User`s guide

RIGOL
© Copyright RIGOL Technologies, Inc. 2007. 2-15
User’s Guide for DS1000 Series
Selecting an FFT Window
DS1000 series oscilloscopes provide four FFT windows. Each window is a trade-off
between frequency resolution and amplitude accuracy. What you want to measure
and your source signals characteristics help determine which window to use. Use the
following guidelines to select the best window.
Tab l e2- 7
Window
Features
Best for measuring
Rectangle
Best frequency
Resolution and worst
magnitude resolution.
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,
poorer magnitude
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 magnitude, worst
frequency resolution.
Single frequency waveforms, to
Find higher order harmonics.
Key points:
FFT Resolution: the quotient between sampling rate and number of FFT
points. With a fixed FFT points, the lower sampling rate results in better
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.