The maximum analysis frequency is always half the sample rate of the input signal.
So if your data is captured at 50kHz the maximum analysis frequency will be 25kHz.
If you want to reduce the maximum analysis frequency then you must downsample the signal. So if you had a signal sampled at 5000Hz and wanted to reduce the maximum analysis frequency to 500Hz you would downsample the time history to 1000Hz.
But you must be careful to only downsample the signal you want to analyse and not the tachometer or speed signal or the quality of the speed information may be affected.
Please note that upsampling is possible, but not recommended as no frequency content will be valid above the original anti-alias point of 0.4 times the sample rate.
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