Frequently Asked Questions (FAQs)

The following are some FAQs about the National Instruments Digital Filter Design Toolkit for LabVIEW.

I'm new to digital filtering and in the process of learning about designing and applying digital filters. Is the Digital Filter Design Toolkit appropriate for me?

The toolkit is shipped with both interactive and programmatic tools for filter design. The interactive tools are excellent teaching tools because they provide immediate feedback after specifying desired filter characteristics.

The Toolkit includes interactive design tools for classical filter design, design through placement of poles and zeros, and for multirate filter design. These tools are LabVIEW Express VIs, which means that the results are immediately available for use within the LabVIEW graphical development environment.

Figure 1. With the classical design Express VI, you can specify filters by typing in passband/stopband frequencies and other parameters. Design results are shown immediately as magnitude response and pole/zero plots.

With the classical design Express VI, you can input desired transition frequencies, sampling rate, and other design parameters. As you modify parameters, the interface immediately shows the magnitude response and pole/zero locations on the Z-plane based on your selections. The pole-zero design tools allow interactive placement of poles and zeros on the Z-plane.

Figure 2. With the pole/zero placement Express VI, you can design a filter by placing poles and/or zeros directly on the complex plane. The results of the design appear immediately as the magnitude response.

With the pole/zero placement Express VI, you can interactively place and move poles and zeros on the Z-plane. Using the tool, you can enter complex value coordinates to specify exact values or click on a graph of the complex half plane to move or place them with the mouse. The magnitude response of the resulting filter updates immediately as you place or move poles/zeros.

In addition to the interactive Express VIs, the toolkit also is shipped with nearly 80 examples, any of which can prove useful to someone learning about digital filters.

Is the Digital Filter Design Toolkit appropriate for someone with experience designing and deploying digital filters?

Yes. Toolkit features that appeal to experienced designers include:

• Comprehensive set of supported filter types (more than 30) and filter topologies (23)
• State-of-the-art filter design tools (more than 25)
• Support for both floating- and fixed-point implementation with automatic code generation for fixed-point designs
• Work within LabVIEW to build, analyze, and deploy designs

Why is LabVIEW an excellent platform for digital filter design?

You can find many benefits to working within LabVIEW to design, analyze, and implement digital filters. The benefits for this particular application include many of the general reasons that make LabVIEW useful in any application. Though there is not room here to list them all, one example involves the extensible nature of the environment. LabVIEW is a programming environment in which you are free to build applications of a size/extent limited only by memory constraints. For filter design, this could mean anything from a simple one-shot application that quickly implements a single design to a larger application that iterates over filter specifications, design methods, and so on to create a filter that exactly fits your needs.

I know LabVIEW includes some digital filtering capability. Why do I need the Digital Filter Design Toolkit?

The toolkit extends LabVIEW with a much more comprehensive set of tools for filter design, analysis, and implementation. Many features are unique to the toolkit - examples include support for multiband design of arbitrary magnitude and/or phase filters, fixed-point quantization modeling, and automatic code generation for fixed-point filters.

Can I design and implement a lowpass/highpass/bandpass/bandstop filter with the Digital Filter Design Toolkit?

Yes. You can also use the included design algorithms to specify arbitrary magnitude and arbitrary magnitude/phase filter designs.

How can I deploy my newly designed digital filter? Can I deploy my filter on an embedded FPGA or DSP?

After you design a digital filter, you have several options for deploying it.

• For any filter that you design with the toolkit, you can export the resulting filter coefficients to plug in to existing code that implements the topology of your filter.
• You can apply any filter that you design with the toolkit to signals within the LabVIEW environment. You can apply filters to any signal source that is available within LabVIEW; examples include simulated signals, signals loaded from a file, and live signals acquired using a PXI- or PCI-based data acquisition card or through an externally connected instrument.
• For fixed-point, single-rate filter designs, the toolkit can automatically generate three types of code to implement your design. Supported code types include:
  • ANSI C
  • Integer LabVIEW code
  • LabVIEW FPGA code
• For fixed-point multirate filter designs, the toolkit can automatically generate the LabVIEW FPGA code that implements your design.

I know that an earlier release of the LabVIEW Signal Processing Toolset (Version 7.0) included a digital filter design component. Is the current Digital Filter Design Toolkit something different?

The older Digital Filter Design Toolkit was a component in the LabVIEW Signal Processing Toolset 7.0 and was also available as a stand-alone product. The new Digital Filter Design Toolkit is available as a stand-alone product and as a component of the upcoming LabVIEW Signal Processing Toolkit release.

The older digital filter design component consisted primarily of a stand-alone application (executable) for interactive classical digital filter design. The current toolkit provides LabVIEW VIs and Express VIs for interactive design and use in your custom VIs. Much of the functionality that was available with the older stand-alone application is now available through the new Express VIs.

As for features, the current toolkit is more powerful and comprehensive. Some examples include:

• New filter design algorithms that help you specify arbitrary magnitude and phase responses for a filter design
• More filter structures (for example, lattice autoregressive moving average (ARMA))
• New code generation capability for fixed-point designs to create integer LabVIEW, FPGA-optimized LabVIEW, and ANSI C code
• New ability to design both fixed- and floating-point multirate filters

How does a digital filter differ from an analog filter?

Digital filters act on discrete signal samples and are implemented algorithmically using software. Analog filters act on a continuously varying (analog) signal and are implemented with discrete electronic components such as capacitors, inductors, and op amps.

With their use of software rather than nonideal hardware, digital filters are more flexible, offer improved performance, and have better stability than analog filters.

What are some common applications of digital filters?

Digital filters are used in a variety of applications, some of which are presented in the following table:

Filter Type	Example Application
FIR bandpass	Extract a radio station from a broadband radio signal
Lowpass	Alias rejection - signal aliases (false frequency components) that result from sampling can be rejected by applying a combination of an analog filter prior to sampling and a digital filter after sampling. The requirements and therefore cost of implementation of the analog filter can be reduced through appropriate design of a digital filter.
Comb	Separate luminance and chrominance components from a composite TV signal.
Lowpass, highpass, bandpass, bandstop, many others	Noise reduction - if you have a band-limited signal (a signal with content of interest that resides in a well-defined frequency range) or a signal where the frequency content is well-defined, you can design a filter or combination of filters that can reduce out-of-band noise.
Rational resampling multirate	Resample a signal to increase or decrease the sampling rate of a signal by a factor that is a rational fraction.
N-stage multirate	To increase or decrease the sampling rate of a signal by a huge factor. A typical application is the delta-sigma ADC.
Group delay compensator (Allpass)	Reduce phase distortion that occurs after applying a filter with a nonconstant group delay.
Raised Cosine	For a digitally modulated signal, limit the occupied bandwidth and reduce intersymbol interference.
Arbitrary Magnitude and Phase	Compensate for nonideal hardware characteristics - for instance, the acquisition front end of oscilloscopes, data acquisition cards, and other devices that acquire dynamic signals will have some variation in their passband frequency responses. You can design a digital filter to calibrate out these variations by flattening the response and linearizing the phase.

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