Nyquist Frequency and Aliasing in the Frequency Domain
The Nyquist frequency is the highest frequency that an instrument can acquire
without aliasing. This frequency is half of the sample rate. Frequencies above the
Nyquist frequency will be under sampled, which causes aliasing. The Nyquist
frequency is also called the folding frequency because aliased frequency
components fold back from that frequency when viewing the frequency domain.
Aliasing happens when there are frequency components in the signal higher than
half the sample rate. Because the FFT spectrum is limited by this frequency, any
higher components are displayed at a lower (aliased) frequency.
Because the frequency span goes from approximately 0 to the Nyquist frequency,
the best way to prevent aliasing is to make sure that the frequency span is greater
than the frequencies of significant energy present in the input signal.
To accurately measure the frequency of higher frequency signals, turn off the checkbox in the
Antialising
softkey on the associated Channel Menu, which appears after you press
[1]
or
[2]
.
FFT 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. The FFT Menu
provides four windows: Hanning, Flat Top, Rectangular, and Blackman-Harris.
Keysight IntegraVision PA2200 Series Power Analyzers Operating and Service Guide 155
