historymars

[Debraj] needed a simple signal generator for a project he was working on, but didn’t have one handy. He found that the easiest and cheapest way to get clean, reliable signaling was by – his PC. He found that the tone generator built into Audacity was quite useful, at least for generating waveforms at less than 20 KHz or so. Upon plugging his scope into his sound card’s audio jack, he observed that the PC had good frequency fidelity, though it required an additional DC offset as most cards are built to remove that offset from the waveform. Using a LM358 as a non-inverting summing amplifier, he was able to apply a steady DC offset and generate usable signals for his micro controller projects. A schematic for his offset circuit is available on his site, should you wish to build one of your own. [Debraj] also notes that though Audacity is a cheap free way to generate simple signals, any number of complex signals can be generated using MATLAB if you happen to own a copy.

Free Download Digital Signal Generator 1.0.0.1 - A simple application that will make it possible for anyone to visualize and save in waveform format. Some very talented people have come up with software. Page all of the links are to freeware or free for non. Powerful audio signal generator. Always thought of making your own PC based signal generator? Do not need to carry one more signal generator. Use PC sound. Once you install the software. Download Pc Signal Generator Software - best software for Windows. Virtins Sound Card Signal Generator: VIRTINS Sound Card Signal Generator is a powerful PC.

Posted in, Tagged, Post navigation. Block the DC offset with a cap? Any reason why he isn’t doing this instead. It would be a 10c fix. It doesn’t isolate the computer from the circuit, but neither does an op amp. An optical isolator or transformer should be used if galvanic separation of the circuits is desired. Sure the cap will have some LF roll-off, but with a properly sized cap you can have it low enough as to not affect anything adversely.

I would just solder a 1uF polypropylene cap in between a male and female 1/8″ stereo jack. This is what I use for testing amplifiers etc. With the fairly low (5 ohms? IIRC) of my laptop it blocks the DC while letting my play the necessary tones to test the amp.

It was actually DC from the amp I was blocking, as it was for a car amplifier and the input floats at 6VDC. For tones I use winISD, but audacity is probably just as good. Going to be honest...if you knew anything about sound card outputs, you’d realize that he didn’t quite figure out what was going on.

Typically your el-cheapo sound card will probably use cheap high voltage offset op amps (which turns into something significant). It all boils down to having a significant DC offset on the output, which can damage headphones and other loads connected to it.

The easy, brute force (but often cheapest) way of solving this problem is to throw the biggest, cheapest output capacitor in series on the output which won’t filter out too much of the low end of the audio spectrum when you form the RC high pass filter with the impedance of your headphones (or audio load). So what’s happening here?

When he says that the DC offset is being slowly “cancelled” out by the sound card, he actually means that the output capacitor I mentioned above was being charged by the oscilloscope input impedance. I admit the numbers from his waveform don’t seem to quite add up (I would have expected it to take VERY long to discharge given the 1M impedance of the scope probe, beyond what’s even observable at his timescale), but this could be due to leakage in the craptastic capacitor they usually use or more likely just that I haven’t crunched the numbers at all out of that waveform. Anyway, when it comes down to it, there is really no need for whatever nonsense he did here with an active op amp. If you run into this problem, the easiest way to deal with it is wire a resistor in parallel (basically load the output with something small, like 1k or 100 ohm resistor). Remember, headphones are usually. It’s not a matter of trimming away VDC.

His circuit is there to be able to add a VDC to the signal, to be able to shift it up or down as desired. The soundcard won’t give you anything that is not centered around 0V, so if you would want a biased signal swinging from, say 0-1V instead of -0.5-0.5V, you’d need to add that voltage yourself.

It’s a summing amplifier which he uses to DC-bias and attenuate the signal to fit his microcontroller projects. Not to block DC, of which there is none on soundcard outputs. This would most certainly affect impedance, and load down the soundcard output. But then again, if you use the headphone output, it is designed to be loaded with an impedance in the 100’eds range of ohms. The op-amplifier has really high input impedance, which is probably what the soundcard line outputs would want to see. Old School Thump Records Rare here. With a resistive network, the signal would see an impedance of R1//R2//Rload, with R1 and R2 being your voltage divider, and Rload being whatever you hook it up to. This amounts to an input signal impedance of less than the smallest of R1,2,load, and could potentially draw more current from the soundcard than it was designed for.