[SOLVED] Frequency/Voltage converter (and vice versa) of input signal to sound card

Hello all

I want to use sound card oscilloscope to make system identification & of some simple systems (simple 1st order and 2nd order systems). the procedure that I intend to do, is to give square wave in put to the system, and then I see the output on the sound card based oscilloscope, and also I may save the values of this output to a sound file, and I can make analysis to it using scilab or matlab (using system identification techniques). My problem now is the following:

mechanical systems are usually slow, and for example if I excite a DC Motor using square wave voltage, the output response of position of this DC motor will be having low frequency, and it will not be suitable to be shown on soundcard. And in the same time, I can not increase the frequency of input to the motor, because t needs some time to respond because its inertia.

I have read somewhere here about voltage to frequency converter (If I understand correctly, I can make this voltage to frequency converter using PWM LM324 circuit), but how can I use this here? I understand that I will make voltage response from mechanical system will be proportional to frequency that will be the input to soundcard, but what is next? how can I recreate the original response again from this frequency signal?

You'll use a FM demodulator.