Direct Digital Synthesis - Research Paper Example

 Direct Digital Synthesis I. Introduction This paper is an exploration of Direct Digital Synthesis or DDS: what it is, what has been accomplished relating to the technology so far, and where the technology is headed. The aim is to get a good grasp of the technology, as a precursor to making use of the technology in design projects. To be specific, the paper aims to investigate what the components of a DDS system are and the interactions of those components; the different component parameters that engineers can select and tweak; and how those parameters impact the overall performance of the DDS systems in general. The name suggests the nature of its function. Direct digital synthesis is about storing waveform data points in digital format, and then recreating them with the use of digital to analog converters from the stored data. Essentially, the speed with which the synthesizer is able to regenerate the waveform is correlated with the waveform frequency. Digital techniques for mapping waveform data point into digital formats for storage and retrieval and are at the heart of DDS systems. In essence the basic components of a DDS system are the phase accumulator, the waveform map, the digital to analog converter or DAC, and the low pass filter, which sums up the DDS process from the mapping of the waveform signal into digital data, to the recreation of the analog waveform via the use of a DAC and a low pass filter to perfect the waveform. The illustration below details this simplified DDS system [1]: Image source: [1] An even more simplified version of the DDS architecture shrinks it into two essential components, the phase generator/accumulator, which is time discrete, and the phase-to-waveform converter, for recreating the appropriate output signal of the DDS [2]: Image source: [2] In the image above, the low pass filters and the waveform map that is part of the previous diagram are assumed to be present in the background, as peripheral components [1] [2]. On the other extreme is a more detailed diagram representing the basic functions of a DDS system, that expands on the first diagram above, and includes the waveform map and low pass filter [3]: Image source: [3] II. Description of Direct Digital Synthesis The idea behind DDS is that waveforms of various kinds are easily implementable and can be easily recreated from digitally stored waveform data, and the freedom that this gives to designers includes that whereas in the past, more and more complex analog components were needed to recreate more and more complex waveforms, in the case of DDS systems all that is needed is stored digital data. This paves the way for the creation of more complex waveforms, including waveforms that are non-standard and generated by users for particular, atypical uses. The advantages of DDS systems over analog wave and function generators, for instance have been firmly established, with cost and design advantages inherent in the simpler design of DDS systems. Also, the inherent characteristics of DDS allow for its widespread use as more appealing alternatives to older analog systems. These characteristics include frequency resolution advantages, and that the technology lends itself to phase, amplitude and frequency modulation being directly implemented on such systems, in comparison to older function and wave generators, which were based on analog technologies [3]. Moreover, lends itself to being tweaked with regard to the different characteristics of its output waveforms, lending themselves to DDS systems being a viable solution for many electronics problems facing modern systems, such as mobile telephones, radio receivers and the like [4] [5]: DDS has the ability to produce and control signal on various frequencies which is a very important requirement in many electronic systems for it to generate any range of local oscillator to a frequencies from a single fixed time base. DDS can work in many modern devices such as, CB radios, GPS systems, mobile telephones, satellite receivers, radiotelephones, walkie-talkies, radio receivers etc. DDS is the kind of frequency synthesizer that creates arbitrary waveforms and frequencies from a single, fixed source frequency [4] Going into the details of the digital storage and recreation of waveforms via a DDS system, the diagram below details the basic DDS components mapped alongside the waveform processing and conversion along every step in the process, in simplified form [6]: Image source: [6] In this mapped DDS diagram, there are essentially four components, adding one to the diagrams presented earlier in this paper. These are the phase accumulator, a converter for turning phase to amplitude, implemented in the usual case via a sine ROM, a DAC, and a low pass filter. The phase to amplitude converter is the additional component in this diagram. In its