Basic Operation 5.3.1.4 FFT Aliasing Problems occur when the time-domain waveform acquired by the oscilloscope contains frequency components higher than the Nyquist frequency. The frequency components above the Nyquist frequency will be undersampled and displayed as lower frequency components that ‘fold back’ from the Nyquist frequency. These erroneous components are called aliases. 5.3.1.5 Eliminating Aliases To eliminate aliases, use the following methods. ➢ Click the TIME/DIV key to set a faster sample rate. Because the Nyquist frequency increases as you increase the sample rate, the aliased frequency components will be displayed correct.
If too many frequency components appear on the screen, you may use the FFT Zoom option to magnify the FFT spectrum. ➢ If there is no need to observe the frequency components above 20MHz, set the Bandwidth Limit option to Limited. ➢ Filter the signal input from outside and limit the bandwidth of the source waveform to lower than the Nyquist frequency. ➢ Identify and ignore the aliased frequencies. ➢ Use zoom controls and cursors to magnify and measure the FFT spectrum. 5.3.1.6 Magnifying and Positioning FFT Spectrum You may scale the FFT spectrum and use cursors to measure it through the FFT Zoom option which enables the horizontal magnification.
To vertically magnify the spectrum, use the vertical controls. Horizontal Zoom and Position You can use the FFT Zoom option to magnify the FFT spectrum horizontally without changing the sample rate. The available zoom factors are X1(default), X2, X5 and X10. When the zoom factor is set to X1 and the waveform is located at the center graticule, the left graticule line is at 0Hz and the right is at the Nyquist frequency. You magnifies the FFT spectrum to the center graticule line when you change the zoom factor. That is, the axis for horizontal magnification is the center graticule line. Click the Horizontal Position Key to move the FFT spectrum to the right.
Vertical Zoom and Position DSO1000B(V) Series HandHeld Oscilloscope User Manual 24 same as no windows. Bartlett Blackman Best magnitude, worst frequency resolution. Single frequency waveforms, to Find higher order harmonics.