:

    • 'Complete' — Displays the complete response of the overall filter

    • 'Individual' — Displays the response of each section separately

    • 'Cumulative' — Displays the response for each section accumulated with each prior section. If your filter has three sections, the first plot shows section one, the second plot shows the accumulation of sections one and two, and the third plot show the accumulation of all three sections.

      You can also define whether to use SecondaryScaling, which determines where the sections should be split. The secondary scaling points are the scaling locations between the recursive and the nonrecursive parts of the section. The default value is false, which does not use secondary scaling. To turn on secondary scaling, use this syntax: set(h.SOSViewSettings,'View','Cumulative',true)

    • 'UserDefined' — Allows you to define which sections to display and the order in which to display them. Enter a cell array where each section is represented by its index. If you enter one index, only that section is plotted. If you enter a range of indices, the combined response of that range of sections is plotted. For example, if your filter has four sections, entering {1:4} plots the combined response for all four sections, and entering {1,2,3,4} plots the response for each section individually.


You can change other properties of FVTool from the command line using the set function. Use get(h) to view property tags and current property settings.

You can use the following methods with the FVTool handle.

addfilter(h,filtobj) adds a new filter to FVTool. The new filter, filtobj, must be a dfilt filter object. You can specify the sampling frequency of the new filter with addfilter(h,filtobj,'Fs',10).

setfilter(h,filtobj) replaces the filter in FVTool with the filter specified in filtobj. You can set the sampling frequency as described above.

deletefilter(h, index) deletes the filter at the FVTool cell array index location.

legend(h,str1,str2,...) creates a legend in FVTool by associating str1 with filter 1, str2 with filter 2, etc. See legend in the MATLAB documentation for information.


collapse all

Display the magnitude response of a 6th-order elliptic filter. Specify a passband ripple of 3 dB, a stopband attenuation of 50 dB, a sample rate of 1 kHz, and a normalized passband edge of 300 Hz. Start FVTool from the command line.

[b,a] = ellip(6,3,50,300/500);

Display and analyze multiple FIR filters, starting FVTool from the command line.

b1 = firpm(20,[0 0.4 0.5 1],[1 1 0 0]); 
b2 = firpm(40,[0 0.4 0.5 1],[1 1 0 0]); 

Display the associated analysis parameters.

Start FVTool from the command line. Display the magnitude response of a 6th-order elliptic filter. Specify a passband ripple of 3 dB, a stopband attenuation of 50 dB, a sample rate of 1 kHz, and a normalized passband edge of 300 Hz.

[b,a] = ellip(6,3,50,300/500);
h = fvtool(b,a);

Display the phase response of the filter.

h.Analysis = 'phase';

Turn on the plot legend and add text.

legend(h,'Phase plot')

Specify a sample rate of 1 kHz. Display the two-sided centered response.

h.Fs = 1000;
h.FrequencyRange='[-Fs/2, Fs/2)';

View the all the properties of the plot. The properties specific to FVTool are at the end of the list.

                     Grid: 'on'
                   Legend: 'on'
          AnalysisToolbar: 'on'
            FigureToolbar: 'on'
               DesignMask: 'off'
          SOSViewSettings: [1x1 dspopts.sosview]
                       Fs: 1000
                 Alphamap: [1x64 double]
          CloseRequestFcn: 'closereq'
                    Color: [0.9400 0.9400 0.9400]
                 Colormap: [64x3 double]
              CurrentAxes: [1x1 Axes]
         CurrentCharacter: ''
            CurrentObject: [0x0 GraphicsPlaceholder]
             CurrentPoint: [0 0]
             DockControls: 'on'
                 FileName: ''
            IntegerHandle: 'on'
           InvertHardcopy: 'on'
              KeyPressFcn: ''
            KeyReleaseFcn: ''
                  MenuBar: 'none'
                     Name: 'Filter Visualization Tool - Phase Response'
                 NextPlot: 'new'
              NumberTitle: 'on'
               PaperUnits: 'inches'
         PaperOrientation: 'portrait'
            PaperPosition: [1.3350 3.3150 5.8300 4.3700]
        PaperPositionMode: 'auto'
                PaperSize: [8.5000 11]
                PaperType: 'usletter'
                  Pointer: 'arrow'
        PointerShapeCData: [16x16 double]
      PointerShapeHotSpot: [1 1]
                 Position: [346.5000 282.5000 583 437]
                 Renderer: 'opengl'
             RendererMode: 'auto'
                   Resize: 'on'
                ResizeFcn: ''
            SelectionType: 'normal'
                  ToolBar: 'auto'
                     Type: 'figure'
                    Units: 'pixels'
      WindowButtonDownFcn: ''
    WindowButtonMotionFcn: ''
        WindowButtonUpFcn: ''
        WindowKeyPressFcn: ''
      WindowKeyReleaseFcn: ''
     WindowScrollWheelFcn: ''
              WindowStyle: 'normal'
             BeingDeleted: 'off'
            ButtonDownFcn: ''
                 Children: [15x1 Graphics]
                 Clipping: 'on'
                CreateFcn: ''
                DeleteFcn: ''
               BusyAction: 'queue'
         HandleVisibility: 'on'
                  HitTest: 'on'
            Interruptible: 'on'
                   Parent: [1x1 Root]
                 Selected: 'off'
       SelectionHighlight: 'on'
                      Tag: 'filtervisualizationtool'
            UIContextMenu: [0x0 GraphicsPlaceholder]
                 UserData: []
                  Visible: 'on'
           FrequencyScale: 'Linear'
          FrequencyVector: [1x256 double]
            PolyphaseView: 'off'
        OverlayedAnalysis: ''
                 Analysis: 'phase'
           NumberofPoints: 8192
           FrequencyRange: '[-Fs/2, Fs/2)'
      NormalizedFrequency: 'off'
               PhaseUnits: 'Radians'
             PhaseDisplay: 'Phase'
            ShowReference: 'on'

Introduced before R2006a