Keysight Übersicht 5991-0852EN | Direct Digital Synthesis (DDS) Generators vs. Trueform Waveform Generators
Technical Overview :: Trueform Waveform Generation Technology.
Trueform waveform generation technology is an exclusive technology found in Keysight Technologies, Inc. 33500B and 33600A Series waveform generators. Trueform technology provides a few advantages over direct digital synthesis (DDS), the incumbent technology used in function generators. These advantages include significantly lower waveform jitter for less test uncertainty and a true representation of the selected waveform, not an approximation. In this overview, we will introduce you to Trueform technology and compare Trueform to DDS technology for waveform generation. Conceptually, the simplest way to generate a waveform is to store its points in memory and then read those points out one after another and clock them into a DAC. After the last point has been read, the generator jumps back to the first point again to begin the next cycle. This is sometimes called "point per clock" (PPC) generation.
Even though this method seems like the most intuitive way to create waveforms, it has two big drawbacks. First, to change the waveform's frequency or sample rate, the clock frequency has to change, and making a good low-noise, variable-frequency clock adds cost and complexity to the instrument. Second, since the stepwise output of the DAC is undesirable in most applications, complex analog filtering is needed to smooth the steps out. Because of its complexity and cost, this technology is used mainly in high-end waveform generators.
DDS uses a fixed-frequency clock and a simpler filtering scheme, so it's less expensive than the PPC method. In DDS, a phase accumulator adds an increment to its output in every clock cycle, and the accumulator's output represents the phase of the waveform. The output frequency is proportional to the increment, so it's easy to change frequency even though the clock frequency is fixed. The output of the accumulator is converted from phase data into amplitude data typically by passing it through some type of look-up table.
The phase accumulator design allows DDS to use a fixed clock, but still execute waveforms at a perceived faster sample rate than the clock. With DDS, not every individual point in the waveform memory is being expressed in the resulting output waveform. Instead, DDS outputs a best approximation of the waveform, which means small features in the waveform can be partially or completely skipped over. At best case, such approximation can lead to added jitter, while at worst, severe distortion can result.
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