The BB60C has a lot of advantages: sweep speed, signal purity, instantaneous bandwidth, etc. But the SA124B is perfectly capable of making good measurements on many types of signals above 6 GHz. Crowded spectrum, signals wider than 40 MHz, or intermittent signals are difficult or impossible to measure with the SA124B.
You could downconvert to any convenient frequency, but there are a lot of off-the-shelf converters for satellite systems that target 1-2 GHz IF, and a lot of mixers that don’t allow an IF much higher than this.
