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LMX2694-EP: output has spur like modulation signal

Part Number: LMX2694-EP


About the LMX2694, the customer found that output signal  has spur like this below curve, he analyzed that CP voltage has noise, but the power supply has no noise.

And when he debugged again, he found that VCO_CAPCTRL configuration would affect, and if VCO_CAPCTRL=96 the output signal is better,  but sometimes it still has a little clutter. 

He also tried another board and has the same phenomenon, it changed the VCO_CAPCTRL value it would be better, modifying different frequencies can also be improved by optimizing VCO_CAPCTRL;

According to the datasheet P17, VCO_CAPCTRL is used to speed up VCO calibration which is the second mode of below, but if force VCO_CAPCTRL, it is the third mode. The following figure states that VCO_CAPCTRL is by choosing by readback, but it would be a waste of time if each frequency point had to read and write to VCO_CAPCTRL.

So the question is that why the second mode isn't feasible? If there is something wrong of the registers configuration?

Best regards