Stanford Research Systems SR780, Operating Manual And Programming Reference

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4-116 Window Menu
SR780 Network Signal Analyzer
side. Thus, signals appear wide but do not leak across the whole spectrum. The Flattop
window is the best window to use for accurate amplitude measurements.
BMH
The BMH window has reasonable off-bin amplitude accuracy (about 0.8 dB) and much
lower side-lobes than the Hanning window. Thus, it has very good selectivity and very
little broadening of non-bin frequencies. The BMH window is a good window to use for
measurements requiring a large dynamic range.
Kaiser
The Kaiser window has the lowest side-lobes and least broadening for non-bin
frequencies. This makes this window the best for selectivity. The off-bin amplitude
variation is the same as the BMH (about 0.8 dB). The Kaiser window is the best window
to use for measurements requiring a large dynamic range.
Force
The Force window is uniform over the beginning of the time record and zero over the
remainder. The force length is user specified. This window is used to isolate impulsive
signals, such as impact excitations, from noise and other oscillations later in the time
record.
Exponential
The Exponential window attenuates the time record with a decaying exponential time
constant. This window is often used in impact testing on the response channel to remove
oscillations which last longer than the time record.
Force-Exponential
Many impact measurements require the Force window for one channel and the
Exponential window for the other channel. Use User Math to define a 2 channel
measurement using different windows for each input channel. For example, to measure a
transfer function with a force window on Ch1 (impulse) and an exponential window on
Ch2 (response), define a user function as FFTb(2)/FFTa(1). In this case, the FFT of Ch2
uses the window of DisplayB and the FFT of Ch1 uses the window of DisplayA,
regardless of which display is showing the function. Simply un-link the
[Window]<Window> entry and assign the Force window to DisplayA and the
Exponential window to DisplayB.
User Defined
The User window is any function that the user provides. The User window is copied from
a stored trace using <Trace to Window>. The trace may contain stored data or may be
loaded from disk or via the computer interface.
<Trace to Window> automatically changes the window of the active display to the User
window. The User window is lost if the window type is changed. To switch back and
forth between a User window and another window, keep using <Trace to Window>.
Remember, window functions have a great deal of impact on the resulting FFT spectrum.
A poorly designed window can result in significant measurement errors.