1、 1 All About Direct Digital Synthesis 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 performing a digital-to-analog conversion. Because operations within a D
2、DS 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 are very compact and draw little power. Why would one use a direc
3、t 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 number of industries. Whether providing agile sources of low-phase-noise vari
4、able-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 design considerations. Many possibilities for frequency generation are open to a
5、 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 frequencies. But the DDS technique is rapidly gaining acceptance for solving frequency- (or
6、 waveform) generation requirements in both communications and industrial applications because single-chip IC devices can generate programmable analog output waveforms simply and with high resolution and accuracy. Furthermore, the continual improvements in both process technolog y and design have res
7、ulted in cost and power consumption levels that were previously unthinkably low. For example, the AD9833, a DDS-based programmable waveform generator (Figure 1), operating Figure 1. The AD9833-a one-chip waveform generator. 2 at 5.5 V with a 25-MHz clock, consumes a maximum power of 30 milliwatts. W
8、hat 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 to generate simple sine waves. DDS devices are now available that can generate frequencies from less than 1 Hz up to 400 MHz (b
9、ased 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 program (and re-program) the output waveform, make DDS devices an extremely attractive solution preferable to less-flexible sol
10、utions comprising aggregations of discrete elements. What kind of outputs can I generate with a typical DDS device? DDS devices are not limited 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
11、wave? Heres a breakdown of the internal circuitry of a DDS device: its main components are a phase accumulator, a means of phase-to-amplitude 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 d
12、epends on two variables, the reference-clock frequency and the binar y number programmed into the Figure 2. Square-, triangular-, and sinusoidal outputs from a DDS. Figure 3. Components of a direct digital synthesizer. 3 frequency register (tuning word). The binary number in the frequency register p
13、rovides 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 to the DAC. The DAC, in turn, converts that numbe
14、r 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 clock cycle. If the phase increment is large, the phase accumulator will step
15、quickly through the sine look-up table and thus generate a high frequency sine wave. If the phase increment is small, the phase accumulator will take many more steps, accordingly generating a slower waveform. What do you mean by a complete DDS? The integration of a D/A converter and a DDS onto a sin
16、gle chip is commonly known as a complete DDS solution, a property common to all DDS devices from ADI. Lets talk some more about the phase accumulator. How does it work? Continuous-time sinusoidal signals have a repetitive angular phase range of 0 to 2 .The digital implementation is no different. The
17、 counters carry function allows the phase accumulator to act as a phase wheel in the DDS implementation. To understand this basic function, visualize the sine-wave oscillation as a vector rotating around a phase circle (see Figure 4). Each designated point on the phase wheel corresponds to the equiv
18、alent point on a cycle of a sine wave. As the vector rotates around the wheel, visualize that the sine of the angle generates a corresponding output sine wave. One revolution of the vector around the phase wheel, at a constant speed, results in one complete cycle of the output sine wave. The phase accumulator provides the equally spaced angular values accompanying the vectors linear rotation around Figure 4. Digital phase wheel.
