Internal Modulation Source Internally, the function generator
incorporates a second, lower speed and lower resolution
DDS arbitrary
waveform generator to produce the modulating signal independent of
the carrier signal. Internal modulation waveshapes range in length from
2,048 points to 4,096 points. User-defined arbitrary waveforms are
automatically expanded or compressed in length as needed to fit within
the required modulation waveform constraints. Linear interpolation is
performed on user-defined arbitrary waveforms while the lengths of
standard waveshapes are varied by decimation. Due to the modulation
sample rate and waveform size limitations, the best case modulation
signal frequency accuracy is approximately 0.05% of setting.
Unlike the main signal output discussed previously, modulation
waveshapes are sampled using a variable “point clock” to sample data
loaded in modulation waveform
RAM. Internally, the modulation
point clock (C) and modulation waveform length are automatically
adjusted to produce the modulation signal frequency desired. For
frequencies greater than C/2048, all modulation shapes are sampled up
to the maximum modulating frequency. A new modulation waveform
is computed and loaded into modulation
RAM each time the modulation
type, modulation waveshape, or modulation frequency is changed. Data
in standard arbitrary waveform memory is not affected by modulation
signal changes (data is expanded or compressed and loaded directly into
separate modulation
RAM following computation). No expansion or
compression is performed on the modulation waveform data for certain
modulation frequencies.
7
Chapter 7 Tutorial
Modulation
293
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