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ADF4158: Limited bandwidth when doing sawtooth modulation via ADIsimPLL

Category: Software
Product Number: ADF4158
Software Version: ADIsimPLL-5.50.04

Dear community,

I am using ADF4158 with ADIsimPLL for saw-tooth modulation. 

Fpfd = 20Mhz, I am trying to generate a frequency sweep up to 93Mhz from 1.86Ghz to 1.953Ghz.  The tool by default generate 20Mhz ramp dev with 128 steps. So I am trying to increase the steps to larger value to achieve higher bandwidth.  When I increase steps to 256, it works fine, the ramp dev increase to 40Mhz.  But when I increase the steps to 512, the ramp dev shows 80Mz, but the frequency figure shows abnormal,  the frequency goes to flat instead of go up linearly. see below pic. 

Any hints what goes wroing?  Appreciate for your response.

Best Regards