【正文】 0dB）。你有更容易進行編程和DDS的性能預測的工具嗎？在線互動設(shè)計工具是一個選拔調(diào)整，給定一個時鐘和期望輸出頻率和階段。選擇所需的頻率，以及理想化的輸出諧波濾波器后重建的外部顯示已被應用。一個例子是如圖11所示。表格數(shù)據(jù)也提供了重要的圖像和諧波。這些工具將如何幫助我的DDS方案？所有這一切需要的是必要的頻率輸出和系統(tǒng)的參考時鐘頻率。該設(shè)計工具輸出的是完整的編程序列所需方案的一部分。以圖12為例，MCLK的是設(shè)置為25MHz和所需的輸出頻率設(shè)置為10MHz。一旦更新按鈕按下時，部分序列初始化。圖12如何評價你的DDS器件？所有DDS器件具有一個評價板可供購買。由專用軟件攜帶，用戶可以測試/評估。每一個技術(shù)說明附評估電路板原理圖包含的信息，并顯示最佳推薦電路板設(shè)計和布局的做法。All About Direct Digital SynthesisBy 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 timevarying signal in digital form and then performing a digitaltoanalog 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, today’s DDS devices are very pact and draw little power. Why would one use a direct digital synthesizer (DDS)? Aren’t there other methods for easily generating frequencies? The ability to accurately produce and control waveforms of various frequencies and profiles has bee a key requirement mon to a number of industries. Whether providing agile sources of lowphasenoise variablefrequencies with good spurious performance for munications, or simply generating a frequency stimulus in industrial or biomedical test equipment applications, convenience, pactness, and low cost are important design considerations. Figure 1. The AD9833a onechip waveform generator.Many possibilities for frequency generation are open to a designer, ranging from phaselockedloop (PLL)based techniques for very highfrequency synthesis, to dynamic programming of digitaltoanalog converter (DAC) outputs to generate arbitrary waveforms at lower frequencies. But the DDS technique is rapidly gaining acceptance for solving frequency (or waveform) generation requirements in both munications and industrial applications because singlechip 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 resulted in cost and power consumption levels that were previously unthinkably low. For example, the AD9833, a DDSbased programmable waveform generator (Figure 1), operating at V with a 25MHz clock, consumes a maximum power of 30 milliwatts.What are the main benefits of using a DDS? DDS devices like the AD9833 are programmed through a high speed serial peripheralinterface (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 (based on a 1GHz clock). The benefits of their low power, low cost, and single small package, bined with their inherent excellent performance and the ability to digitally program (and reprogram) the output waveform, make DDS devices an extremely attractive solution—preferable to lessflexible solutions prising aggregations of discrete elements.What kind of outputs can I generate with a typical DDS device? Figure 2. Square, triangular, and sinusoidal outputs from a DDS.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 wave? Here’s a breakdown of the internal circuitry of a DDS device: its main ponents are a phase accumulator, a means of phasetoamplitude conversion (often a sine lookup 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 referenceclock frequency and the binar y number programmed into the frequency register (tuning word). Figure 3. Components of a direct digital synthesizer.The binary number in the frequency register provides the main input to the phase accumulator. If a sine lookup table is used, the phase accumulator putes a phase (angle) address for the lookup 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 number to a corresponding value of analog voltage or current. To generate a fixedfrequency 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 quickly through the sine lookup 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 plete DDS? The integration of a D/A converter and a DDS onto a single chip is monly known as a plete DDS solution, a property mon to all DDS devices from ADI. Let’s talk some more about the phase accumulator. How does it work? Continuoustime sinusoidal signals have a repetitive angular phase range of 0 to 2.The digital implementation is no different. The counter’s carry function allows the phase accumulator to act as a phase wheel in the DDS implementation.Figure 4. Digital phase wheel.To understand this basic function, visualize the sinewave oscillation as a vector rotating around a phase circle (see Figure 4). Each designated point on the phase wheel corresponds to the equivalent 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 plete cycle of the output sine wave. The phase accumulator provides the equally spaced angular values acpanying the vector’s linear rotation around the phase wheel. The contents of the phase a