allaboutdirectdigitalsynthesis-外文文献(编辑修改稿)

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ALGORITHMD/ACONVERTERFigure 5. Signal flow through the DDS architecture.What are popular uses for DDS? Applications currently using DDSbased waveform generation fall into two principal categories: Designers of munications systems requiring agile (., immediately responding) frequency sources with excellent phase noise and low spurious performance often choose DDS for its bination of spectral performance and frequencytuning resolution. Such applications include using a DDS for modulation, as a reference for a PLL to enhance overall frequency tunability, as a local oscillator (LO), or even for direct RF transmission.Alternatively, many industrial and biomedical applications use a DDS as a programmable waveform generator. Because a DDS is digitally programmable, the phase and frequency of a waveform can be easily adjusted without the need to change the external ponents that would normally need to be changed when using traditional analogprogrammed waveform generators. DDS permits simple adjustments of frequency in real time to locate resonant frequencies or pensate for temperature drift. Such Analog Dialogue 3808, August (2020) 3applications include using a DDS in adjustable frequency sources to measure impedance (for example in an impedancebased sensor), to generate pulsewave modulated signals for microactuation, or to examine attenuation in LANs or telephone cables. What do you consider to be the key advantages of DDS to designers of realworld equipment and systems? Today’s costpetitive, highperformance, functionally integrated DDS ICs are being mon in both munication systems and sensor applications. The advantages that make them attractive to design engineers include:• digitally controlled microhertz frequencytuning and subdegree phasetuning capability,• extremely fast hopping speed in tuning output frequency (or phase)。 phasecontinuous 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.MARK10SPACETIMEtDATASIGNALAMPLITUDEf0f1Figure 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.TUNINGWORD 1TUNINGWORD 2FSKDATA10CLOCKMUXDACDDSFigure 7. A DDSbased FSK encoder.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 im。