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Applications using DDS-based waveform generation can be segmented into two categories. Industrial and biomedical apps use DDS for programmable waveform generation to locate resonant frequencies or compensate for temperature drift. And RF communications systems that require agile frequency sources with excellent phase noise and spurious performance often include a DDS device as a local oscillator or as a reference to a phase-locked loop (PLL) to enhance frequency resolution.
Many industrial and biomedical applications stimulate a sensor or network with a signal that has known amplitude, frequency and phase; they then measure and analyze the system response. The response signal is compared with the original to calculate the amplitude and phase shifts through the system. Since a single excitation frequency is usually not adequate to obtain the desired information, a range of frequencies may be swept through the system. In the past, those frequencies would have been generated with discrete components and a complex switching circuit-a power-hungry solution that was expensive, error-prone and susceptible to temperature drift.
A DDS chip can control the frequency and phase of the stimulus with very tight resolution and can replace the whole discrete circuit. No external components are required for frequency control. The DDS also has the flexibility to control the output phase with 10-bit resolution (0.35 degrees ).
The system works by forcing a known amplitude, frequency and phase at V1, as shown in the figure. The signal at V2 will exhibit amplitude and phase shifts that depend on the network characteristics. Time and frequency domain data, and therefore the network characteristics, can be calculated based on those shifts.
In operation, the AD9834 DDS chip, driven with a 50-MHz crystal oscillator, provides the stimulus to the system. Its frequency resolution is 28 bits, or about 0.2 Hz, and its amplitude is controlled by varying an external resistor to ground. The RC low-pass filter reduces clock feed-through, images and higher frequencies. A gain stage drives the network, which is represented by an LRC circuit. The reference signal for the network (V1) is connected to channel 1 of a simultaneously sampling A/D converter; the response signal (V2) is connected to channel 2. The DSP is used as both system controller and data-processing engine.
In communications apps, two basic approaches are commonly used for frequency synthesis: PLLs or DDS. The choice between them is not always clear-cut. A hybrid PLL/DDS circuit, however, uses the assets of each technique, eliminating most of the trade-offs and outperforming the individual options in frequency resolution, switching speed, settling time, bandwidth, power consumption, phase noise and spurious noise.
In hybrid synthesizers, the DDS can be used as the reference to a PLL or can be used to generate a frequency offset for the PLL. The main benefit of using a DDS is its fine-tuning capability. The PLL output frequency resolution is limited to the reference frequency. By using a DDS instead of a fixed oscillator, the reference frequency can be varied in very small steps by writing to the frequency register. The AD9834 can be tuned to 0.2 Hz with a 50-MHz clock rate, resulting in very fine tuning of the hybrid PLL/DDS.
A hybrid synthesizer can also provide better frequency resolution.The PLL provides the coarse steps; its output frequency has the same resolution as the reference frequency. The DDS provides fine steps between the coarse steps.
Read more about DDS at www.analog.com/analogdialogue.
Eva Murphy (firstname.lastname@example.org) and Colm Slattery (email@example.com), applications engineers at Analog Devices Inc. (Limerick, Ireland)