PC based scope

[peterr]

I am considering buying one of these or similar:

PC based Scope



Before I do so I would like to know if anybody has any experience with these, positive or negative.
There are many sellers on ebay offering these and they all look about the same. Price depends mostly on sampling rate.
I am especially interested in the FFT possibilities as I have a normal non pc based scope for other measurements.

All comments appreciated

[Pjotr]

It is 8 bits resolution. If you want to use that for serious audio measurements with FFT that is way too crappy.

Usually the FFT software that comes with these devises is too limited for audio and do you need post processing with more powerful stuff.

[tommak]

For audio, try this first. http://www.sillanumsoft.org/

Tom

[Gary P]

I'm with Pjotr on this. 8 bits makes a decent 'scope but is not nearly enough resolution for FFT work. Even 16 bit sound cards are on the limited side for high accuracy measurements.

As you already have a 'scope and are looking for FFT capabilities look into sound card based acquisition systems. You need to setup a buffer that protects the sound card inputs from over voltage to use sound cards but the performance available is quite good.

With the performance available from a 24 bit sound card you will need to use a card with balanced inputs unless you want to spend all you time chasing ground loops. I'm currently using the M-Audio Audiophile 192 sound card. With this sound card you can measure out to ~95Khz.

Using the sound card in loopback mode the distortion residual is ~.0025%. Using a Tektronix SG505 low distortion oscillator with balanced outputs the distrotion residual is .0016%.

The input side of the M-Audio card is very clean but the output side is quite a bit noisier. The noise floor goes up ~18dB when the output of the sound card is connected back to the input. This noise is not present when using the SG505. Comparing loopback mode to the output of the SG505 the noise floor is 13dB higher using the sound card as the source.

Gary

[peterr]

Thank you all for the information.
I will do some more research

[peterr]

After all the advice given above I decided take the soundcard route and I have in the meantime acquired an
M Audio USB Transit soundcard

My main concern now is the buffer I need to make to protect the soundcard. I have found this simple schematic in the Arta manual:



The resistors act as a voltage devider. I thought I'd make a simple switchable
x1 (R1=0 and R2=10k)
x10 (R1=9k and R2=1k)
x100 (R1=9.9k and R2=100)

Is this right and all that is needed?

[Pjotr]

Mainly yes, but keep in mind that the input resistance of the card is in parallel with R2. With most soundcards Ri is 10K. So you have to adjust R2 to take that into account.