implementation, it is a sine lookup table, with the output characterized mathematically as such [6]: [6] The filter at the end is necessary to smooth out the components of the output signal that are the result of sampling at high frequencies, so that the output is a sine wave without impurities[6]. As can be gleaned from the above discussion, the limitations of DDS are to be seen only in the ability of the DDS design to sample at higher frequencies, and to recreate waveforms from tweaking the component parameters in various ways [7]. The modulation possibilities arise from DDS systems being in essence digital signal processing devices as well, with the modulation being done as add-ons to the basic DDS architecture, as implemented below [6]: Image Source: [6] In the diagram above, modulation by frequency, by phase and by amplitude is effected via the use of a modulation control bus, which then tweaks the signal as it passes through the different components of a DSS system, in between the major components. Adders are inserted to modulate frequency and phase before the phase accumulation and the phase to amplitude conversion steps, respectively, while a multiplier effects amplitude modulation control prior to the feeding of the digital signal data into the DAC [6]. III. DDS Architecture Parameters The preceding diagram tells us that at every stage in the DSS process, there are tweaks to the values that can be inputted into the components, with varying results. For instance, the frequency of the signal can be modulated prior to feeding the signal to the phase accumulator. The changes in the frequency do not materially impact the performance of the DDS system, unless the frequencies are modulated to upper limits that stress the ability of the phase accumulator to gather enough data points to characterize the waveform properly. In other words, the frequency limitations of the DDS limits the frequency to which the signals can be modulated in both the upper and lower limits of the DDS [6]. Phase noise is an inherent characteristic of the DSS oscillator used, as is jitter and spurious-free dynamic range, that can also affect the way it is able to handle varying input frequencies. Related to this, the reference clock is ideally also within the reach of the engineer for tweaking, with consequences for the way the DSS is able to code and recreate waveforms at different input signal frequencies [7]. Sampling frequencies, for one, have implications for the ability of the DSS system to account for aliasing, and the ability of the DSS to compensate for DSS aliasing [8]. Phase modulation allows for the signal to be modulated as such prior to the signal being fed into the phase to amplitude converter. Again, the limits of the phase to amplitude converter limits the size of the amplitude modulation that can be achieved, without compromising the ability of the DDS to perform its job and convert analog wave signals into digital data. As well, amplitude modulation affects the performance of the DAC, and at more finely-tuned higher amplitudes outside the DAC range, the wave recreation capabilities degrade. Modulating the signals at these different points therefore have implications for the ability of the DSS to accommodate the modulations, based on the inherent performance limits of the components in general [6] [7] [8]. Works Cited Eva Murphy and Colm Slattery. “All About Direct Digital Synthesis”. Analog Dialogue 38/Analog Devices http://www.analog.com/library/analogdialogue/archives/38-08/dds.html August 2004 [26 September 2012] Ian Poole. “Direct digital synthesis DDS tutorial”. Radio-Electronics.com. Internet: http://www.radio-electronics.com/info/rf-technology-design/pll-synthesizers/direct-digital-synthesizer-dds-tutorial.php 2012 [26 September 2012] Jouko Vankka. “Direct Digital Synthesizers: Theory, Design and Applications. Helsinki University of Technology. http://lib.tkk.fi/Diss/2000/isbn9512253186/isbn9512253186.pdf November 2000 [26 September 2012] Lau Kai Hsiung James. Eng499 Capstone ICT Project Final Report. Sim University. http://sst.unisim.edu.sg:8080/dspace/bitstream/123456789/179/1/09_Lau%20Kai%20Hsiung%20James.pdf March 2009 [26 September 2012] Lionel Cordesses. “Direct Digital Synthesis: A Tool for Periodic Wave Generation (Part 1)”. IEEE Signal Processing Magazine. http://people.chu.edu.tw/~chlee/Audio2004/R3_1.pdf July 2004 26 September 2012] National Electronics. “Understanding Direct Digital Synthesis”. National Instruments website. http://www.ni.com/white-paper/5516/en 10 April 2012. [26 September 2012] Stanford Research Systems. “Direct Digital Synthesis”. ThinkSRS.com. Internet: http://www.thinksrs.com/downloads/PDFs/ApplicationNotes/DDS.pdf [26 September 2012] Tom Wiltshire. “Direct Digital Synthesis”. Electric Druid. Internet: http://www.electricdruid.net/index.php?page=info.dds 2008 [26 September 2012] Read More