It is possible to realize a very good low frequency oscilloscope (until 15 KHZ) by using the sound card of the PC and the freeware
WinOscillo
which transforms your PC in oscillo. You have only to apply the signal to the " line in " input of your sound card.
To protect the sound card of surges, it is necessary to insert between the signal to measure and the sound card input a very simple small circuit which attenuates the signal (I use a 100:1 ratio) and cuts the DC componant of the signal.
You will find below the scheme that I use (it is really very simple).
Sound card interface
In addition WinUscillo allows to implement a signal generator but 2 PCs are needed if you want to use the generator and the oscilloscope at the same time.
You can see below the PWM power supply output signal displayed by WinOscillo.