Strange traveling spur problem seen in ADF4113HV

Dear Analog Engineering Community,

I am using the ADF4113HV to generate a phase locked signal on 7.784-7.786GHz from a 48MHz crystal reference.

The simplified loop circuit is

The settings are as shown:

The current setting resistor is 4.7Kohm.

The resulting signal is contaminated with a travelling spur much like that seen in the fractional N PLL synthesizers.  I tried adding a 470k-ohm negative bleed resistor, but the spurs were still observed with no chance.  I notice that the spurs are often strongest at power up and move away with time.  However, if I am unlucky, the spurs move close to the carrier and stay there, causing severe phase disturbance.  Shifting the reference frequency slightly causes them to move about.   What is going on here?

Is there something about my settings that is causing this or is it an intrinsic problem with the phase detector of the 4113HV chip.  I tried running on battery power, and the results were the same. Help!!!!!!! I am doing some sensitive precision timing measurements and this is really leaving me in a bind!

Thanks and best regards,


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