Keysight Technologies N8241A, User Manual

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108 Chapter 5
Direct Digital Synthesis Option 330
Theory of Operation
Theory of Operation
The Direct Digital Synthesis, Option 330, is a powerful tool for those
customers who are using the N8241A Arbitrary Waveform Generator
(AWG) to synthesize waveforms best expressed in the frequency domain.
Traditionally, waveforms are expressed in the time domain, sampled, and
then stored in waveform memory for eventual playback. This approach is
completely generic and applicable to any describable waveform. However,
many waveforms can be described as a combination of information content
and simple sinusoids. For these waveforms, most of the available waveform
memory gets used up storing the sinusoids, leaving little space for the
information content.This is an inefficient utilization of waveform memory.
For example, in communications, the waveform can be described as a carrier
(sinusoid) modulated with data (information content). Because the AWG has
such high dynamic range, the modulated carrier can be generated with very
good equivalent error vector magnitude (EVM) performance. But, because
the carrier has to be stored in waveform memory along with the modulation,
limited playback time can be achieved. Another important example of a
frequency domain waveform is wideband radar chirps. Again, the waveform
consists of a combination of a sinusoid and a frequency chirp profile, both of
which must be traditionally stored in waveform memory, resulting in limited
playback time.
To address this issue, Option 330 allows the AWG to generate the sinusoidal
portion of the waveform real time, and then modulate the sinusoid with the
information content stored in waveform memory (see
Figure 5-7 ). This is
done by adding a direct digital synthesizer (DDS) to the main FPGA in the
AWG. The DDS implemented has a frequency resolution of 1.1369 mHz,
and can synthesize sinusoids from DC to 400 MHz (using the 1.25 GHz
internal clock). The DDS can be linearly ramped in frequency, with a
frequency ramp rate resolution of 1.3552 Hz/s, and a maximum frequency
ramp rate of 46.566 GHz/s. The frequency ramp rate can be positive or
negative.
The initial phase of the DDS can be set to a known value, with a phase
resolution of 21.458
μ
degrees. Alternatively, through the use of the initial
phase control field, the DDS can be operated in phase continuous mode; for
example, the initial phase not initialized. This is useful for applications