LTC6946 Leakage of Phase Comparison Frequency


We have the LTC6946 designed into a product and have shipped >30 units over the last couple of years. In a recent batch of products we have noticed that the PFD frequency leaks into the PLL output much more in LTC6946 devices bought recently compared to ones from earlier batches i.e a couple of years ago.

We are sure it is the device as we have taken an old LTC6946 device from a working board and replaced on a non working new board and we can see the problem follows the device, so rules out our PCB's

This is a working device - old device from a couple of years ago..

And with the device replaced with a new device

and another new device on another board..

The spur frequency is 220kHz which is our PFD.

We are a little stuck as to what the problem may be.  

This is the schematic of the PLL

The reference is not particularly clean as it comes from an FPGA, but the first plot you see is sufficiently good performance.

Something appears to have changed with the devices, but there is also the possibility we are doing something wrong which makes our design susceptible/sensitive to small changes.

Is there anything you think we could do to resolve the PFD leakage into the output? and is this a problem seen before? and have there been any revisions of the hardware over the last few years?