LabVIEW DSP allows the user to implement
standard IIR digital filters designed using a filter designsupplemental program, called Digital Filter Design Toolkit. The DFD
toolkit is an add-on module to LabVIEW (not LabVIEW Embedded). Thismodule allows the user to design complex filters, save them and
implement them using LabVIEW DSP.
New Block Diagram with DFD Filter
Go to the Functions palette and add the DFD
Filter Express VI to your existing Block Diagram: Functions»SignalProcessing»Filters»DFD Filter
Wire the Express VI as shown in Figure 6.
The output of the Add function is the input “Signal” of the ExpressVI and the “Filtered Signal” goes into the Analog Output. Set the
function generator’s signal to a frequency of 500 Hz.
Follow the steps below to implement alowpass filter that will be implemented on the DSP. Launch LabVIEW
by going to Start»All Programs»National Instruments»LabVIEW7.1»LabVIEW. Let’s try to understand what is happening. We first
launched LabVIEW Embedded Edition which allowed us to target theDSP hardware and run our program. Now, we are launching LabVIEW,
not LabVIEW Embedded Edition. The program you built earlier shouldstill be open. We will use the Digital Filter Design Toolkit
available in the LabVIEW environment to design the filter.
Create a blank VI (New»Blank VI).
Switch to the Block Diagram and drop a DFD Classical Filter
Design on the Block Diagram: It is in Functions»AllFunctions»Digital Filter Design»Filter Design»DFD Classical Filter
Design.
In the DFD Classical Filter Design Express VI, enter the
following settings:
Filter Type: Lowpass
Sampling Frequency: 16000 Hz
Passband Edge Frequency: 1000 Hz
Passband ripple: 3 dB (standard for Butterworth
filters)
Stopband Edge Frequency: 1500 Hz
Minimum stopband attenuation 20 dB
Design Method: Butterworth
Leave the other settings to their default values.
DFD Classical Filter Design Configuration Window
Check the graph to the left to make sure the
filter response appears as expected and click “OK” to set theconfigurations.
Some filter parameters are described
below:
Sampling Frequency – self-explanatory. Make sure to set it to
the sampling frequency that you plan on using!
Center Frequency – (not used for lowpass filters). Set this
to the middle frequency of the passband for Bandpass/Highpassfilters or the middle frequency of the stopband for Bandstop
filters.
Bandwidth – Set this to the frequency width of the passband
(stopband for Bandstop filters). Recall that Lowpass filters have alower passband frequency of 0 Hz and that Highpass filters has an
upper passband frequency of Fs/2.
Stopband Attenuation – Set this to the desired attenuation in
dB at the edge of the stopband. For Bandstop filters, thisparameter must be negative (because the stopband is really the
passband). For all other filter types, this parameter must bepositive.
Passband Ripple – Set this to the desired attenuation (in dB)
at the edge of the Passband. For Butterworth filters, the standard(but not required) value to use is 3 dB.
Filter Order N – Set this to the desired filter order.
However, after you press the Apply button, the program calculateswhich filter order you need to meet the given specs. For most
applications, then, you can put any number here and then change itto the necessary value when instructed. There may be some
situations, however, when you want to force the filter order to besome set value. In this case, the desired specifications may not be
met.
Regarding calculation of filter order N: This program uses the same design equations that we use in EE 453. For IIR filters, the program automatically does prewarping.
Build the following Block Diagram to save
the filter settings: Drop a “DFD Save to File” VI on the BlockDiagram. This VI is located at Functions»All Functions»Digital
Filter Design»Utilities»DFD Save to File. Wire the Block Diagram asshown in Figure 6.
Filter Design
Run the filter design VI you just built.
When prompted to save your design, navigate to your folder and saveyour filter under any chosen name. Save and close this VI.
