Direct Digital Synthesis (DDS) is a method for generating arbitrary frequency sine waves with high accuracy and spectral purity. The ability to generate spectrally pure sine waves at programmable arbitrary frequencies is useful for instrumentation, communications products, and other applications. Today’s Direct Digital Synthesis components (DDS) provide highly accurate, spectrally pure sine wave generation with reasonable power requirements. Modern DDS components have added communications features such as chirp programming, FM modulation, AM modulation, programmable gain settings, and the ability to generate period digital data strobes. All terrific features for an electronic engineer’s bag of tricks.
Orchid Technologies recently applied two Analog Devices 48-bit data word DDS components in one of its newly completed instrument designs. The 48-bit components replaced an older product generation’s 32 Bit DDS system. The new 48-bit data word device enabled us to combine the previous discrete waveform generation and gain functions into a single higher performance component.
Removal of the external multiplying digital to analog converter enabled us to remove a significant source of spectral noise and impurity while simplifying the digital data signal path.
New DDS component technology utilizes a 48 bit data word to synthesize output sine wave frequency. Use of a 48 bit data word reduces the output sine wave frequency error by a factor of 10E4. 48 bit DDS output frequency is calculated as follows:
Output F = (Programmed Data Word * Base Frequency) / 281474976710656.0
Older 32 Bit DDS devices operated with a base frequency of about 75MHz, today’s modern DDS devices operate with a frequency of 300MHz. The base frequency increase permits easier low pass filter design for modern applications. Crunching the math and comparing 32-Bit 75MHz devices with 48-Bit 300MHz devices yields the following results:
|Target Frequency (Hz)||Percent Error 48 Bit DDS||Percent Error 32 Bit DDS|
Qualitatively, one can see that both DDS devices have increased frequency error at lower frequencies. One can also see that at higher target frequencies, the 48 Bit DDS is approximately 105 times more accurate than its older 32 bit counterpart. In the design of precision instrumentation, the reduced frequency error results in far better instrument performance.
Achieving the most out of our 48-bit DDS components required careful circuit board layout and routing execution. The image below shows the care the we took during circuit board design.
Paul Nickelsberg is President and CTO of Orchid Technologies Engineering and Consulting, Inc. and has 20 years experience in electronic product design and development.
Orchid Technologies designs and fabricates custom solutions for demanding control needs.