If you need an accurate output level, you will have to feed (a portion of the) signal back into the SA and correct it. TG output level is only specified accurate within +-3dB of adjusted value. Moreover, signal quality is only half-way decent with the first harmonic at round about -35dB (tested 10...500MHz) and a measured THD between 2.5 and 3%. Yet, in many situations the TG comes in handy as a signal gen, I frequently use it as such.
Cheers,
Thomas