We are seeing a bunch of large spurs at the output of ADF5355 eval board. Here is our test setup info:
ADF5355 Board with SDP controller board.
Reference signal is supplied externally from an Agilent E8257D signal generator. We tried different reference frequencies ranging from 10MHz to 125 MHz, the spur level is reduced with reduced reference frequency.
Fout: 4 GHz
ADI supplied control software (v0.55.6) is utilized to program the ADF5355.
Please see the first screenshot in the attachment. There is a -55dBc signal around 1300 MHz. As you can see there are many spurs and there are separated by PFD rate. The level of the spurs seem to be independent of RF output divider ratio. You can see that in the second attachment. It shows the output spectrum for a fundamental VCO freq of 4 GHz and RF output divider of 4, e.g., Fout = 1 GHz.
How can we get rid of these spurs?