Waveform analysis
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310User Manual 1326.1032.02 ─ 20
You can set up a FIR-filtered math waveform in the basic math editor, or in the
advanced editor. Both ways are described below in this chapter.
7.3.5.1 FIR filter in the math setup
The finite impulse response filter ("Operator" = FIR) is a filter to create filtered wave-
forms with lowpass, highpass, or bandpass. The filter requires additional settings.
●
"Type": defines whether the FIR filter is a highpass, lowpass, or bandpass/band-
stop (= "User defined") filter.
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"Cut-Off": sets the limit frequency for the highpass or lowpass FIR filter.
●
"Characteristics": relevant for lowpass filter. Defines whether it has a Gaussian or a
rectangular shape. The highpass is always Gaussian.
●
"Select filter file": relevant for bandpass/bandstop filter. Opens a file dialog to select
the file with the filter coefficients.
Cut-off frequency for lowpass FIR filter
The cut-off frequency depends on the horizontal resolution and the filter characteris-
tics. The frequency for the lowpass filter can only be set in this range:
f_g_3dB = (0.001 ... 0.2)* f_a_in for Gaussian FIR filter
f_g_3dB = (0,001 ... 0.4)* f_a_in for rectangular FIR filter
Where: f_g_3dB = cut-off frequency to be set for the lowpass filter, and f_a_in = recip-
rocal of the resolution, or sample rate.
Cut-off frequency for highpass FIR filter
To check limit frequency for the highpass filter, convert it to an equivalent lowpass fre-
quency:
f_LP = f_a_in/2 - f_HP
Where f_HP is the requested highpass limit frequency and f_LP the equivalent lowpass
frequency that has to comply with the limits given above.
Bandpass and bandstop FIR filter: CSV file
To define a bandpass or bandstop, you need a CSV file that contains the comma-sepa-
rated filter coefficients. The maximum number of filter coefficients is 2 048 000 taps. No
other parameters are allowed in the file.
To create the CSV file, we recommend using the Matlab Filter Design & Analysis tool.
In the tool, enter the filter type, filter order and filter frequencies. Make sure to set the
Mathematics
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