Three criteria are necessary for implementing a phase-coherent zero-crossing switch:
1. The ability to determine the smaller GRR of the mark and space FTWs associated with CH0 of figure 2.
2. A second DDS channel (CH1 of figure 2) synchronized to CH0 of figure 2 and programmed with an FTW having all zeros except for the one bit corresponding to the smaller GRR.
3. The capability to use the rollover of the second channel to trigger a frequency change on CH0 of figure 2.
Unfortunately, the latency between when a DDS accumulator hits zero and when that zero phase is represented at the output further complicates the solution. Fortunately, this latency is constant. The ideal solution necessitates that the auxiliary channel be phase adjusted to compensate for this latency. For instance, both channels on the AD9958 have a phase-offset word that can be used to fix this problem.
The AD9958 two-channel DDS produced the results shown in Figure 4, Figure 5, and Figure 6. Figure 4 and Figure 5 exhibit phase-continuous FSK switching versus zero-crossing FSK switching. Figure 5 shows both phase continuous switching and phase coherent switching. Figure 6 shows the results from a pseudorandom sequence (PRS) data stream that toggles between multiple frequencies.
Figure 4. Phase-continuous FSK transition
Figure 5. Zero-crossing FSK transition
Figure 6. Zero-crossing with multi-FSK transitions
The AD9958 two-channel DDS produced the results shown in Figure 7 and Figure 8. These figures exhibit phase-continuous BPSK switching versus zero-crossing BPSK switching.
Figure 7. Phase-continuous BPSK transition
Figure 8. Zero-crossing BPSK transition
1. Brandon, David. Multichannel DDS Enables Phase-Coherent FSK Modulation
About the Authors
David Brandon has supported DDS products since the first DDS released back in 1995. His career spans 28 years at ADI, with the last 11 years as an applications engineer in the Clock and Signal Synthesis Group. He has authored a number of application notes and a couple of magazine articles.
Jeff Keip has nearly 20 years of experience in the semiconductor industry; over 15 of those have been spent working on and with frequency synthesis products. For the past nine years, Jeff has had primary responsibility for the high-speed DDS product portfolio at ADI.