PC capacitor meter

[Guest AngeloFranca]

Hi people!
I' m sure you know about PC oscilloscopes: they use soundcard as an input and show the signal waveform on the screen. I' d like to use this oscilloscope and a sinusoidal generator,(alsopreset in some of this freewave software), to make a setup to measure capacitors. I hope to plot capacitor's impedence in the range 20Hz-20kHz. My major troubles are:
-how to compare fase shitf beetween input and output;
-how soundcard input impedence,(which I don't really know), must be considered.

Any idea is well accepted!
Thank you so much

[TestGuru]

1.  Two input channels of the sound card must be used, one to capture the signal from the DUT (in your case, a capacitor), the other to capture the signal directly from the sound card output channel. By comparing these two signals, the phase shift introduced by the DUT can be obtained. This method assumes that no phase shift difference is introducted by the two sound card input channels, otherwise some sort of compensation is required.


2. Sound Card input impedance is a unknown during the tests. It can be measured by two tests, i.e. with and without a known resister in series.