Data AcQuisition And Real-Time Analysis
Scope - Spectrum - Spectrogram - Signal Generator
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Sound Card Spectrum Window Function Select

Controls: Spectrum Dialog >> Hann...Flat Top
Macro: SpectWind

This set of six radio buttons selects the window function to be applied when the Window button is active. Window functions reduce sidelobe leakage "skirts" at low levels for input signals that don't have an integer number of cycles in the 1024-point FFT data set.

When the button is not active, the effective function is often termed "rectangular", which simply means no window function at all.

Hann (often called Hanning) offers good resolution of spectral peaks and good rejection of sidelobe leakage skirts. It is the only window that has no peaks in its sidelobe skirts, making it suitable for Fundamental Track operation. A good choice for most uses. This is the initial default when Window is toggled on.

Hamming (not to be confused with Hanning) provides finer resolution of spectral peaks than Hann, but poorer rejection of sidelobe skirts at low levels.

Blackman gives peak resolution that is not as fine as Hann, but the response shape flares out less at lower levels and rejection of sidelobes is better.

Blackman Exact and Blackman-Harris are fairly similar to each other, but are both included for compatibility with existing data sets or user preferences. The peak resolution is similar to the regular Blackman, but the response shape flares out even less at lower levels.

Flat Top has the broadest peak resolution, but the best amplitude accuracy of any window here. It has a fairly straight-sided response shape at high and middle levels, then flares out like the Hamming at lower levels with similarly poor sidelobe rejection. Use this to measure spectral peaks where interference from adjacent frequencies is not a problem. It is the window of choice for swept frequency response measurements.

Macro Notes:

You may use either mnemonics or numeric values to set the Spectrun Window type. SpectWind=Hann or SpectWind=0 sets the window to Hann. Numeric values allow use of any numeric variable or expression. This allows you (for example) to save the current value to a variable, change it during a test, and then restore the initial value from the variable afterward.

The values and corresponding mnemonics are:

    0 = Hann = Hann
    1 = Hamm = Hamming
    2 = Bkmn = Blackman
    3 = BkEx = Blackman Exact
    4 = BkHr = Blackman-Harris
    5 = Flat = Flat Top

Note that if you use a mnemonic in an {\b IF} test, it must be in quotes: IF.SpectWind="Bkmn".

See also Spectrum Control Dialog


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