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Multichannel DDS Enables Phase-Coherent FSK Modulation
Common single-channel direct-digital synthesizers (DDS) produce phase-continuous frequency transitions, as shown in Figure 1. In applications such as coherent pulse Doppler radar and NMR/MRI spectrometry for medical and material analysis, however, phase-coherent transitions are preferred. This article demonstrates how to configure the AD9958/AD9959 multichannel DDS as a robust phase-coherent frequency-shift keyed (FSK) modulator by summing the DDS outputs.
A multichannel DDS virtually eliminates channel-to-channel temperature and timing issues encountered when synchronizing multiple single-channel devices. Multichannel DDS outputs, though independent, share the same system clock, allowing them to track better over temperature and power-supply deviations than the outputs of multiple single-channel devices. As a result, a multichannel DDS is better suited for producing phase-coherent frequency transitions at the summed output.
Figure 1. Phase-continuous and phase-coherent frequency transitions.
The AD9958's two independent channels operate at preprogrammed frequencies F1 and F2. Wiring the outputs together sums them. The profile pins, which drive the multiplier at each DAC input to control the output amplitude, switch the channel outputs on or off to select the desired frequency. To accomplish this, each multiplier has two preprogrammed profile-selectable settings, zero- and full scale. Logic low on the profile pins shuts off the sine wave while logic high passes it to the output. The operation requires two complementary input data streams to alternate between frequencies. Note that the two DDS channels continuously generate F1 and F2. The off feature mutes the appropriate DDS output, thereby producing a phase-coherent FSK signal.
Figure 2. Setup for phase-coherent FSK modulator.
The AD9959 4-channel DDS produced the results shown in Figure 3. Its two additional channels serve as a phase reference for the two switched frequencies at the summed output, making it easier to demonstrate the phase-coherent switching. The summed output, shown in the upper trace, exhibits phase-coherent switching. The middle two traces show reference signals F1 and F2. The bottom trace shows the pseudo-random sequence (PRS) data stream that selects between the two frequencies. Note the edges of the PRS data stream do not align exactly with the frequency transitions of the summed outputs due to the pipeline delay within the device.
Figure 3. Measured phase-coherent FSK transition.
Figure 4 shows an example of phase-continuous FSK switching, also produced by the AD9959. This type of operation requires less bandwidth, but does not maintain phase memory between transitions.
Figure 4. Measured phase-continuous FSK transition.
Analog Devices offers a variety of direct digital synthesizers, clock distribution chips, and clock buffers to build a DDS-based clock generator. Refer to www.analog.com/dds and www.analog.com/clock for more information.
10-Bit, 500-MSPS Direct Digital Synthesizers
Figure 5. AD9958 functional block diagram.
Generator Has 12 LVPECL/24 CMOS Outputs
Figure 6. AD9520 functional block diagram.
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