Creating Arbitrary Waveforms
For most applications, it is not necessary to create a waveform of any
specific length since the function generator will automatically sample
the available data to produce an output signal. In fact, it is generally
best to create arbitrary waveforms which use all available data
(16,000 points long and the full range from 0 to 4,095
DAC codes). For the
33120A, you do not have to change the length of the waveform to change
its output frequency. All you have to do is create a waveform of any
length and then adjust the function generator’s output frequency.
Remember, if you create an arbitrary waveform that includes three
cycles of a waveshape (for example), the output frequency will be three
times the value displayed on the function generator’s front panel.
When creating arbitrary waveforms, you have control of both the
amplitude quantization and phase truncation errors. For example,
phase truncation harmonics will be generated when a waveform is
created using the full amplitude range of the
DAC (12 bits) but is
created using only 1,000 waveform data points. In this case, the
amplitude quantization errors will be near the noise floor while the time
quantization error will produce harmonics near the -60 dBc level.
Similarly, amplitude quantization harmonics will be generated when
you create a waveform using less than the full amplitude resolution of
the function generator. For example, if you use only one-fifth of the
available amplitude resolution, amplitude quantization will produce
harmonics below the -60 dBc level.
When importing data from instruments such as oscilloscopes, the data
will generally range between 1,024 and 4,096 time points and between
64 and 256 amplitude points.
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. As shown on the next page,
it is possible that the shape and phase of a signal may be such that a
transient is introduced at the end point. When the waveshape is
repeated for all time, this end-point transient will introduce leakage error
in the frequency domain because many spectral terms are required to
describe the discontinuity.
Chapter 7 Tutorial
Creating Arbitrary Waveforms
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