还有的DDS就像AD9850/AD9851那样，只能输出正弦波和方波，三角波需要外围电路。
http://en.wikipedia.org/wiki/Direct_digital_synthesishttp://zh.wikipedia.org/wiki/DDS
http://www.analog.com/library/analogDialogue/archives/38-08/dds.html
http://www.thinksrs.com/downloads/PDFs/ApplicationNotes/DDS.pdf
http://www.radio-electronics.com/info/receivers/synth_basics/dds.php
DDS Direct Digital Synthesis
Direct Digital Synthesis (DDS) is an electronic method for digitally creating arbitrary waveforms and frequencies from a single, fixed source frequency.
Overview
A 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 running.
Programming
In 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 data.
Running
In 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 waveform.
To 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 kHz.
This 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 counter.
Implementation details
Practical 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 MHz.
The 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 signal.
Implementations exist in both software and hardware. Due to the realtime nature of DDS, software implementations are usually limited to audio frequencies.
Applications of DDS are: function generators, mixers, modulators, and sound synthesizers.
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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 devicesthough they often have artistic uses as well.
There 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 applications.
Traditionally, signal generators have been embedded hardware units, but since the age of multimedia-PCs, flexible, programmable software tone generators have also been available.
General purpose signal generators
Function generators
A 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 waveform.
function generators are typically used in simple electronics repair and designwhere 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 generators
Main article: Arbitrary waveform generator
Arbitrary 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 waveformsan 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 bandwidthas a result, they are generally limited to higher-end design and test applications.
[edit] Special purpose signal generators
In addition to the above general-purpose devices, there are several classes of signal generators designed for specific applications.
[edit] Tone generators and audio generators
A 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 analyzers.
Many 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 errorsthey also often provide ways to manipulate the metadata associated with digital audio formats.
The term synthesizer is used for a device that generates audio signals for music, or that uses slightly more intricate methods.
[edit] Video signal generators
Main article: Video signal generator
A 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 formatsmany 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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