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Chapter 7 Tutorial
Creating Arbitrary Waveforms
For example, consider an arbitrary waveform consisting of 10 cycles of a
sine waveform. When you set the function generator's frequency to
1 MHz, the actual output frequency will be 10 MHz and the amplitude
will be attenuated by approximately 3 dB. As you increase the frequency
above 1 MHz, more attenuation will occur. At approximately 2.5 MHz,
waveform distortion due to aliasing will become evident. Some aliasing
will be present in most arbitrary waveforms, but whether or not it will be
troublesome depends on your specific application.
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When creating arbitrary waveforms, the function generator will always
attempt to replicate the finite-length time record to produce a periodic
version of the data in waveform memory. However, as shown below, it is
possible that the shape and phase of a signal may be such that a
discontinuity is introduced at the end point. When the waveshape is
repeated for all time, this end-point discontinuity will introduce
leakage errors in the frequency domain because many spectral terms
are required to describe the discontinuity.
Leakage error is caused when the waveform record does not include an
integral number of cycles of the fundamental frequency. Power from the
fundamental frequency, and its harmonics, is transferred to the spectral
components of the rectangular sampling function. You can reduce
leakage errors by adjusting the window length to include an integer
number of cycles or by including more cycles within the window to reduce
the residual end-point discontinuity size. Some signals are composed of
discrete, non-harmonically related frequencies. Since these signals are
non-repetitive, all frequency components cannot be harmonically related
to the window length. You should be careful in these situations to
minimize end-point discontinuities and spectral leakage.
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1 Cycle
Arbitrary Waveform with Discontinuity
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