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Old 21st May 2004, 05:15 PM   #1
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Question Windows Based Osilloscopes

Anyone have experince with a PC based Scope? Software is said to work with the soundcard as an input device. Some programs also have tone generators and analyzers. I tried it on Win 2000 Pro and it seemed to function. Both are free to use for personal use. Could this be sufficent for audio circuit testing?

Another question is- what would one use as a probe?

Here's a like to one:

http://www.electronics-lab.com/downloads/pc/001/

http://www.electronics-lab.com/downl...002/index.html

Vince
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Old 21st May 2004, 06:04 PM   #2
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Well, it depends on what you're measuring.

If you just want to observe the waveforms to test things like gain, clipping, etc. then you the soundcard solution is fine. However the bit depth, noise, and sampling rate of your soundcard may limit or more precise kinds of measurement.

Providing you're mostly interested in the former, then alligator clips with an 1/8" jack at the other end will be okay for your first probe.

P.S. You may want an interface that allows you to attenuate the input for measuring voltages higher than 1V. A simple resistor divider will do the trick too. Also, look for software that has a calibration feature.
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Old 21st May 2004, 06:59 PM   #3
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Thanks for the reply.

I'm mostly interested in troubleshooting audio amp problems, e.g., tracing the signal paths for damaged components. I understand that most limitations are due to the sound card, like a freq. range of 20Hz to 20Khz.

At least one of the 2 programs I downloaded lets you use a reference signal to calibrate the software.

mrothacher,

Do you know if it is best to use a scope with the circuit powered up or down? Or, can a scope work with both?

Thanks,

Vince
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Old 21st May 2004, 08:43 PM   #4
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Hi Vince,

If it is for trouble shooting, do yourself a favour and buy a good second hand analog 50 MHz oscilloscope on EBay. An amp that suffers from instability can oscillate in the MHz range. Besides that it has neat input attenuators and you donít have to fiddle to connect decent probes.

Cheers
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Old 21st May 2004, 10:03 PM   #5
Tobbe_L is offline Tobbe_L  Sweden
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If your try to troubleshot an amp using just a voltage divider and a soundcard you WILL burn the input of the soundcard and in the worst case the whole computer.

PC-based software if fine if you want to looka t low-voltage audio signals but for higher voltages and frequencies you need a proper oscilloscope or DAQ.
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Old 22nd May 2004, 01:47 AM   #6
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Tobbe:

I believe your advice on getting a good scope is very sensible, and since these units can be purchased cheaply it seems that any serious hobbyist should have one.

However, I disagree entirely with the notion that one WILL burn out his soundcard using a proper attenuator and diode protection. I have been conducting precisely these kinds of experiments for years without loosing a card.

In fact, I'm not sure why you would make such a claim with such apparent authority. One need only obey Ohm's law to make proper use of this technique.

Mike
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Old 22nd May 2004, 03:26 AM   #7
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The real problem with sound card oscilloscopes is sample rate. Nyquist says you need to sample at twice the fundamental frequency to reproduce the original wave shape. If you are going to monitor a 20KHz waveform the theory says you need to sample at 40KHz. Ok, no problem, a lot of the sound cards out there sample at 44KHz. Nyquist falls a bit short though because when sampling at 44KHz and looking at a 20KHz signal we only get two points on the waveform. This is hardly enough to accurately reproduce the waveform. In order to accurately reproduce the waveform we must delay the sample points at a random interval at every trigger so we do not measure at the same point every time and build a table of points in memory, plot them to the screen and reconstruct the waveform. As long as the signal is repetitive we can keep filling the memory with new points and discarding the old ones. After a few triggers we have enough points measured at different places along the wave to accurately reproduce the waveform. You can do a sin x/ x interpolation to fill in the blanks. This is basically how a sampling oscilloscope works.

Now suppose you are troubleshooting and have a random oscillation you need to capture. This is a scenario in which the sound card falls short. We need a real time or "one shot" oscilloscope to make the measurement. In the sampling oscilloscope scenario, every time we got a trigger we simply made our measurements updated memory and went on our merry way. In a one shot scenario, at a trigger event we capture all the points we can (in this case two), stuff them into memory and display them to the screen. Remember, there was only one event to trigger the oscilloscope and capture the two data points.

What this boils down to is that sound cards make great sampling oscilloscopes easily capable of monitoring a repetitive waveform. If you need an oscilloscope to aid you in troubleshooting, the sound card oscilloscope may not be the best choice because of the limited sample rate available. In your situation, I would recommend an analog oscilloscope of appropriate bandwidth with good analog persistence to allow you to capture one shot events. I hope this helps.
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Old 22nd May 2004, 01:01 PM   #8
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Very clear explanation!

Thanks
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