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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
Start | Design & Hardware | Assembly | Firmware | Calibration | Operating Procedure | Project Complete | Sources & PDF
OPERATING PROCEDURE
The commands, which may be listed by entering either H1 or H2 following the NimbleSig sign-on message, are listed in Table 1. The AF command sets the amplitude scale factor (ASF) register, which controls the RF output relative voltage level. For maximum output, enter AF1000000. For 50%, enter AF500000. For 10%, enter AF100000.
The B command, from B0 to B4, decreases the output level in 10-dB steps relative to maximum output. B5 turns the carrier off completely by setting the ASF register to 0.
The reciprocal C and S commands clear or set most active bits in the CFR1 and CFR2 registers, respectively. For example, the command C112 clears CFR1 bit 12, while the command S112 sets CFR1 bit 12. The changing of any bits that could disrupt control of the DDS is prevented and a “Cmd Error!” message is echoed.
New frequencies can be entered in hertz, kilohertz, or megahertz with the FH, FK, and FM commands. To set the frequency to 123,456,789 Hz, key in the following at the ">" prompt: FH123456789<enter>. To set the frequency to 150 MHz, simply key in: FM150<enter>.
MA and MF turn on AM and FM modulation, respectively, in accordance with the settings of the MD, MP (as appropriate), and MQ commands.
R permits the reading of the DDS registers. SP permits the setting of the phase offset in millidegrees. Z can be used to put the MPU to sleep.
project complete
In general, the NimbleSig design has met my objectives. I found that Keil’s IDE and Luminary Micro’s EVM worked well together. Luminary Micro’s API library for its LM3S811 greatly simplified the writing of the firmware. I think it should be quite easy for others to pick up where I left off to customize the firmware to a specific application.
My only regret was my selection of a power regulator in a package that was difficult to solder without an oven. If there is sufficient interest demonstrated in NimbleSig, I may redesign the PCB artwork for power regulator chips in packages that are easier to work with.
I did my best to make the overall content of this article as factual as possible; however, there are no guarantees implied about anything presented here. Thus, anyone who uses my NimbleSig design or any portion of it must accept all of the associated risks.
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