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hmc838 phase noise and loop filter BW

The loop filter's BW is so strange on my HMC838 board, The BW gets so large at lower frequency(1.62GHz) and get smaller at high frequency(1.67Ghz). and also the phase noise get worse at low frequency, and actually it is not good enough at high frequency. the settings of the design are: Fref:19.2MHz, PFD:19.2MHz, Fout:16.GHz~1.7GHz, BW:50KHz. there are two plots were attached. Any suggestions? Thanks!