System Transfer Function: Obtained by modulating the RF attenuation of the AOM driver with different frequencies and measuring the corresponding signal on the Photodiode. Do not get confused by the absolute Magnitude of the Gain values: A sinusoidal signal with 1mV amplitude going into the AOM driver does not correspond to a signal with 1mV on the photodiode. This is for example dependent on the offset (because of the slope) or the photodiode amplification. The relative numbers however give you insight in attenuation at higher frequencies and therefore System bandwidth.