Stanford Research Systems SR785, Operating Manual And Programming Reference

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Window Menu 4-131
SR785 Dynamic 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.
If the Analyzer Configuration is set to Independent Channels, the two displays can have
different Windows. If the Analyzer Configuration is set to Dual Channel, both displays
use the same Window.
The following windows are available for FFT and Order 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.
The uniform window is also useful in the order analysis measurement group when the
harmonics of interest are integer multiples of the shaft rotation speed. This guarantees
that these order fall on exact bin frequencies.
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