Rigol DS1202/4B - Page 45

RIGOL
© 2008 RIGOL Technologies, Inc.
User’s Guide for DS1000B Series
2-17
Selecting an FFT Window
DS1000B series oscilloscope provides four FFT windows. Each window is a trade-off
between frequency resolution and amplitude accuracy. It depends on the desired
measurement and the source signals characteristics to determine the window to use.
Use the following guidelines to select the best window.
Table 2-9 FFT Windows
Window
Features
Best for measuring
Rectangle
Best frequency resolution
and
worst amplitude
accuracy
.
This is essentially the
same
as no
window.
Transients or bursts, the signal
levels before and after the event
are nearly equal.
Equal-amplitude sine waves with
fixed frequencies.
Broadband random noise with a
relatively slow varying spectrum.
Hanning
Hamming
Better frequency resolution,
poorer
amplitude accuracy
than Rectangular.
Hamming has slightly
better
frequency
resolution than
Hanning.
Sine, periodic, and narrow
-band
random noise.
Transients or bursts where the
signal levels before and after the
events are significantly different.
Blackman
Best amplitude accuracy,
worst frequency resolution.
Single frequency waveforms, to
find higher order harmonics.
Key points:
FFT Resolution:
The FFT resolution is the quotient between
sampling rate and
the number of FFT points
.
With a fixed FFT points, the lower sampling rate, the
better the resolution.
Nyquist Frequency
The highest frequency that any real
-
time digitizing oscilloscope can acquire
without aliasing. It’s normally half of the sample rate. This frequency is called
the Nyquist frequency. Frequency above the Nyquist frequency will be under
sampled, causing a situation known as aliasing.