PC SOUND-CARD SCOPE INTERFACE FACILITATES DC RESTORATION

To start with I must appreciate acknowledge and thank ‘Christian Zeitnitz’ for the fantastic Sound-card Oscilloscope & signal generator software available at his website  on which this Instructable is based.

While working with the PC sound-card oscilloscope and signal generator I found the following limitations:

1.   A low Input Impedance of the order 10 Kilo Ohms
2.   Input voltage range limited to 2.8 V p-p
3.   AC coupling of the I/O signals
4.   Output voltage limited to 2V p-p
5.   Sampling rate limited to 44kbps

Not much can be done about the sampling rate as it is a limitation of the sound card but the scope interface presented here attempts to improve the other factors.

Most significant is the addition of circuitry to estimate the positive and negative peak DC values of the signal and use this to offset the waveforms providing a realistic DC-coupled scope display.

Let me illustrate this with an two Examples

Step 1: 555 Timer Example

Waveform before correction

The 555 Timer IC when operated as an astable oscillator from 5V provides a square wave of 0-5V and the waveform at the timing capacitor varies from 1/3 to 2/3 of 5V.

The first screen shot shows the timer IC 555  AC coupled waveforms as captured by the PC sound-card. It can be seen that the zero settles at the average value. (The input has been scaled by 1:10 to remain within the input voltage limits.)

The square wave shows a peak value of 171mV instead of 5V and the capacitor waveform appears to be centered at 0V with a peak value of 66.59mV.

The interface circuit provides two peak-hold circuits which provide the positive and negative peak DC value of the input waveforms. Using a multimeter to measure these values gave 468mV and 283mV as the positive peak values for the square and capacitor waveforms respectively.

We can compute that the square wave needs to be offset by 468mV -171mV = 297mV and the capacitor waveform by 283mV - 66mV = 217mV.

Waveform after DC restoration

The second  figure shows the waveforms after entering the offset values 297mV and 217mV into the offset boxes for CH1 and Ch2.

After DC restoration the square wave varies from 0 to 455mV and the capacitor waveform from 1/3 to 2/3 of 5V.

This would be the display we would see on a scope with DC coupling.

Comments

[Raul_77]

(removed by author or community request)

[boingx]

You need to click on the  " i " in the corner of the picture of the schematic and select the original high res size of the image.

Great Instructible! Another project goes on the list.

[tmetford]

Please forgive "numptie" status here but I can't find where to download the high res image of this schematic!
Boingx says click on the "i" but I can't find the "i" to click on!
Suggestions welcome (related, of course to the download of the schematic)!
Thanks

[ajoyraman]

There is a little 'i' seen on the left hand top corner of the schematic page, and every page. Check it out !

Thanks for your interest.

[Raul_77]

O.K. Thank you very much

[deserticus]

how can i get the schematic from here?

[ajoyraman]

Thank You for your interest. I am providing a higher resolution version of the schematic file here.

[ajoyraman]

Thanks for the great response!

I find the DC offset in the X 10 gain is high and have changed the LM747, U1 to the LF353 using an 8Pin to 14Pin converter. The LF353 pins need to be bent upwards delicately and the converter leads soldered on. Plug this into the U1 socket. It works fine now.

[Brunomaster]

Some video of the device working?
download links?
the project is good but needs some more work

You're doing great, keep it up ^^