Internal resistance of Function Generator
The 50 Ohm internal resistance changes the behavior from that of an ideal signal source is one simple manner: It behaves just like adding a 50 Ohm resistance in series with the voltage source, completing the circuit through your load:







simulate this circuit &ndash; Schematic created using CircuitLab

As the function generator is specified with an internal resistance of 50 Ohms, its signal specifications would be for a 50 Ohm load. 

For example, for a 5 Volt signal output setting, and using a matched 50 Ohm load for X above, the current through the loop will be: I = V / R = 5 / 100 = 50 mA. This is the nominal, specified case. As load 'X' is changed, the current drawn will change, and thus the voltage across the load changes correspondingly.