Math FFT
Nyquist Frequency
The highest fre
quency that any real-time digitizing oscilloscope can measure
without errors is one-half the sample rate. This frequency is called the Nyquist
frequency. Frequency information above the Nyquist frequency is undersampled,
which causes FFT aliasing. (See page 66, FFT Aliasing.)
The math function transforms the center 2048 points of the time-domain
waveform into an FFT spectrum. The resulting FFT spectrum contains 1024
points that go from DC (0 Hz) to the N yquist frequency.
Normally, the display compresses the FFT spectrum horizontally into 250 points,
but you can u se the FFT Zoom function to expand the FFT spectrum to more
clearly see the frequency components at each of the 1024 data points in the FFT
spectrum
.
NOTE. The oscilloscope vertical response rolls off slowly above its bandwidth
(100 MHz or 200 MHz, depending on the model, or 20 MHz when the Bandwidth
Limit option is set to ON). Therefore, the FFT spectrum can show valid frequency
information higher than the oscilloscope bandwidth. However, the magnitude
information near or above the bandwidth will not be accurate.
TPS2000B Series Digital Oscilloscope User Manual 63
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