With limitation of Keysight E5052B, it can't measure phase noise when the carrier frequency less than 250MHz. 

If one application need to see the phase noise at 100MHz while the carrier frequency is less than 250MHz, then they may need below two methods:

1. Test the frequency directly, and simply use the phase noise number at 40MHz or 20MHz, usually the far end to 100MHz phase noise trace is the extensions.

2. Double or quadruple the carrier frequency to make sure the carrier frequency larger than 250MHz and now you can the span of E5052B to 100MHz,  and then plus 6 and 12 to get the actual carrier frequency phase noise. (Note: phase noise decreased by 6db every twice, it comes from a simplified function 20lg(f2/f1) ).