Tutorial
81150A and 81160A User’s Guide 605
An example of the 81150A: Consider an arbitrary waveform consisting of 10
cycles of a sine waveform. When you set the function generator’s frequency
to 24 MHz, the actual output frequency will be 240 MHz and the amplitude
will be attenuated by approximately 11 dB. As you increase the frequency
above
24 MHz, more attenuation will occur. At approximately 60 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.
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 wave shape 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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