5-26
Making Measurements
FFT Measurement
The following figure illustrates aliasing. This is the spectrum of a 1 kHz square
wave, which has many harmonics. The sample rate is set to 200 kSa/s, and the
oscilloscope displays the spectrum. The displayed waveform shows the
components of the input signal above the Nyquist frequency to be mirrored
(aliased) on the display and reflected off the right edge.
Aliasing
Since the frequency span goes from ≈0 to the Nyquist fr equency, th e best wa y
to prevent aliasing is to make sure that the frequency sp an is greater than the
frequencies of significant energy present in the input s ignal.
Spectral Leakage The FFT operation assumes that the time record repeats.
Unless there is an integral number of cycles of the sampled waveform in the
record, a discontinuity is created at the end of the record. This is referred to
as leakage. In order to minimize spectral leakage, windows that approach zero
smoothly at the beginning and end of the signal are employed as filters to the
FFT. T h e FF T me n u p ro v i d es t h r ee w i n do w s : H an n i n g, f l a tt o p , a n d r e ct a n g ul a r.
For more information on leakage, see Agilent Application Note 243, "The
Fundamentals of Signal Analysis" (Agilent part number 5952-8898).