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Direct Digital Synthesis (DDS) Generators versus Trueform Waveform Generators

Technical Overviews

The incumbent technology used in most function generators is known as direct digital synthesis (DDS). Unfortunately, DDS has a number of problems that can cause unexpected and seemingly unexplainable problems in the lab. Trueform technology avoids many of these traps with its lower jitter and true representation of waveforms – no more approximations! Read more to learn about the differences between DDS and Trueform waveform generators and how those differences will affect your testing.

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.

Keysight’s Trueform technology represents the next leap in waveform generation technology, providing you the best of both worlds. It gives you a predictable low-noise waveform with no skipped waveform points like PPC technology, but at the price point of DDS technology. Trueform works by employing a patented virtual variable clock with advanced filtering techniques that track the sample rate of the waveform. In the following sections, we will look at some of the waveform generation advantages Trueform provides over DDS.

Improved signal integrity

One of the key advantages Trueform provides over DDS is overall better signal integrity. You can see the advantage in the frequency domain by comparing spectra and the time domain with a jitter measurement comparison. Figure 1 shows a frequency domain view of a 10-MHz sine wave generated using Trueform technology. Figure 2 shows the same frequency domain view of a 10-MHz sine wave generated with DDS technology.

The waveform you create is the waveform you get

As we mentioned earlier, DDS uses a fixed clock and a phase accumulator so it cannot guarantee that every point or feature in a waveform will be played. The higher the frequency, the more gaps you will see in the output waveform compared to the ideal waveform. Trueform, on the other hand, plays every waveform point regardless of the set frequency or sample rate. This becomes critical when you are dealing with a waveform that may have a small detail that is critical to the test you are performing. 

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