Direct digital synthesis (DDS), as used to denote a direct digital synthesizer device, is the use of digital-to-RF conversion to directly generate RF signals from digital signals. In some cases, DDS RF signals are fully modulated and ready to be amplified and sent along to an RF antenna for transmission. In other cases, a DDS device/system may be tasked to create tones that are later modulated and conditioned by RF hardware. Often with DDS systems, the modulation, phase tuning, amplitude control, etc. are performed prior to RF synthesis by antenna processors, multi-input multi-output (MIMO) processors, or other digital signal processors (DSPs) that feed the digital-to-analog (DAC) converter capable of outputting high fidelity RF signals.
DDS is used in a variety of ways. More recently, DDS have been used instead of a variety of analog hardware for communications transmission circuits. With these circuits a superheterodyne architecture would be used, where a communication signal is frequency converted to a higher frequency, an intermediate frequency (IF) stage, at least one time prior to amplification and transmission. In this case the DDS is replacing virtually all the RF hardware prior to the amplifier, antenna selection hardware, and antenna in the transmission chain. This leads to a much more configurable, compact, and possibly lower cost system. These are key concerns for designers of communications and sensing system designers working on 5G, WiFi, or the latest radar hardware that employs advanced/active antenna systems (AAS).
Though DDS hardware is by no means inexpensive, it provides some size, weight, power, and cost (SWAP-C) benefits compared to the repeated filtering, mixing, local oscillator, and power systems needed to run the active components a DDS system can replace. In essence, a DDS device can replace all of the analog/RF hardware from the frequency synthesizer to the transmitter, which could also result in other signal quality benefits, such as linearity, harmonics, and spurs, if the DDS device doesn’t generate comparable signal quality impairments itself.
With high performance applications, such as complex radar and test equipment, there are other advantages to DDS. A DDS system, if designed using a numerically controlled oscillator, can often be made to change the frequency of the output in a single clock cycle. Moreover, these changes are phase continuous and repeatable, resulting in inphase signals at multiple frequencies. This results in a decrease in uncertainty in regards to phase errors, which is especially critical for test devices or sources that use multiple source generators, such as multiport vector network analyzers (VNAs).
In general, DDS can potentially outperform analog/RF hardware in terms of frequency agility, phase noise, and controllability. However, DDS systems are also known to generate substantial cross spurs from high order Nyquist images, a higher noise floor at large frequency offsets, and requires an analog reconstruction low-pass filter, which may not be trivial to implement. These spurs may result in intermodulation distortion in some applications, which may require thorough filtering and other precautions to prevent.
The major limitation of DDS devices, until recently, has been the frequency limitations of digital-to-analog converter technology. However, there are now DDS-based frequency synthesizers capable of generating RF signals in the tens of gigahertz with very high bandwidths. Though DDS devices are taking the traditional place of RF hardware in some signal chains, this does not mean that DDS are a good fit for every application and are often designed for specific communications or sensing use cases that may not allow for their wide applicability. Moreover, applications with extreme performance demands will still likely default to precision RF hardware that has been established over decades, in some cases.