What is Direct Digital Synthesis?

Direct digital synthesis (DDS) is a method of producing an analog waveform -- usually a sine wave -- by generating a time-varying signal in digital form and then performing a digital-to-analog 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 compact draw little power.

Why would one use a direct digital synthesizer (DDS)? Aren't there other methods for easily generaing frequencies?

The ability to accurately produce and control waveforms of various frequencies and profiles has become a key requirement common to a number of industries. Whether providing agile sources of low-phase-noise variable-frequencies with good spurious performance for communication, or simply generating a frequency stimulus in industrial or biomedical test equipment applications, convenience, compactness, and low cost are important design considerations. Many possibilities for frequency generation are open to a designer, ranging form phase-locked-loop (PLL) -based techinques for very high-frequency sunthesis, to dynamic programming of digital-to-analog converter (DAC) outputs to generate arbitrary waveforms at lower frequencies. But the DDS technique is rapidly gaining acceptance for solving frequency- (or waveform) generation requirements in both communications and industrial applications because singal-chip IC devices can generate programmable analog output waveforms simply and with high resolution and accuracy. Furthermore, the continual improvements in both process technology and design have resulted in cost and power consumption levels that were previously unthinkably low. For example, the AD9833, a DDS-based programmable waveform generator, operating at 5.5V with a 25MHz clock, consumes a maximum power of 30 milliwatts.

What are the main benefits of using a DDS?

DDS devices like the AD9833 are programmed through a high speed serial peripheral interface (SPI), and need only an external clock to generate simple sine waves. DDS devices are now available that can generate frequencies from less than 1 Hz up to 400 MHz (based on a 1-GHz clock). The benefits of their low power, low cost, and single small package, combined with their inherent excellent performance and the ability to digitally program (and reprogram) the output waveform, make DDS devices an extremely attractive solution -- preferable to less-flexibel solution comprising aggregations of discrete elements.

How does a DDS device create a sine wave?

Here's a breakdown of the internal circuitry of a DDS device: its main components are a phase accumulator, a means of phase-to-amplitude conversion (often a sine look-up table), and a DAC. A DDS produces a sine wave at a given frequency. The frequency depends on two variable, the reference-clock frequency and the binary number programmed into the frequency register (tuning word). The binary number in the frequency register provides the main input to the phase accumulator. If a sine look-up table is used, the phase accumulator computes a phase (angle) address for the look-up table, which outputs the digital value of amplitude -- corresponding to the sine of that phase angle -- to the DAC. The DAC, in turn, converts that number to a corresponding value of analog voltage or current. To generate a fixed-frequency sine wave, a constant value (the phase increment -- which is determined by the binary number) is added to the phase accumulator with each clock cycle. If the phase increment is large, the phase accumulator will step quickly through the sine look-up table and thus generate a high frequency sine wave. If the phase increment is small, the phase accumulator will take many steps, accodingly generating a slower waveform.

What do you mean by a complete DDS?

The integration of a D/A converter and a DDS onto a single chip is commonly known as a complete DDS solution, a property common to all DDS devices from ADI.