Complete Direct Digital Synthesis (DDS) is a technique for producing an analog waveform by manipulating a fixed system clock digitally then running the output through a DAC. This allows for fine frequency resolution over a wide range of frequencies and quick switching between those frequencies.
The Process of Generating a Clock from a DDS, taken from Analog Devices application note AN-823
How Direct Digital Synthesis Works: Digging Deeper
Most fundamental circuit components like microcontrollers require a single clock frequency for timing and control purposes. This can be achieved using a standard oscillatorthat emits one pre-determined frequency. This may sound a little new-age, but it comes up in very different conversations for a pretty piece of quartz emitting a frequency believed to resonate with a particular chakra. The frequency of a crystal oscillator is dependent on piezoelectric properties of the physical material in the device. While ICs like clock dividers and multipliers exist and it is possible to make slight adjustments to the frequency by changing the surrounding components, you are generally locked into one frequency per crystal.
Some devices like RF exciters require multiple frequencies. Even if these frequencies are not required concurrently, each frequency that is not an exact multiple of another necessitates its own oscillator. Most of these multi-frequency applications also require the ability to hop around between frequencies quickly and dynamically, which demands a level of control that can prove impossible for a system that must rapidly select between multiple distinct crystals.
Beyond Direct Digital Synthesis: Phase Locked Loop
Direct digital synthesis is not the only way to generate an arbitrary waveform. Phase Locked Loop (PLL) based frequency synthesizers are fine when the latency and size of analog components are not a concern and low frequency synthesis can be achieved using just a digital to analog converter (DAC) and some clever programming. However, applications that require the agility and precision of digital control almost exclusively rely on DDS.
Analog Devices has a phenomenal document by Eva Murphy and Colm Slattery that explains the math and logic around direct digital synthesis in more detail, but the basic principle can be explained with a picture:
Components of a Direct Digital Synthesizer from Analog Devices’ Ask the Application Engineer #33
DDS uses a known system clock and a number provided by the digital processor to create a sine wave based on the position in a look up table to which the number corresponds.
Analog Devices dominates the conversation around the practical implementation of DDS with families of low-power ICs that perform all the stages of DDS in a single package.
The functional block diagram of the AD9833, one of their most popular synthesizers, bears clear resemblance to the image above.
Functional Block Diagram of Analog Devices’ AD9833
This small device is capable of producing sine, triangular, and square output waves, and the output frequency and phase are fully software programmable. The 10-pin automotive rated IC communicates with a microcontroller or DSP via 3-wire SPI and consumes only milliwatts of power.
Understanding Tunable Precision of Direct Digital Synthesis
The tunable precision of a direct digital synthesis IC depends on the frequency registers and supplied clock frequency. The wider the registers are, the more unique slices can be made from the clock frequency. For example, the AD9833 has a bus width of 28. This means it can store 28 bits of binary data, or split a clock frequency up 268,435,455 ways in base 10.
If the supplied clock is running at 1MHz, the device would be capable of an output precision of 1MHz divided by 268,435,455, or about 0.004Hz. At 25MHz, the maximum input frequency for this particular device, the output can be controlled to 0.1Hz nearly instantaneously.
Applications like agile local oscillators (LO) often require higher frequencies, but can accept lower resolution.
The AD9914 from Analog Devices only uses a 16-bit wide frequency register, but it can use an input frequency up to 3.5GHz. This gives coarser resolution of achievable frequencies but enables frequency hopping with the speed of digital technology across high frequencies with high accuracy.
Direct digital synthesis is not the ultimate solution for every application. If you are building an IoT device that you want to enable for both 2.4GHz and 5GHz, you do not need ultra-fast frequency hopping. However, if you are building something that needs to fly though a wide range of frequencies like a chirp source or polar modulator, direct digital synthesis ICs make it easy to integrate this highly complex synthesis method into your designs.
Popular DDS Synthesizers
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Want to know more? See how Direct Digital Synthesis compares to Direct Analog Synthesis.
See how we got to direct digital synthesis with this infographic showing frequency generation over time.