Stanford Research Systems SR785, Руководство по эксплуатации и программированию

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FFT Windowing 2-13
SR785 Dynamic Signal Analyzer
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 or exact bin sine frequencies.
In the Order Analysis measurement group the uniform window can be used in situations
where most of the harmonics of interest will be at integer multiples of the shaft rotation
speed. In this case, these harmonics will fall on exact bin frequencies and will be most
accurately reproduced with the uniform windows.
Hanning
The Hanning window is a commonly used window. However, it has an amplitude
variation of about 1.5 dB for signals which are not at exact bin frequencies 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.
The Hanning window function is
w
i
N
i
= −




1 0 2 . cos
π
for i = 0 .. N-1 and N = number of time record points.
Flattop
The Flattop window has the best amplitude accuracy of any window. The amplitude
variation is only about 0.02 dB for signals between exact frequency bins. However, the
selectivity is worse. Unlike the other windows, the Flattop window has a very wide pass
band and very steep rolloff on either 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.
The Flattop window function is
w
i
N
i
N
i
N
i
N
i
= − ⋅




+ ⋅




− ⋅




+ ⋅




1 0 1 93 2 1 29 4 0 388 6 0 028 8 . . cos . cos . cos . cos
π π π π
for i = 0 .. N-1 and N = number of time record points.
BMH
The BMH window combines good selectivity and reasonable accuracy (about 0.8 dB for
signals between exact frequency bins). The BMH window has much lower side-lobes
than the Hanning window and very little broadening of non-bin frequencies.
The BMH window is a good window to use for measurements requiring a large dynamic
range.
The BMH window function is