Thanks for the reply! I only checked back now ... the project has been on hold due to other issues.

As I understand it, the thermal noise floor in a 50 ohm system is at an absolute level of -174dBm/Hz.

However, the Holzworth spec says -175dBc/Hz (max -170dBc/Hz), i.e. it's relative to the carrier (and specified at a particular offset frequency from the carrier). I don't think thermal noise would be at a level that's relative to the carrier, would it?

Anyway, thanks for the link to the article, I'll have a look at it.