Oscilloscope Reference Selecting a spectral window
Hanning
Kaiser-Bessel
Blackman Harris
Gaussian
Tek Exponen
tial
Flattop2
The windows are listed in the order of their ability to resolve frequencies (resolution bandwidth). You
can easily observe the shape of a window in the frequency domain by feeding a sine wave into the
instrumen
t and setting the center frequency of the spectral analyzer to the same frequ ency. Then reduce the
resolution bandwidth to spread out the lobe horizontally. You can then select different window functions
and observe their shape.
Window attributes
In the time domain, windows are typically bell shaped and go to zero at the ends of the record. For
cases where you may be doing impulse response testing, the impulse should be centered at the zero
phase reference point (for most windows, this is the 50% position of the gate and 20% for the Tek
Expon
ential window).
Different window functions affect the resolution bandwidth of the spectral analyzer.
Various window shapes affect the scallop loss (see page 907).
The shape of the frequency domain lobe is determined by the window function. Some windows have
better resolution bandwidth, but they do so at the expense of side lobe attenuation and energy leakage
(see page 908) into adjacent bins. For example, a rectangular window typically spills energy into
man
y bins showing signals that don’t exist; but it has the best frequency resolution. If you are using
a window where leakage is occurring, you may want to use spectral gating to reduce the amplitude
errors when measuring a given frequency.
Scallop loss
Scallop loss is the difference between the actual magnitude and the computed magnitude of a signal
that is halfway between two frequency bins in the spectral output data. Scallop loss is only noticeable
when the spectral analyzer is not using zero-fill such as when it is set to full span.
If zero-fill is in use, frequency domain interpolation occurs and there is minimal scallop loss. Zero-fill
cannot be directly controlled; it is affected by changing settings of resolution bandwidth or gate width.
When in use, a minimum of 20% of the FFT input is always zero, effectively removing scallop loss
error by interpolating in the frequency domain.
NOTE. For most settings, descriptions of amplitude accuracy due to scallop loss (as discussed in
other publications) do not apply to this oscilloscope when used as a spectral analyzer because of
zero-fill. Full span is the most likely setting where scallop loss might occur.
DSA/DPO70000D, MSO/DPO/DSA70000C, DPO7000C, and MSO/DPO5000 Series 907
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