Stanford Research Systems SR780, Operating Manual And Programming Reference

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Window Menu 4-115
SR780 Network Signal Analyzer
Window Menu
The Window menu allows the user to choose the window function for FFT
measurements. See ‘Windowing’ in Chapter 2 for more information.
Window
Select the Window Function for the active display.
The two displays can have different Windows. This entry field can be linked to both
displays by using the [Link] key.
The following windows are available for FFT, Windowed Time, Cross Spectrum,
Transfer Function, Coherence and User Function measurements:
Command: FWIN (?) d {, i}
Uniform
The time record is used with no windowing. This window provides amplitude accuracy
only for exact bin frequencies and very poor frequency selectivity. This window is a poor
choice for continuous signals. In general, this window is only useful when looking at
transients which do not last the entire time record. The uniform window may also be used
with signals which are exactly periodic in the time record such as a chirp.
Hanning
The Hanning window is the most commonly used window. However, it has an amplitude
variation of about 1.5 dB for off-bin signals and provides only reasonable selectivity. Its
side-lobes are very high and broad for off-bin frequencies. As a result, the Hanning
window can limit the performance of the analyzer when looking at signals close together
in frequency and very different in amplitude. The Hanning window is most often used in
noise measurements since it has the lowest noise floor.
Flattop
The Flattop window has the best amplitude accuracy of any window. Its off-bin
amplitude variation is about 0.02 dB. However, the selectivity is worse. Unlike the other
windows, the Flattop window has a very wide pass band and very steep rolloff on either