Saturday, May 16, 2015

Better performance for function generators by Sifi technology –

                 15/05/2015 11:20 |
                  signal synthesis

                Technical Articles
 Thomas Rottach

Traditional function generators use a digital image of the output signal, which sets the signal for each discrete value. Thus, arbitrary functions can be generated. However, the clock is fixed; to change the output frequency of the generator has to skip individual interpolation points or more to spend. Sifi, however, always uses the entire image and fit for the pace.

Rigols Generator series utilizes DG1000Z Sifi to precisely output arbitrary waveforms at each frequency. (image: Rigol)

Most of function and signal generators are based for many years on DDS (Direct Digital Synthesis ) as a common technology for signal generation. Simplistically, we can say that DDS each phase value of the reference clock each assigns a digital amplitude value. The allocation (phase to digital value) is usually stored as a table, like in a digitized audio signal. If the function generator for example, spend a sinusoidal waveform, the values ​​of a sine function are stored in the table. The digital-to-analog converter converts these values ​​into analog voltage values, and the device it is according to a level of adjustment. Since the clock is fixed, other output frequencies are possible only by changing the number of output points. The DDS technology has generations of function generators allow that generate signals with good quality at a reasonable price.

Key data

Arbiträrfunktionsgeneratoren save the signal to be generated as a time sequence of feature values. These lie in a simple table. The traditional DDS technology is, however, before a fixed cycle, the device outputs the signals. To play the course faster, thereby increasing the frequency, these generators skip individual values. The Sifi technology Rigol fits, sampling at: The DA converter is always each individual support point and therefore significantly improves the signal fidelity. In addition, the devices generate desired harmonics.

Emerging technologies enable generators, which integrate the advantages of DDS and also improve signal purity. This makes them suitable for other applications. Rigol has called Sifi technology integrated into the generator series DG1000Z, thereby improving the fidelity and additional new functions. In this generation of instruments at the point-by-point signal Arbitrary signals and a redesigned output stage combine for a very versatile signal generator. Arbitrary curves with high signal integrity can easily create.

Rigol also has features such as internal summing of curves or the generation of harmonics implemented and a large memory integrated, which in turn increases the possibilities for arbitrary wave generation.

signal fidelity

There are a number of ways to evaluate the quality of a signal. As the most important and critical for engineers key parameters of an arbitrary signal are reproducibility, to name the accuracy and the noise level.

Figure 1 shows the spectrum of a 20 MHz sine wave. The focus is on the harmonics: The purple line shows the peaks of the signal of a conventional DDS generator, while the blue curve represents the spectrum of the same signal from one DG1000Z generator with Sifi functionality. Overall, the specified total harmonic distortion (THD, Total Harmonic Distortion) of DG1000Z by more than half less than that of a conventional DDS generator.

Image 1: to identify the spectrum of a 20 MHz sine wave are the Klirrsignale of conventional DDS generator (purple) , The Sifi variant (blue) clearly generates less interference. (image: Rigol)

While THD says a lot about the noise and the accuracy of a generator, can be combined with the jitter rate repeatability. Again the DG1000Z shows an approximately two and a half fold improvement compared to traditional generators.

quality issues

While DDS selectively chooses only the nearest amplitude value a Arbiträrwelle at a fixed sampling rate and outputs, Sifi adjusts sampling and displays all points on the curve. This creates an exact replica of the predetermined Arbiträrkurve. If the function generator is, for example, spend a 100,000 dots long Arbitrary with a repetition rate of 500 Hz, the following equation applies:

  • Number Arbiträrpunkte × desired frequency = to be set sampling

For the above example, this results in an output rate of 50 MSPS / s. From the number of arbitrary curve points and the sampling rate is the frequency of the output signal is determined. The generator are all points with their exact amplitude value at the defined time out.

