天一布艺镇海店
DDS and converter form signal generatorMany applications require low-frequency signal generators that can deliver high-performance, high-resolution signals. This Design Idea presents a circuit that generates frequencies of 0 to 1 MHz. Sinusoidal, triangular, and square-wave outputs are available. You can achieve frequency resolution of better than Hz and phase resolution of better than °; thus, you can program exact coherent frequencies. This feature is useful in digital modulation and frequency-tuning applications. The circuit uses the ADµC831 and AD9834 to generate the required frequencies (Figure 1). You can program the microcontroller from either a PC or a Unix-based workstation. You then program the AD9834 using a three-wire serial interface via the microcontroller. The interface word is 16 bits can program the AD9834 to provide sinusoidal, triangular, and square-wave outputs using the DDS (direct-digital-synthesis) architecture. The chip operates as an NCO (numerically controlled oscillator) using an on-chip, 28-bit phase accumulator, sine-coefficient ROM, and a 10-bit D/A converter. You typically consider sine waves in terms of their magnitude form, A(t)=sin(ωt). The amplitude is nonlinear and is, therefore, difficult to generate. The angular information, on the other hand, is perfectly linear. That is, the phase angle rotates through a fixed angle for each unit in time. Knowing that the phase of a sine wave is linear, and, given a reference interval (clock period), you can determine the phase rotation for that period: ΔPhase=ω dt; ω=ΔPhase/dt=2πf, and f=(ΔPhase×fMCLK)/(2π), where dt=1/fMCLK, and fMCLK is the master this formula, you can generate output frequencies, knowing the phase and master-clock frequency. The phase accumulator provides the 28-bit linear phase. The amplitude coefficients of the output sine wave are stored in digital format in the sine-coefficient ROM. The DAC converts the sine wave to the analog domain. If you bypass the ROM, the AD9834 delivers triangular waveforms instead of sinusoidal waveforms. A square-wave output is also available on the part. Figure 2 shows the various waveforms available from the system. As shown in Figure 1, the sinusoidal/ triangular output waveforms are available on the IOUT pin (Pin 19); and the square wave output is available on the SIGN_BIT_OUT pin (Pin 16). You program the DDS by writing to the frequency registers. The analog output from the part is then: fOUT=fMCLK/228×(frequency-register word).The outputs of the DDS have 28-bit resolution, so effective frequency steps on the order of Hz are possible to a maximum of approximately 1 MHz. Figure 2 shows the typical waveform outputs. Two phase registers are available that allow 12-bit phase resolution. These registers phase-shift the signal by: Phase shift=2π/4096×(phase-register word).A 50-MHz crystal oscillator provides the reference clock for the DDS. The output stage of the DDS is a current-output DAC loaded by an external resistor. A 200Ω resistor generates the required peak-to-peak voltage range. The output is ac-coupled through capacitor C1. The MicroConverter contains two on-chip, 12-bit DACs. DAC1 varies the current through R5, adjusting the full-scale current of the DDS via the FSADJUST pin. The equation to control the full-scale current of the DDS DAC is: IOUT (full-scale)=18×I×, the internal reference of the MicroConverter, and op amp 2 allow for offset control of the output voltage of the DDS. You can program this dc offset to ±10V at 10-bit resolution. When R1=R2 and the gain of op amp 2=8, then the output of op amp 2 is: VOUT=(DAC output–(VREF/2))×8, yielding a ±10V R6 through R9 allow for control of gain through op amp 3. The gain of the op amp is a function of resistor switching, which you enable using the RDRIVE pin available on the MicroConverter. This operation allows for an effective programmable-output amplitude of approximately ±10V p-p. Thus, the circuit allows for programmable sinusoidal and triangular waves, including dc offsets, and the ability to set peak-to-peak amplitude of approximately ±10V. The square wave output on the SIGN_BIT_OUT pin has 0 to 5V amplitude. For low-frequency operation, a lowpass filter normally serves to filter reference-clock frequencies, spurs, and other images. For applications in which the output signal needs amplification, you should use a narrowband filter to filter out unwanted noise before the gain stage. A third-order filter would be good enough to remove most of the unwanted noise. Figure 3 shows a typical spectral plot of the output. Applications for this circuit range from signal-waveform generation to digital modulation. You can use the system in frequency-sweeping and -scanning applications and in resonance applications that use the frequency as an excitation signal to determine circuit resonance. Another useful application is as a reference oscillator for a PLL system.
