Direct digital synthesis (DDS)
Direct Digital Synthesis (DDS) is a modern method of generating multiple frequencies from a single reference frequency source, primarily used in instrumentation and communication systems. It operates by digitally creating arbitrary waveforms, such as sine waves, and then converting these digital signals into analog form through a digital-to-analog converter (DAC). DDS offers several advantages over traditional analog systems, including rapid frequency switching, fine frequency resolution, and a wide operational frequency range, making it suitable for various applications such as function generators, local oscillators, and digital sound synthesis.
The core of a DDS system consists of a frequency reference, a numerically controlled oscillator (NCO), and a DAC. The NCO allows for precise control of output frequency and phase, while the DAC is responsible for producing the final analog output signal, which is influenced by factors such as spectral purity and phase noise. Despite its flexibility, a basic DDS system has limitations in frequency adjustment, which can be improved in more advanced implementations. Overall, DDS is an essential technology in contemporary electronic design, providing efficiency and precision in signal generation across multiple fields.
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Direct digital synthesis (DDS)
Direct digital synthesis (DDS) developed as a result of the widespread use of digital techniques in instrumentation and communications systems. It is a method that is digitally controlled and generates multiple frequencies from a reference frequency source. More precisely, it is a type of frequency synthesizer utilized for creating arbitrary waveforms from a single, fixed-frequency reference clock.
DDS is a means of producing an analog waveform (i.e., a sine wave) by generating a time-varying signal in digital form and thereafter carrying out a digital-to-analog conversion. Operations within a DDS device are primarily digital in nature. Therefore, DDS allows fast switching between output frequencies, fine frequency resolution, and operation over a broad spectrum of frequencies. Modern DDS devices are very compact and exert little power due to advances and developments in design and process technology.
Overview
DDS has multiple applications that involve signal generation, local oscillators in communication systems, function generators, mixers, modulators, sound synthesizers, and part of a digital phase-locked loop. The method sometimes plays a role in digital sound recording.
A DDS system possesses much better frequency agility, improved phase noise, and precise control of the output phase across frequency switching transitions than its analog counterparts. A DDS system is a sampled data system, which is why all the issues involved in sampling are considered, including quantization noise, aliasing, filtering, and so on.
A basic direct digital synthesizer consists of a frequency reference (referred to as a crystal or SAW oscillator), a numerically controlled oscillator (NCO), and a digital-to-analog converter (DAC).
In the basic architecture of DDS, a stable clock drives a programmable-read-only-memory (PROM), a type of computer memory, that stores one or more integral number of cycles of a sine wave (or other arbitrary waveform). As the address counter steps through each memory location, the corresponding digital amplitude of the signal at each location drives a DAC. The latter in turn generates the analog output signal. The spectral purity of the final analog output signal is determined primarily by the DAC. The phase noise is the reference clock.
The major problem with a simple DDS system is that the final output frequency can be changed only by changing the reference clock frequency or by reprogramming the PROM, making it rather inflexible. A practical DDS system implements this basic function in a much more flexible and efficient manner and uses an NCO. The value stored in the frequency control register (FCR) determines the output frequency of a DDS. The former in turn controls the NCO’s phase accumulator step size. The NCO operates in the discrete-time domain. Therefore, it changes frequency instantaneously at the clock edge coincident with a change in the value stored in the FCR.
The phase response of the reconstruction filter determines the DDS output frequency settling time. An ideal reconstruction filter with a linear phase response (meaning the output is simply a delayed version of the input signal) would allow instantaneous frequency response at its output.
Bibliography
Brandon, David. Direct Digital Synthesizers in Clocking Applications. Norwood: Analog Devices, 2006. Print.
Edgar, Thomas F., and Duncan A. Mellichamp. Process Dynamics and Control. Hoboken: Wiley, 2010. Print.
Jouko, Vankka, and Kari A. I. Halonen. Direct Digital Synthesizers: Theory, Design and Applications. Boston: Kluwer, 2010. Print.
Surber, Jim, and Leo McHugh. “Single-Chip Direct Digital Synthesis vs. the Analog PLL.” Analog Dialogue 30.3 (1996): n. pag. Web. 20 Dec. 2013.
Zumbahlen, Hank. Basic Linear Design, Analog Devices, 2006. Amsterdam: Elsevier-Newnes, 2008. Print.