ADF5610, undesired FM modulation on RF

I have an Up Converter design with a ADF5610. I generate an LO of 14 to 14.5 GHz. My board has a 250MHz SAW oscillator for an on board reference. 

RF 14.25 GHz

N = 57

R = 5

RF out is fine, but for some applications the close in phase noise of the SAW is a problem.

I provided an input for an external reference in case we need higher stability. I us an NEL O-C15-xxx type OCXO 10MHz reference. 

RF = 14.24 GHz

N = 356

R = 1

This is a highly stable oscillator, the ADF5610 will lock with this signal, but I get a random FM modulation on the RF. It varies in time, but is typically 20KHz FM modulation on the RF. I do not see this with the high frequency reference.

I have two stage LDO for the power distribution, the final one is a LT3045 for the 5.0 and 3.3 VDC. I do not believe I have noise on the DC.

The reference power level is in spec. The RF out goes into a MACOM X1007QT doubler, it is specified to have good return loss at this frequency.

My suspicion is that the problem is thermal since it seems to vary over several minutes from no FM to significant. Has any one else seen problems with this chip? It is my first design with this one, I have used many of the other PLL chips from Analog over the years. 


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