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2-3个月。因为converter论文从见刊到检索，需要一定的时间，具体多久与各converter期刊有关，一般是2-3个月左右。此时期刊方会把发表见刊的论文，送检converter数据库收录。
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; Direct Digital SynthesisDirect Digital Synthesis (DDS) is an electronic method for digitally creating arbitrary waveforms and frequencies from a single, fixed source basic DDS circuit consists of an electronic controller, a random-access memory, a frequency reference (usually a crystal oscillator), a counter and a digital-to-analogue converter (DAC). Two operating steps are required to make the device work: we shall call these programming and the programming step, the electronic controller fills the memory with data. Each item of data is a binary word representing the amplitude of the signal at an instant of time. The array of data in the memory then forms a table of amplitudes, with time implied by the position in the table. If, for example, the first half of the table were filled with zeroes and the second half with values of 100%, then the data would represent a square wave. Any other wave shape can be created simply by altering the the running step, the counter (properly called the phase accumulator) is instructed to advance by a certain increment on each pulse from the frequency reference. The output of the phase accumulator (the phase) is used to select each item in the data table in turn. Finally, the DAC converts this sequence of data to an analogue generate a periodic waveform, the circuit is set up so that one pass through the table takes a time equal to the period of the waveform. For example, if the reference frequency is 1 MHz, and the table contains 1000 entries, then a complete pass through the table with a phase increment of 1 will take 1000 / 1 MHz = 1 ms, so the frequency of the output waveform will be 1/(1 ms) = 1 system can generate a higher output frequency simply by increasing the phase increment so that the counter runs through the table more quickly. In the example above, the phase increment is equal to 1, so the next possible frequency is obtained by setting the increment to 2, resulting in a doubling of output frequency. To obtain a finer control of frequency than this, the standard phase increment can be set to, say, 10. This then allows slightly higher or lower output frequencies. For example, increasing the increment to 11 would increase the output frequency by 10%, and reducing it to 9 would decrease the output frequency by the same proportion. The more precision required over the frequency, the more bits are needed in the detailsPractical implementations usually set the size of the lookup table to be a power of 2 and work with 32-bit phase accumulators and phase increments. Usually the upper 8 or 10 bits of the counter are used as lookup table index (lookup table size is 256 or 1024, respectively). The remaining lower bits can be used as a parameter or index to interpolate between the adjacent entries in the lookup table. Often linear interpolation suffices. The source frequency usually comes from a crystal of 1 MHz to 100 highest frequency that can be generated this way depends on the size of the lookup table and the frequency. In order to generate a reasonable representation of the waveform, at least a minimum number of samples must be taken from it. If the phase increment becomes too large, then the counter would step through the lookup table too fast and the result may be a severe distortion of the output exist in both software and hardware. Due to the realtime nature of DDS, software implementations are usually limited to audio of DDS are: function generators, mixers, modulators, and sound 信号发生器 Signal Generator A signal generator, also known variously as a test signal generator, function generator, tone generator, arbitrary waveform generator, or frequency generator is an electronic device that generates repeating electronic signals (in either the analog or digital domains). They are generally used in designing, testing, troubleshooting, and repairing electronic or electroacoustic devices; though they often have artistic uses as are many different types of signal generators, with different purposes and applications (and at varying levels of expense); in general, no device is suitable for all possible , signal generators have been embedded hardware units, but since the age of multimedia-PCs, flexible, programmable software tone generators have also been purpose signal generatorsFunction generatorsA function generator is a device which produces simple repetitive waveforms. Such devices contain an electronic oscillator, a circuit that is capable of creating a repetitive waveform. (Modern devices may use digital signal processing to synthesize waveforms, followed by a digital to analog converter, or DAC, to produce an analog output). The most common waveform is a sine wave, but sawtooth, step (pulse), square, and triangular waveform oscillators are commonly available as are arbitrary waveform generators (AWGs). If the oscillator operates above the audio frequency range (>20 kHz), the generator will often include some sort of modulation function such as amplitude modulation (AM), frequency modulation (FM), or phase modulation (PM) as well as a second oscillator that provides an audio frequency modulation generators are typically used in simple electronics repair and design; where they are used to stimulate a circuit under test. A device such as an oscilloscope is then used to measure the circuit's output. Function generators vary in the number of outputs they feature, frequency range, frequency accuracy and stability, and several other parameters.[edit] Arbitrary waveform generatorsMain article: Arbitrary waveform generatorArbitrary waveform generators, or AWGs, are sophisticated signal generators which allow the user to generate arbitrary waveforms, within published limits of frequency range, accuracy, and output level. Unlike function generators, which are limited to a simple set of waveforms; an AWG allows the user to specify a source waveform in a variety of different ways. AWGs are generally more expensive than function generators, and are often more highly limited in available bandwidth; as a result, they are generally limited to higher-end design and test applications.[edit] Special purpose signal generatorsIn addition to the above general-purpose devices, there are several classes of signal generators designed for specific applications.[edit] Tone generators and audio generatorsA tone generator is a type of signal generator optimized for use in audio and acoustics applications. Tone generators typically include sine waves over the audio frequency range (20 Hz–20 kHz). Sophisticated tone generators will also include sweep generators (a function which varies the output frequency over a range, in order to make frequency-domain measurements), multitone generators (which output several tones simultaneously, and are used to check for intermodulation distortion and other non-linear effects), and tone bursts (used to measure response to transients). Tone generators are typically used in conjunction with sound level meters, when measuring the acoustics of a room or a sound reproduction system, and/or with oscilloscopes or specialized audio tone generators operate in the digital domain, producing output in various digital audio formats such as AES-3, or SPDIF. Such generators may include special signals to stimulate various digital effects and problems, such as clipping, jitter, bit errors; they also often provide ways to manipulate the metadata associated with digital audio term synthesizer is used for a device that generates audio signals for music, or that uses slightly more intricate methods.[edit] Video signal generatorsMain article: Video signal generatorA video signal generator is a device which outputs predetermined video and/or television waveforms, and other signals used to stimulate faults in, or aid in parametric measurements of, television and video systems. There are several different types of video signal generators in widespread use. Regardless of the specific type, the output of a video generator will generally contain synchronization signals appropriate for television, including horizontal and vertical sync pulses (in analog) or sync words (in digital). Generators of composite video signals (such as NTSC and PAL) will also include a colorburst signal as part of the output. Video signal generators are available for a wide variety of applications, and for a wide variety of digital formats; many of these also include audio generation capability (as the audio track is an important part of any video or television program or motion picture).
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