If you are using Spirent SR5500-M to verify the power settings, you need below equipment and accessaries to conduct your evaluations:
1, Signal generator.
2, Spectrum Analyzer.
3, RF cables.
If you are using only one signal generator, please do not just using splitter to feed into the two CH1& CH2 of the fader, you may get different result if you set RF OUTPUT of CH1 and CH2 in the same level, the result is that the same signal you feed may have some phase sum up or combinations, this will result in the signal level change due to the addition.
The best way is to feed the signal separately or using different signal generator in the same time.