Part Number: LMX2594
When I let LMX2594 work in Integer-N mode with Phase-Sync enabled,I find the PFD spur is about -46dBc，and Integer-N mode with Phase-Sync disabled, the PFD spur is about -76dBc。The OSCIN frequency is 100MHz ，PFD is 50MHz with Phase sync enabled.
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In reply to Hao Zheng72:
In reply to ldx dfdx:
Please see the attachment for the configuration file。it's not convenient to measure this clock right now..But it seems look like the following reslult
I agree that there is a high spur (not as high as -46dBc in my measurement, though. What I saw was close to -56dBc for 4GHz output). The reason is that the loop filter on the EVM board is quite wide, because it was designed for optimum jitter performance in integer mode. Therefore, it does not have as good spur suppression outside of the band. In order to better suppress the spurs, narrower loop bandwidth is needed. Since the roll-off slope of the loop filter is about -40dB/dec, if the loop bandwidth is halved, the PFD spur can be reduced by 12dB. If you reduce the phase margin, the roll-off rate can be further increased. Please refer to the below file for measurement results of wide loop bandwidth board (default version) versus that of the board which has a narrower loop bandwidth. To test the effect of reducing loop bandwidth on spur suppression, you can simply narrow the loop bandwidth by reducing the charge pump current. But ultimately, the loop filter should be re-designed. Please understand that different use cases require different loop filter designs for optimum performance. To design loop filter, use our free simulation tool Platinumsim http://www.ti.com/tool/PLLATINUMSIM-SW.
Thank you for your detailed reply.According to forth slide in the ppt attached,it seems that the phase margin of the narrow bandwidth board is not enough.
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