User manual

Analyze Wavefor
morTraceData
7. The FFT will app
ear on the display.
Quick Tips
Use short record lengths for faster instrument response.
Use long record lengths to lower the noise relative to the signal and increase the frequency resolution.
If desired, use the zoom feature along with the horizontal Position and Scale controls to magnify and position the
FFT waveform.
Use the default dBV RMS scale to see a detailed view of multiple frequencies, even if they have very different amplitudes.
Use the l
inear RMS scale to see an overall view of how all frequencies compare to each other.
The math
FFT feature provides four windows. Each offers a trade-off between frequency resolution and magnitude
accuracy. The choice of which window to use depends upon what you want to measure and your source’s signal
characteristics. Use the following guidelines to select the best window:
Description Window
Rectan
gular
The frequency resolution when using the Rectangular (a.k.a. boxcar or none) window is very
good, the spectral leakage is high, and amplitude accuracy is poor.
Use th
e Rectangular window for measuring transients or bursts where the signal levels
before and after the event are nearly equal. Also, use this window for equal-amplitude sine
waves with frequencies that are very close together, and for broadband random noise with a
rela
tively slow varying spectrum. This window is the best type for measuring the frequency
spectrum of non-repetitive signals, and measuring frequency components near DC.
Hamm
ing
The frequency resolution when using the Hamming window is good (slightly better than
Hanning), the spectral leakage is moderate, and amplitude accuracy is fair.
Use
the H amming window for measuring sine, periodic, and narrow band random noise. This
window works well on transients or bursts where the signal levels before and after the event
are significantly different.
142 MDO3000 Series Oscilloscopes User Manual