Erratic behavior

In contrast, when a DDS generator, the sampling rate is set. Depending on the desired output frequency, he must skip points or more to spend. Thus, there is no guarantee that all details of the arbitrary curve or exactly the right voltage at any given time are output.


Image 2: In this window, users can generate harmonics artificially and thus generate new signals. (image: Rigol)

Some of these errors are not visible when viewed with an oscilloscope, because the analog bandwidth and the limited rise time of the device to hide the discrepancies. That’s one of the reasons why one to evaluate the accuracy of the signal spectral purity draws on (THD, total harmonic distortion). Signal fidelity is crucial to engineers using waveform generators for tests. The use of a generator with Sifi technique improves the accuracy of the reproduced waveforms and enables developers maximum flexibility in setting the output rate their Arbiträrkurven.

More functions and types

In addition to better signal fidelity enables Sifi technology also means that DG1000Z generators on request generate harmonics. In this phase and amplitude are to define each of these harmonic frequencies. Figure 2 shows the display representation of the definition window of harmonics. Developers can set up to eight harmonics in magnitude and phase.

3: Using the values ​​from Figure 2 that here measured by oscilloscope spectrum up to the eighth harmonic is created. (image: Rigol)

had the case of traditional generators users define these harmonics individually then integrate into a new Arbiträrkurve and this curve then spend. Any amendment moved back a complete preparation cycle by itself. Now let the harmonics create much simpler: Users only need to set the amplitude and phase of each frequency (based on the fundamental); the instrument automatically combines them and gives them back.

Figure 3 shows the appropriate range for the defined signal in Figure 2. In Figure 4, the signal can be seen in the time domain on an oscilloscope. This analog curve would Users previously create manually, load it into the generator and have to spend it.

Figure 4: With the additional harmonics from 3 image is created from a simple sinusoidal signal that waveform. (image: Rigol)

The memory use

The deep memory of DG1000Z- Class further increases the flexibility of the device. In combination with the Sifi technology developers to create longer and more precise than with DDS arbitrary waveforms. In Customizable sample rate mode users can define a signal output from the generator with up to 60 MSPS / s.

People use the maximum memory depth of 16 million points (DG1kZ-Mem option) provides the a device on the maximum sampling rate and can still generate a 280ms lasting signal. The difficulty with such a long waveforms is in progress. For example, in Microsoft Excel 2013 are only slightly more than one million rows. Thanks Sifi technology allows users to retain the original loaded waveform and only need to adjust the output sample rate.

5: In Ultra-station users can enter waveforms via PC software and change each individual value. (image: Rigol)

curves shapes

In addition, but is also a reliable method required in order to generate long waveforms new, edit, and upload to the device. The PC software Ultra-Station offers a solution here to edit the curves. Ultra station dominates the importing, combining and freehand machining even of Arbiträrkurven with millions of points. Files stores the program as a TXT, CSV or RAF. RAF files to store user on a USB flash drive and then download it to the instrument, or send it directly via USB or LXI to the instrument. Figure 5 shows the freehand processing in ultra-station. In addition to time-domain view, the software provides a frequency range view (amplitude and phase representation) can be seen in Figure 6.


6: The software from Figure 5 calculates the timing of the signal and the spectrum view with amplitude and phase. (image: Rigol)

Additional windows and filter functions are available as part of Ultra-Advanced station. The above-mentioned features are already included in the bundled, free version. Users can expand the Advanced version of their software at any time. Figure 7 shows an image formed with the Rigol software Eight-million-point signal output from the function generator at the maximum rate of 60 MSPS / s. The combination of memory depth and Sifi to create and spend more flexible and more precise curves allows as with a conventional DDS-based generator.

7: The via software (Figure 5) produced signal the function generator faithfully recreated. (image: Rigol)


Rigols Sifi technology in DG1000Z- Series arbitrary function generators enables developers able to generate more signals with improved signal integrity, flexibility and usability. The Deep-memory functions and the hardware design of the instruments work seamlessly with the Sifi sampling technology together.



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