hi pelican2,
The datasheet specifies that the HMC679LC3C are Current Mode Logic (CML) logic levels but I guess it needs to be more clear and improved. I have attached the application note which describe in details this versatile I/O interface in section 3. Figure 3.2,3.3 give you the min.max levels of the CML Input. For full speed of 26GHz,the I/O should be interfaced with widely used advance 1.2V CML FPGA's. Section 4 of the document gives all possible interface circuits to different I/O standards.