Originally posted Oct 10, 2014
RF and microwave applications get their own benefits from semiconductor advances
Gordon Moore is well known for his 1965 prediction that the number of transistors in high-density digital ICs would double every two years, give or take. While the implications for processors and memory are well understood, perhaps only RF and microwave engineers recall Moore’s other prediction in that same paper: “Integration will not change linear systems as radically as digital systems.”
Though it’s hard to quantify, it seems that the pace of advances in combined digital/analog circuits is somewhere in the middle: slower than that of processors and memory, but faster than purely analog circuits. To many of us, the actual rate means a lot because analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) bring the power, speed and flexibility of digital circuits to real-world challenges in electronic warfare (EW), wireless, radar, and beyond.
Direct digital synthesis (DDS) technologies are an excellent example, and they’re becoming more important and more prominent in demanding RF and microwave applications. The essentials of DDS are straightforward, as shown in the diagram of a signal source below.
In DDS, memory and DSP drive an RF-capable DAC, and its output is filtered to remove harmonics and out-of-band spurs. Other spurs must be kept inherently low by the design of the DAC itself.
Deep memory and high-speed digital processing—using ASICs and FPGAs—have long been used to drive DACs. Unfortunately, most DACs have been too narrowband for frequency synthesis, and wideband units lacked the necessary signal purity. The Holy Grail has been a DAC that can deliver instrument-quality performance over wide RF bandwidths.
Engineers at Keysight Technologies (formerly Agilent) realized that new semiconductor technology was intersecting with this need. They used an advanced silicon-germanium BiCMOS process to fabricate a DAC that is the perfect core for a DDS-based RF/microwave signal generator. Signal purity is excellent even at microwave frequencies, as shown in the spectrum measurement below.
A 10 GHz CW signal measured over a 20 GHz span, showing the output purity of DDS technology in the Keysight UXG agile signal generator.
Compared to traditional phase-locked loops (PLLs) and direct analog synthesizers, DDS promises a number of advantages:
- Frequency and amplitude agility. With no loop-filter settling, new output values can be implemented at the next DAC clock cycle. As a result, the UXG can switch as fast as 250 ηs.
- Multiple, coherent signals can be generated from one source. DDS can generate both CW and complex or composite signals, continuously or in a changing sequence. This enables generation of scenarios instead of just signals, making the technology well-suited to EW or signal-environment simulation.
- No phase noise pedestal from a PLL, and no need to trade phase noise performance for agility. PLLs provide a wide frequency range, high resolution and good signal quality, but often require tradeoffs between frequency agility and phase noise performance.
- Signal coherence, phase continuity and signal generator coordination. Multiple signals from a single generator can be aligned in any way desired, and switching can be phase continuous. Triggers and a shared master clock allow multiple DDS generators to produce coordinated outputs easily and with great precision.
With sufficient DAC performance, DDS is clearly a good fit for signal generation in radar and EW, which need agility and wide bandwidth. DDS also can be valuable in signal analyzers and receivers because fast sweeping/tuning and the lack of a phase noise pedestal enables LO designs with better performance and fewer tradeoffs.
DDS implementations are generally more expensive than PLL technologies. However, as Moore predicted, technological evolution creates a dynamic environment in which the optimal solutions change over time. It seems clear that DDS will have an expanding role in better RF measurements, even if it doesn’t happen at the pace of Moore’s law.
For more about the use of DDS in a specific implementation, go to www.keysight.com/find/UXG and download a relevant app note.