a. Simple PC digital oscilloscope

This simple adapter enables connection of an external signal source to a standard PC audio line input. The correct values of R₁ and R₂ resistors have to be calculated from the maximum external source signal range and the maximum signal range of the PC sound chip line input. The line input resistance (R_i) must also be considered in most cases. If R_i value is less than 10 times greater than R₂, you should first calculate the R_2i, which is a combined resistance of R₂ and R_i. Afterwards R₂ may be calculated from R_2i. The electrical current (I) through the voltage divider should also be limited to a few miliamps (mA) to prevent draining a significant current from the external signal source. Therefore R₁ or R₂ should be initially chosen to have values in range of kilo ohms. A good suggestion is to select R₂ = R_i/10. R_i is typically around 10 k ohms (ex. Sound Blaster X-Fi sound card).

If external signal source has a maximum range of 24 V_pp (a symmetrical signal range should be presumed) then U_c = 24 V_pp (alternates between -12 V to +12 V). A typical line input maximum signal range is U_i = 1.4 V_pp (alternates between -0.7 V to +0.7 V). Now, R1 and R2 vales can be calculated as follows:

R_2i = 1 / (1 / R₂ + 1 / R_i) à R₂ = 1 k ohm, R_i = 10 k ohm à R₂ = 1 / (1 + 0.1) k ohms = 990 ohm (in this case R_i could be neglected which would give us R_2i resistance of 1 k ohms)

Now, let us calculate the R1 resistance:

U_c = (R₁ + R_2i) I , U_i = R_2i I , I = U_i / R_2i

U_c = (R₁ + R_2i) U_i / R_2i à R₁ = (U_c / U_i -1) R_2i

In our example we get: R₁ = (24 V_pp / 1.4 V_pp -1) 990 ohm = 16 k ohms. It is not a bad idea to select a bit higher R₁ value to have some margin on sound chip A/D converter range. In this case we could select R₁ = 25 k ohms.

Now, we only need a Windows application to display the input signal. There are many Windows sound recording applications on the internet (ex. Cool Edit 2.0). Many of them enable you to capture and analyze signals up 100 kHz. A typical audio line input A/D converter of a new PC has conversion rate up to 192 kHz or more, which is enough for low frequency AC signal analysis. There are sample digital oscilloscope applications in the Downloads section.

NOTE: Read 1 Msps modular oscilloscope for everyone on how to build an oscilloscope that goes up to 1 MHz.