About failure of Phase error and Frequency error .

Dear all,

I have designed a GSM cell phone , when i do the RF measurement using HP 8960, the peak phase error and freqeuency error failed.

I wonder that when the phase error display in graph mode, only the first few bits got failed (i.e. bit 0 to 2 in a burst), but all other bits got pass. Could anyone explain this to me, and how to solve it?

Thanks a lot.

Best rgds,

M