sound card voltages

If you use the Line In, you can typically apply around 1 Vrms (just under 3 V peak-to-peak). The mixer input slider controls will affect this to a certain extent. If you reduce the slider setting, you may be able to handle somewhat higher voltages. The sound card chipset can probably handle 15 V, but better safe than sorry.

The Mic input is typically more sensitive, but usually has a fairly crappy preamp that rolls off frequencies above 8 or 9 kHz.

You can use my (shameless plug) Daqarta for Windows software for viewing signals as waveforms, spectra, or color spectrograms. It has an Auto-Calibration feature to determine the relative sensitivity of the mixer's internal volume controls, so you can adjust sensitivity in dB. Also has instructions for performing a simple absolute calibration using only a cheap DMM (no sensitive AC volts range) and a battery.

(Note: Version 3.03 should be out in a few more days.)

Best regards,

Bob Masta D A Q A R T A Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, Signal Generator Science with your sound card!

Thanks for the information, Bob.

BTW, do you know if a sound card can accurately measure a DC voltage?

Also, do you know if you can usually use a sound card to measure the power level of a high frequency (i.e. 30 MHz) sine wave?

Thank you.

No, they are AC coupled. In fact, the microphone input has a DC bias for electret microphones.

Anything over 20 KHz is pushing your luck with most sound cards, 30 MHz is 1500 times that.

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

To add to Michael's post, you can measure DC by converting it to AC which the card can accept. The simplest way to do this would be to chop the DC signal at a kHz or so. This can be as simple as 2 elements of CD4016 (etc) that connect the input to the signal or ground when driven by a simple CMOS oscillator.

The rectangular wave will easily be accepted by the sound card. Then you just measure the peak-to-peak voltage to get the original DC value.

If you have a spectrum analyzer mode in your software, you can read the peak height of the fundamental as a single measurement instead of 2 separate waveform measurements for peak-to-peak.. This isn't as accurate for a couple of reasons: The peak height will be subject to "leakage" of its energy into sidelobes unless the sample rate just happens to be a multiple of the chopper clock (which effect can be partially cured by applying a window function), and also there is the fact that the fundamental amplitude of a square wave is not the same as the peak-to-peak amplitude (it's actually higher!). But if you have a lot of measurements to make, both of these effects can be calibrated for.

Daqarta simplifies the true peak-to-peak measurement by providing triggered operation (so you get a stable trace, which isn't essential for the above spectrum method), and by allowing you to place separate cursors at two places on the waveform. Besides readouts of the two waveform values, there is a 3rd readout that shows their difference, which is the desired peak-to-peak value. In general, the chopper frequency won't need to be particularly stable for this (unlike the spectral method) as long as the drift is not so great that the cursors "fall off" their waveform phases. A simple RC oscillator is way better than good enough.

In principle, you could use the above approach with your

30 MHz signal by first rectifying it to DC and then applying it to the chopper.

Best regards,

Bob Masta D A Q A R T A Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, Signal Generator Science with your sound card!