The output impedance of the FG085 function generator that I assembled yesterday is supposed to be 50Ω, so I decided to measure it by plotting RMS voltage as a function of load resistance:
The RMS voltage measurements are consistent with a series resistance of 46.66Ω (slightly less than the 50Ω spec) and with a peak-to-peak voltage of 5.018V (slightly more than the 5V requested).
It would be interesting to determine whether there is a parallel capacitance or series inductance that affects the output impedance. I may have difficulty measuring that, as the values are likely to be small, and so their effects will only be visible at high frequency, which the function generator is not very good for generating.
With the Fluke 8060A meter set to AC voltage, I should be providing a 10MΩ load with at most 100pF in parallel, and with that load I see no drop in voltage until about 200kHz. The power drops by 2 (amplitude by ) at around 277kHz, after which the voltage drops as a second-order filter (that is as approximately (277kHz/f)^2). Since the 277kHz is a lower frequency than what I saw as a cutoff with the Bitscope oscilloscope (385 kHz and a first-order rolloff), I believe it is internal to the meter.
I could take the function generator into the lab used for the circuits class, and characterize it a bit more precisely at the high frequency end, but I don’t think that will help much in determining whether there is extra capacitance or inductance in the output impedance of the function generator. The problem is simply that the function generator doesn’t produce high enough frequencies for the inductance or capacitance to matter.
At 100kHz, a 33nF capacitor would have an impedance of –48j Ω, so small parallel capacitances (say, <100pF) would have negligible effect on the impedance of the series resistor. Similarly, at 100kHz, 80µH has an impedance of around 50j Ω, so small series inductances (say, < 1µH) would have negligible effect on the impedance of the function generator.
In short, I don’t see any easy way to improve the model of the function generator as a 46.66Ω source.