Creating and Using Math Waveforms
3- 204
CSA7000B Series & TDS7000B Series Instruments User Manual
There are only two samples per cycle of a signal that have a frequency equal to
one half of the sample rate. This is the highest nonaliased signal that can be
output from the spectral analyzer. Thus, at this frequency, one sample of
acquisition jitter will show up in the spectrum as 180 degree phase variations.
The phase spectrum is dejittered by using the fractional trigger value of the
instrument to correct all phases in the spectrum. Therefore phase is accurately
measured from DC to the Nyquist frequency.
H Effects of Average and High Res acquisition modes. The result of averaging
the time domain acquisition using either average mode or Hi Res acquisition
mode affects the frequency response of the instrument. This is due to the one
sample of jitter in the acquisition system. Both High Res and average
acquisition modes have the same affect on the frequency response. These
modes cause the response to roll off from a magnitude value of one at DC to
a magnitude value of 0.63 at Nyquist which is the frequency equal to one
half of the sample rate. This is true regardless of the real time sample rate
setting.
H Frequency Domain Averaging. You may turn on average for a math
waveform by editing the math expression. Sometimes it is more desirable to
average in the spectrum than in the time domain. For example, consider a
signal that has time domain components that are asynchronous to the trigger.
If you turn on averaging in the time domain, these components may go to
zero or produce strange non-deterministic effects on the resultant waveform.
Thus, these signal components may end up not appearing in the spectrum.
However, if averaging is done in the frequency domain instead, then these
components will be present. An example is:
Math1 = AVG(SpectralMag(Ch1)).
Aliasing occurs when the input frequency of a signal is greater than one half of
the sampling frequency (the sample rate).
Set the sample rate high enough so that the signals in the spectrum appear at the
correct frequencies 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.
Recognizing Aliasing
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