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Issue #208 November 2007
Analog Techniques
NimbleSig
A Compact DDS RF Signal Generator
by Thomas Alldread
Second Prize Luminary Micro DesignStellaris2006 Contest
The NimbleSig is a versatile DDS RF signal generator built around a Luminary Micro LM3S811 microcontroller. This handy system, which provides a frequency-agile RF output signal source with 1-Hz step resolution, is also capable of low-level (–50 to 10 dBm) RF power measurement.
Start | Design & Hardware | Assembly | Firmware | Calibration | Operating Procedure | Project Complete | Sources & PDF
Modern integrated circuits make the generation of stable RF signals a relatively trivial chore compared to the technology of earlier times. Typical 1960s-era VHF signal generators were relatively huge (about the size of carry-on-luggage) and somewhat impractical to use because of poor frequency stability. The manual process of tweaking the signal to within 1000 Hz of the desired frequency took some tedious fine-tuning. The low-level RF power meters of that era often needed to be renormalized to zero every few minutes.
With current technology, it is now possible to construct a shirt-pocket-size VHF/RF signal source/power meter module that maintains 10-Hz stability at room temperature for days on end and can be readily shifted in frequency in 1-Hz step increments. It takes only one additional IC to add a stable RF power meter capable of measuring levels lower than a nanowatt. The $100 component cost for such a modern-day, dual-function module seems like a techno-economic boon when you consider the performance and original cost of legacy RF test equipment. RF instrumentation is yet another area of technology where there have been huge improvements during the past few decades.
From the introduction of relatively low-priced direct digital synthesis (DDS) and logarithmic RF detector ICs, I have built various low-cost RF signal sources and detectors based on new semiconductors as they have become available. During the past decade, I have witnessed a lot of improvement in DDS technology. The maximum output frequency of my first DDS generator was limited to about 20 MHz. Current technology now supports 10 times that. Recently developed DDS ICs that are capable of generating 400-MHz signals are now becoming available at more attractive costs as technology continues to evolve.
In this article, I will describe a DDS-based RF signal generator/level detection module. It is intended to be used either with a PC as a signal generator or integrated into equipment that requires an integral RF signal source. I named the module NimbleSig for its quick-frequency agility, which enables it to act as a signal source across more than 11 octaves of spectrum (see Photo 1). The NimbleSig design uses a Fox Electronics FOX924B TCXO for a reference oscillator source, which provides good frequency stability. DDS control is provided by a 50-MHz, 32-bit Luminary Micro LM3S811 microcontroller linked to the DDS chip at a data rate of 25 Mbps, which is fast enough to accommodate many modulation needs.
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| Photo 1a—This is the completed NimbleSig module. It is framed with a caliper to illustrate its small size in both English and metric units. b—The NimbleSig output spectrum analyzer display spanning 30 to 50 MHz. A 40-MHz full signal is at –10 dBm in the center of the screen. |
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