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Offline levettpTopic starter
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« on: October 24, 2010, 09:59:16 am »
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I'm trying to use my PC soundcard as an oscilloscope to do some coil testing.

The software I use is Zelscope.

The only additional hardware that I use is a simple over voltage protection circuit that trims the input voltage to the soundcard to +/- 0.7Volt so that I don't damage the card.

When I send a square wave signal to the soundcard it initially measures it correctly but after approximately 1 second the signal changes to a sinusoidal shape and then diminishes completely.

Any advice on what I'm possibly doing wrong?

My only hunch is that there might be some Windows based noise filter that eliminates the signal.

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« Reply #1 on: October 24, 2010, 10:12:09 am »
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Your sound card is the filter.

It's designed for Audio, Not Square Waves.

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Offline GoldDigger1950
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« Reply #2 on: October 24, 2010, 01:56:41 pm »
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You need an isolation probe, not a voltage limiter. Try one of these.

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http://www.dealextreme.com/details.dx/sku.34221


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« Reply #3 on: October 24, 2010, 02:00:17 pm »
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Hi levettp
          tray The connection & the soft name BIP Electronics Labs 3.0 - Oscilloscope
best time
j.

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Offline GoldDigger1950
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« Reply #4 on: October 24, 2010, 02:20:09 pm »
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The resistor in that probe needs to be 10Megohms or better, not 10k.

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« Reply #5 on: October 24, 2010, 02:27:03 pm »
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Thank you everyone for your inputs so far.

As far as I could see from my research on this concept, I do not understand the 10K or 1M input resistor. It appears that any resistance will influence the reading from something like a coil that produces low amps e.g. RX coil for PI MD.

"Sound cards have an input impedance that varies around 1KOhm. This means that a circuit with a small signal will get sucked up and you will not be able to read it. (but we can fix this with a pre amp)"

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« Reply #6 on: October 24, 2010, 02:32:42 pm »
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The Program I use can Not Produce a square wave out put. It will Not except one ether.

I'm not sure how but it can produce a Triangle, Saw Tooth and  Sign!




 It will Not take one input. If it does it converts to Sinusoidal wave.

How would a probe fix that?

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http://www.zeitnitz.de/Christian/scope_en


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Offline GoldDigger1950
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« Reply #7 on: October 24, 2010, 02:37:02 pm »
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You're forgetting the back EMF from the coil field collapse. Use the 10M. The input impedance is a rating given for matching audio components. If you are not a technician, at least listen to reason here. You could damage your sound card and nowadays, that often means your motherboard.

You are not using your sound card in a standard way. You are making it do things it was not designed to do and you need to take precautions. A series resistor will NOT affect the voltage. It will affect the current. With the 1k input impedance, the low current will limit the voltage. Use the 1M resistor in series.

Posted on: October 24, 2010, 11:35:43 PM
Quote:Posted by homefire
The Program I use can Not Produce a square wave out put. It will Not except one ether.

I'm not sure how but it can produce a Triangle, Saw Tooth and  Sign!




 It will Not take one input. If it does it converts to Sinusoidal wave.

How would a probe fix that?

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http://www.zeitnitz.de/Christian/scope_en

The probe isn't to "fix" what he's doing, it replaces what he is doing. Xavier has been using one for a long time now. Just ask what he does.

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Offline levettpTopic starter
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« Reply #8 on: October 24, 2010, 02:59:34 pm »
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The problem with the square wave and the probe are 2 different issues that I must address.

As far as the square wave problem is concerned I think that I understand it now:

"Sound cards are AC coupled, meaning that you will not be able to read signals with frequencies lower than around 10Hz. Also anything lower than 20Hz will get distorted by the AC coupling capacitor."

The square wave was just a test signal for me to verify the scope readings. I will now rather use the speaker output with a sawtooth or sine signal and read it with the scope software on the mic input.

Posted on: October 24, 2010, 03:50:34 PM
I did not forget about the back EMF but my perception was that the current is very weak ... so since I'm wrong here, I will definately use the 1M resistor.

Does anyone have practical experience on the accuracy of these sound card A/D converters? I have a 16Bit sound card but also want to try the newer 24Bit versions. 24Bit A/D conversion at rates of 44kHz however sounds like a fairytale to me!

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Offline GoldDigger1950
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« Reply #9 on: October 24, 2010, 03:04:21 pm »
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Quote:Posted by levettp
Does anyone have practical experience on the accuracy of these sound card A/D converters? I have a 16Bit sound card but also want to try the newer 24Bit versions. 24Bit A/D conversion at rates of 44kHz however sounds like a fairytale to me!

Contact a user here by the name of Xavier. He uses one regularly and loves it. The frequency of metal detector projects are in the audio range so for our use, it's perfect.


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