Analog filter design is one of the most important areas of electronic design. Although analog filter design books featuring tested filter designs exist, filter design is often reserved for specialists because it requires advanced mathematical knowledge and understanding of the processes involved in the system affecting the filter.
Modern sampling and digital signal processing tools make it possible to replace analog filters with digital filters in applications that require flexibility and programmability. These applications include audio, telecommunications, geophysics, and medical monitoring.
Digital filters have the following advantages over their analog counterparts:
You can use digital filters in LabVIEW to control parameters such as filter order, cutoff frequencies, amount of ripple, and stopband attenuation.
The digital filter VIs described in this section follow the virtual instrument philosophy. The VIs handle all the design issues, computations, memory management, and actual data filtering internally, transparent to the user. You do not have to be an expert in digital filters or digital filter theory to process the data.
The following discussion of sampling theory is intended to give you a better understanding of the filter parameters and how they relate to the input parameters.
The sampling theorem states that you can reconstruct a continuous-time signal from discrete, equally spaced samples if the sampling frequency is at least twice that of the highest frequency in the time signal. Assume you can sample the time signal of interest at t equally spaced intervals without losing information. The
t parameter is the sampling interval.
You can obtain the sampling rate or sampling frequency fs from the sampling interval
which means that, according to the sampling theorem, the highest frequency that the digital system can process is
The highest frequency the system can process is known as the Nyquist frequency. This also applies to digital filters. For example, if your sampling interval is
t = 0.001 sec,
then the sampling frequency is
fs = 1,000 Hz,
and the highest frequency that the system can process is
The following types of filtering operations are based upon filter design techniques: