IntuiLink Waveform Editor

Figure 6 shows my filter's display screen,
from which you can select a filter and enter
its parameters. The "Filter Type" tab lets you
select from low-pass, high-pass, or movingaverage
filters. For the frequency scale in
most data-acquisition applications, you
should select "Show frequency as a fraction
of sample frequency." If you select "Show
frequency as a multiple of window
frequency," you'll see a scale from 0 to one
half the number of points in the waveform
that you want to filter. Remember, to meet
the Nyquist criterion, the "window" that you
apply to your waveform must have fewer
than half of the waveform's number of points
(Ref. 1).
Next, use the Parameters tab (Figure 7) to
select the cutoff frequency, transition
bandwidth, and ripple. In the "Frequency
Response" tab (Figure 8), the calculated
frequency response shows 100 dB of
attenuation, but as I explained, you won't get
that much. When you click OK in any tab, the
application will apply the filter to the data and
then plot the filtered waveform. You can save
the filtered waveform in a text file.
Figure 6. A digital-filter application program lets
you select filter type, frequency scale, and
destination.
If you choose to download the ActiveX
control, you can use it to filter data in your
own program. Listing 1 shows an
example in Visual Basic. In the sample code,
the variable data (an array of doubleprecision
variables contained in a variant)
represents the waveform that you want to
filter. The ActiveX control uses the function
GetWaveForm to process the data and then
return the filtered waveform into the variable
data. You can use the waveform in your
application or you can store it to a file. For a
more detailed example of how to use the
control, download the application program,
which also comes with Visual Basic source
code. T&MW
Figure 7. Enter a fraction of your sampling
frequency for values of cutoff frequency (fc/fs),
transition frequency (ft/fs), and percent ripple.
Listing 1
Dim data As Variant
Dim dummy As Double
Dim fir As agtFirFilter.CWaveForm
Dim result As Long
Set fir = New agtFirFilter.CWaveForm
With fir
.getWaveform data, dummy
result = .WaveFormResult
End With
Figure 8. The filter software calculates the number
of terms needed to meet your needs and then
plots the filter’s frequency response.
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