HMC733LC4B

Hi
We use the HMC733 VCO.

I have a question.

1. The frequency was 9.7 GHz. If we watched it for 1 hour, we could see that it was moved to the left little by little.
At 18GHz, the frequency will continue to move to the right.
This is the case with the Frequency Drift Rate parameter.
The question here is that the frequency stops and shifts little by little. Why does the frequency continue to shift after 2 hours and 3 hours?
  Could you help me?
 
Thank you.