I connected a signal generator to a 5.1 ohm resistor, which may be too low comparing to the internal resistance of the generator, so I got only several hundreds millivolts amplitude for frequencies below a hundred. However, when the frequency was increased beyond a hundred and something, the voltage could rise to about 4.5Vamp (maximum). Why dose this happen?

(The specified maximum voltage amplitude of the signal generator is 20Vp-p)

Thank you!