Creating and Using Math Waveforms
3- 228 CSA7000 Series, TDS7000 Series, & TDS6000 Series Instruments User Manual
Set the sample rate high enough so that the signals in the spectrum appear at
their correct frequency as opposed to a lower aliased frequency value. Also,
complex signal shapes that have many harmonics in them, such as a triangle or
square wave, can appear to be OK in the time domain when in fact many of the
harmonics in that signal are aliased.
One way to check for aliasing is to increase the sample rate and observe whether
any of the harmonics unwrap to different frequency locations.
Higher order harmonics usually have decreasing magnitudes compared to lower
order harmonics. Thus, if you see a series of increasing harmonic magnitude
values as frequency increases then you can suspect that they may be aliased. In
the spectral math waveform, the actual higher frequency components are
undersampled, and therefore, they appear as lower frequency aliases that “fold
back”around the Nyquist point. (See Figure 3--61.) You can test by increasing
the sample rate and observing if aliases unwrap to different frequency positions.
Frequency
Amplitude
Aliased frequencies Actual frequencies
0Hz
Nyquist frequency
(½sample rate)
Figure 3- 61: How aliased frequencies appear in a spectral waveform