ADRF6821 2xLO frequency is not as expected.

I am using the ADRF6821 with a REF in of 10 MHz and set it's up with internal VCO. The intended freq at the LO-out pin should be 900 MHz. This is according the datasheet the minimum that's possible. This set-up was working on the evaluation board + GUI software of analog devices. Now i am using it with my own firmware, by programming via SPI all the low level registers. What i have observed sofar is that:

The 2xLO freq is too low e.g. 718 MHz and it's not locked since it does drift over time.

Even when i enable the 2xLO with NO 10 MHz REF in connected i still get the 718 MHz on the 2xLO pin.

My question: 

Is the sequence in with the low level registers are programmed critical?

What is the prefferred low level registers programming sequence?

I hope that some can provide me a good idea to solve this issue. Thanks Leo

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