外文文献1原文：DDS devices to produce high-quality waveform: a simple, efficient and flexibleSummaryDirect digital frequency synthesis (DDS) technology for the generation and regulation of high-quality waveforms, widely used in medical, industrial, instrumentation, communications, defense and many other areas. This article will briefly describe the technology, on its strengths and weaknesses, examine some application examples, and also introduced some new products that contribute to the promotionIntroductionA key requirement in many industries is an exact production, easy operation and quick change of different frequencies, different types of waveforms. Whether it is broadband transceiver requires low phase noise and excellent spurious-free dynamic performance of agile frequency source, or for industrial measurement and control system needs a stable frequency excitation, fast, easy and economical to produce adjustable waveform while maintaining phase continuity capabilities are critical to a design standard, which is what the advantages of direct digital frequency synthesis.Frequency synthesis taskThe growing congestion of the spectrum, coupled with lower power consumption, quality of never-ending demand for higher measuring equipment, these factors require the use of the new frequency range, requires a better use of existing frequency range. A result, the search for better control, in most cases, by means of frequency synthesizer for frequency generation. These devices use a given frequency, fC of to generate a target frequency (and phase) fOUT the general relationship can be simply expressed as:fOUT = x fCAmong them, the scale factor x, sometimes known as the normalized frequency.The equation is usually gradual approximation of the real number algorithms. When the scale factor is a rational number, two relatively prime numbers (output frequency and reference frequency) than the harmonic. However, in most cases, x may belong to a broader set of real numbers, the approximation process is within the acceptable range will be truncatedDirect Digital Frequency SynthesizerThe frequency synthesizer a practical way to achieve is the direct digital frequency synthesis (of DDFS), usually referred to as direct digital synthesis (DDS). This technique using digital data processing to generate a frequency and phase adjustable output, the output anda fixed frequency reference clock source fC. related. DDS architecture, the reference or the system clock frequency divided by a scale factor to produce the desired frequency, the scale factor is controlled by the binary tuning word programmable.In short, direct digital frequency synthesizer to convert a bunch of clock pulses into an analog waveform, usually a sine wave, triangle wave or square wave. Shown in Figure 1, its main parts: the phase accumulator (to produce the output waveform phase angle data), relative to digital converter, (above the phase data is converted to the instantaneous output amplitude data), and digital-to-analog converter (DAC) (the magnitude of data into a sampled analog data points)Figure 1.DDS function of the system block diagram.For the sine wave output, relative to digital converter is usually a sine lookup table (Figure 2). Phase accumulator unit count N a relative to the frequency of fC, according to the following equation:The number of pulses of the fC:M is the resolution of the tuning word (24-48) N corresponds to the smallest increment of phase change of the phase accumulator output wordFigure 2. Typical DDS architecture and signal path (with DACs).Changing N will immediately change the output phase and frequency, so the system has its own continuous phase characteristics, which is one of the key attributes of many applications. No loop settling time, which is different from the analog system, such as phase-locked loops (PLLs). DAC is usually a high-performance circuit, designed specifically for the DDS core (phase accumulator and phase amplitude converter). In most cases, the results of the device (usually single-chip) is generally referred to as the pure DDS or the C-DDS.Actual DDS devices are generally multiple registers, in order to achieve a different frequency and phase modulation scheme. Such as phase register, their storage phase of increase in the output phase of the phase accumulator. In this way, the corresponding delay output sine wave phase in a phase tuning word. This is useful for phase modulation applications for communication systems. The resolution of the adder circuit determines the number of bits of the phase tuning word, therefore, also decided to delay the resolution.Integrated in a single device on the engine of a DDS and a DAC has both advantages and disadvantages, however, whether integrated or not, need a DAC to produce ultra-high purity high-quality analog signal. DAC will convert digital sinusoidal output to an analog sine wave may be single-ended or differential