【正文】 ctrum. For the very best SFDR, it is essential to begin with a highquality oscillator. SFDR is an important specification in an application where the frequency spectrum is being shared with other munication channels and applications. If a transmitter’s output sends spurious signals into other frequency bands, they can corrupt, or interrupt neighboring signals. Typical output plots taken from an AD9834 (10bit DDS) with a 50MHz master clock are shown in Figure10. In (a), the output frequency is exactly 1/3 of the master clock frequency (MCLK). Because of the judicious choice of frequencies, there are no harmonic frequencies in the 25MHz window, aliases are minimized, and the spurious behavior appears excellent, with all spurs at least 80 dB below the signal (SFDR = 80 dB). The lower frequency setting in (b) has more points to shape the waveform (but not enough for a really clean waveform), and gives a more realistic picture。 phase continuous frequency hops with no overshoot/undershoot or analogrelated loop settlingtime anomalies, ? the digital architecture of DDS eliminates the need for the manual tuning and tweaking related to ponent aging and temperature drift in analog synthesizer solutions, and ? the digital control interface of the DDS architecture facilitates an environment where systems can be remotely controlled and optimized with high resolution under processor control. How would I use a DDS device for FSK encoding? Binary frequencyshift keying (usually referred to simply as FSK) is one of the simplest forms of data encoding. The data is transmitted by shifting the frequency of a continuous carrier to one of two discrete frequencies (hence binary). One frequency, f1, (perhaps the higher) is designated as the mark frequency (binary one) and the other, f0, as the space frequency (binary zero). Figure 6 shows an example of the relationship between the markspace data and the transmitted signal.Figure 7. A DDSbased FSK encoder.Figure 6. FSK modulation.This encoding scheme is easily implemented using a DDS. The DDS frequency tuning word, representing the output frequencies, is set to the appropriate values to generate f0 and f1 as they occur in the pattern of 0s and 1s to be transmitted. The user programs the two required tuning words into the device before transmission. In the case of the AD9834, two frequency registers are available to facilitate convenient FSK encoding. A dedicated pin on the device (FSELECT) accepts the modulating signal and selects the appropriate tuning word (or frequency register). The block diagram in Figure 7 demonstrates a simple implementation of FSK encoding.And how about PSK coding? Phaseshift keying (PSK) is another simple form of data encoding. In PSK, the frequency of the carrier remains constant and the phase of the transmitted signal is varied to convey the information. Of the schemes to acplish PSK, the simplestknown as binary PSK (BPSK)—uses just two signal phases, 0 degrees and 180 degrees. BPSK encodes 0 phase shift for a logic 1 input and 180 phase shift for a logic 0 input. The state of each bit is determined according to the state of the preceding bit. If the phase of the wave does not change, the signal state stays the same (low or high). If the phase of the wave reverses (changes by 180 degrees), then the signal state changes (from low to high, or from high to low). PSK encoding is easily implemented with DDS ICs. Most of the devices have a separate input register (a phase register) that can be loaded with a phase value. This value is directly added to the phase of the carrier without changing its frequency. Changing the contents of this register modulates the phase of the carrier, thus generating a PSK output signal. For applications that require high speed modulation, the AD9834 allows the preloaded phase registers to be selected using a dedicated toggling input pin (PSELECT), which alternates between the registers and modulates the carrier as required. More sophisticated forms of PSK employ four or eight wave phases. This allows binary data to be transmitted at a faster rate per phase change than is possible with BPSK modulation. In fourphase modulation (quadrature PSK or QPSK), the possible phase angles are 0, +90, –90, and 180 degrees。外文翻譯（原文）畢業(yè)設(shè)計(jì)（外文翻譯材料）學(xué) 院：專 業(yè)：學(xué)生姓名：指導(dǎo)教師：電氣與電子工程學(xué)院電子信息工程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 va