are amplifier and attentuator bascially of the similiar function?
It is extremely unlikely that the sole purpose of the amplifier is to provide attenuation.  Attenuation can be achieved with a bank of resistors that will provide more accuracy, higher bandwidth, lower noise and better reliability than an amplifier using active devices like vacuum tubes (if the generator is that old) or transistors.  Resistors also don't require any power. The most common design for signal generators is to have a basic oscillator to provide the signal followed by a power amplifier designed to deliver the maximum desired output power.  A resistive attenuator at the amplifier output than is used to select the desired output level and provide the desired output impedance (usually 50 or 600 ohms). At full power, the attenuator would be switched out entirely. As for the digital switch, could you provide a more detailed description.  It's possible that the attenuator resistors are built into the switch.