A couple of years ago I presented a preamplifier on my homepage which allowed it to use standard oscilloscope probes on the line input of a soundcard. This small circuit solved the problem that a soundcard has quite a low impedance on its line inputs which makes it unusable for measuring high-impedance signals.
Already at that time I found quite a few application programs to use the soundcard as a simple and cheap storage oscilloscope, but nobody seemed to bother about the impedance issue or any way to connect oscilloscope probes to a soundcard.
My previous design used a single-supply operational amplifier and a 9 V battery to offer a standardized input impedance of 1 MΩ at the input and buffer the input signal which is then capacitively coupled into the line input of a soundcard. A directly coupled signal would have required a surgical procedure on the soundcard itself, which I deemed too risky.
Now, you can buy external USB soundcards for cheap money and about a year ago I got the idea to modify such a card for a DC connection of the input signal. In 2008 also the electronics journal Elektor took again up the idea of using the soundcard for electronic measurements. So now I finally went all the way and designed a universal input/output circuitry which can be adapted to different soundcards and offers two input channels with a standardized input impedance of 1 MΩ and two low-impedance, DC connected output channels which can drive an amplitude of 1 V into a 40 Ω load.
The USB soundcard
My design is based on the USB soundcard SC-5500P from Trust which is based on the CMedia USB sound chip CM106. From the datasheet of this chip you can see that it uses a single +5 V supply voltage and handles negative amplitudes of sound signals by offsetting these with a reference voltage of +2.25 V which is available on pin 38 of the QFP100-package for the connection of external bypass capacitors.
In order to connect a signal to the inputs of this chip without coupling capacitors you have to add this reference voltage to the signal, when taking the output signal you would have to subtract this voltage from the signal – this can easily be done with operational amplifiers.
The 5.1 soundcard comes with 2 stereo output channels, 1 mono output channel, 1 stereo line-in channel and 1 mono microphone input channel (the chip itself actually supports stereo microphone input, but this is not externally available).
Since the soundcard itself is powered from USB, a single +5 V supply voltage is available on the card. Also the current which can be drawn from this supply is limited, but since I am only adding a handful of operational amplifiers and limit the output current from the signal output this is not a problem.
In order to work with directly coupled positive and negative signals, a negative supply voltage is needed. I chose to create this supply voltage with a small DC/DC converter based on the MC34063 chip.
A first also showed that it was necessary to buffer the reference voltage from the sound chip itself, because the sound chip was not able to drive the four inputs where I needed the reference voltage (one for each input channel and one for each output channel). Therefore I had to add a voltage follower based on a single operational amplifier.
The input circuit
The input circuit consists of a voltage follower with an input impedance of 1 MΩ followed by a voltage adder which takes the 2.25 V reference voltage from the soundcard and adds it to the input signal. This way a symmetric sine wave with an amplitude of 1 V would be transformed into a sine wave swinging between 1.25 V and 3.25 V which is then fed into the sound chips analog-to-digital converter inputs. All the conversion for both channels is performed in a single quad operational amplifier package. I chose the TL074, but for this application the exact type of operational amplifier is not critical. The total amplification of the two stages is 1 – i.e. the input signal is presented to the sound chip with unchanged amplitude. At the very input you can easily implement an attenuator as found on conventional oscilloscopes, allowing you to take only 1/10 or any other fraction of the input signal – I realized a selectable 1/10 attenuation in this circuit.
In order to connect it to the soundcard with no interference with the original input circuitry I removed the coupling capacitors (C22 and C27, SMD 3 μF) and soldered the connection cable to one of the solder pads of these capacitors.
The output circuit
Apart from an oscilloscope a signal generator is also a very important tool in an electronics workshop. And some soundcard oscilloscope software also provides means to send output signals to the soundcard, emulating a signal generator. Again the original output circuit of a soundcard might not be up to the task and especially its specifications are almost certainly unknown.
I therefore even designed an output stage. It also consists of two operational amplifiers per channel – one to subtract the 2.25 V reference voltage from the sound chips output signal, and the second one to drive a complementary pair of bipolar transistors. This is not a real power amplifier and the signal amplitude is limited to the actual output voltage of the sound chip, since the two operational amplifier stages have an amplification of 1. But it is able to drive a signal with a 1 V amplitude into a standard 50 Ω load with low distortion.
In order to connect it to the soundcard, I removed the original coupling capacitors (C10 and C12, 470 μF through-hole) and attached wires to one of these capacitors pads on the circuit board.
Putting it all together
I built up the circuit on a piece of stripboard small enough to fit into a box together with the circuit board of the soundcard. In order to optimize the design I first drew the layout in XCircuit and then soldered it together starting with the wire bridges. The layout on the stripboard looks like this:
There seem to be several applications around on the internet, which can use the soundcard as an oscilloscope. Earlier I was advertising a program called audioTester from http://www.audiotester.de/ on my homepage. It also has seen an upgrade in the recent years, but I haven’t tested it again, yet.
Recently I found another program from Germany, Soundcard Oscilloscope from http://www.zeitnitz.de/. This program allows to control the soundcard output as quite a versatile signal generator, including a noise source, frequency sweep and adjustable phase delay between the two channels. It offers a two channel scope display with cursors and various trigger settings, an independent X-Y display and a spectrum analyzer. It also allows to highpass/lowpass/bandpass/notch filter the input signal and display the result on the scope screen.
The Soundcard Oscilloscope software is free for private and educational use, but I chose to register my copy of the software and plan to contact the author soon with some additional suggestions, but also some flaws which I think I have discovered.
|USB sound card||Kjell & Co.||SEK 299||SEK 299|
|plastic box||Kjell & Co.||SEK 69||SEK 69|
|stripboard||Kjell & Co.||SEK 45||SEK 45|
|TL074||Electrokit.se||SEK 10||SEK 20|
|TL071||Electrokit.se||SEK 6||SEK 6|
|MC34063||Electrokit.se||SEK 15||SEK 15|
|BNC socket||Electrokit.se||SEK 8||SEK 32|
|switch||Electrokit.se||SEK 6||SEK 24|
|capacitors, resistors, diodes, transistors||own stock||SEK 50||SEK 50|