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Issues with AD9164 Phase Continuous Frequency Hopping

Question asked by sivart64 on Oct 4, 2017
Latest reply on Oct 12, 2017 by danf

I'm having issues with the output of the AD9164 in phase continuous, NCO only(DC Test) mode. I am trying to use it as a frequency reference for an AD9361, but sometimes when I change the main FTW value, I get a large discontinuity which causes the channels of the AD9361 to go out of sync with each other. I've double checked that register 0x800 is set for phase continuous operation. Below is a pic of the output changing frequency with a discontinuity. It doesn't always do this and cause the AD9361 to loose sync but usually every couple of frequency hops. 


I'm triggering off the blue trace below which is the Slave select line for the SPI interface loading the new frequency value. The yellow trace is the output of the DAC. 



My understanding of the NCO and DDS output in phase continuous mode is that the NCO will continue counting with the new frequency programmed into FTW0  so the DDS output shouldn't have any glitch in the output like I am seeing. Am I misunderstanding how this should work? Are there other settings on the part that could be causing this?


Thanks in advance for any help.