1、PDF外文：http:/ - 1 - All About Direct Digital Synthesis By Eva Murphy Colm Slattery What is Direct Digital Synthesis? Direct digital synthesis (DDS) is a method of producing an analog waveform usually a sine wave by generating a time-varying signal in digital form and then perform
2、ing a digital-to-analog conversion. Because operations within a DDS device are primarily digital, it can offer fast switching between output frequencies, fine frequency resolution, and operation over a broad spectrum of frequencies. With advances in design and process technology, todays DDS devices
3、are very compact and draw little power. Why would one use a direct digital synthesizer (DDS)? Arent there other methods for easily generating frequencies? The ability to accurately produce and control waveforms of various frequencies and profiles has become a key requirement common to a
4、number of industries. Whether providing agile sources of low-phase-noise variable-frequencies with good spurious performance for communications, or simply generating a frequency stimulus in industrial or biomedical test equipment applications, convenience, compactness, and low cost are important des
5、ign considerations. Many possibilities for frequency generation are open to a designer, ranging from phase-locked-loop (PLL)-based techniques for very high-frequency synthesis, to dynamic programming of digital-to-analog converter (DAC) outputs to generate arbitrary waveforms at lower frequenc
6、ies. But the DDS technique is rapidly gaining acceptance for solving frequency- (or waveform) generation requirements in both communications and industrial applications because single-chip IC devices can 外文翻译（原文） - 2 - generate programmable analog output waveforms simply and with high re
7、solution and accuracy. Furthermore, the continual improvements in both process technolog y and design have resulted in cost and power consumption levels that were previously unthinkably low. For example, the AD9833, a DDS-based programmable waveform generator (Figure 1), operating at 5.5 V wit
8、h a 25-MHz clock, consumes a maximum power of 30 milliwatts. Figure 1. The AD9833-a one-chip waveform generator. What are the main benefits of using a DDS? DDS devices like the AD9833 are programmed through a high speed serial peripheral-interface (SPI), and need only an external clock t
9、o generate simple sine waves. DDS devices are now available that can generate frequencies from less than 1 Hz up to 400 MHz (based on a 1-GHz clock). The benefits of their low power, low cost, and single small package, combined with their inherent excellent performance and the ability to digitally p
10、rogram (and re-program) the output waveform, make DDS devices an extremely attractive solution preferable to less-flexible solutions comprising aggregations of discrete elements. What kind of outputs can I generate with a typical DDS device? 外文翻译（原文） - 3 - DDS devices are not limit
11、ed to purely sinusoidal outputs. Figure 2 shows the square-, triangular-, and sinusoidal outputs available from an AD9833. How does a DDS device create a sine wave? Heres a breakdown of the internal circuitry of a DDS device: its main components are a phase accumulator, a means of phase-to-amp
12、litude conversion (often a sine look-up table), and a DAC. These blocks are represented in Figure 3. A DDS produces a sine wave at a given frequency. The frequency depends on two variables, the reference-clock frequency and the binar y number programmed into the frequency register (tuning word). &nb
13、sp;The binary number in the frequency register provides the main input to the phase accumulator. If a sine look-up table is used, the phase accumulator computes a phase (angle) address for the look-up table, which outputs the digital value of amplitude corresponding to the sine of that phase angle t
14、o the DAC. The DAC, in turn, converts that number to a corresponding value of analog voltage or current. To generate a fixed-frequency sine wave, a constant value (the phase increment which is determined by the binary number) is added to the phase accumulator with each Figure 2. Square-, triangular-, and sinusoidal outputs from a DDS. Figure 3. Components of a direct digital synthesizer.
