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Building a Cheap Oscilloscope Using Your PC?
JohnMadison asks: "As a engineering student, I have a lot of projects, but not much test equipment at home. I was wondering if anybody has advice on using my PC as an oscilloscope. I've downloaded a couple of shareware programs that use the sound card for input, but they weren't really useful. I am looking for a good way to make a cheap, yet decent scope. Any sugestions?" While something like this would be an interesting hack, I'm at a loss as to what you would use for probes. The submitter mentions using the sound card as an input, but would that be the best solution? If you were going to make a custom add-on to the PC to do this, what would it need? Does such an add-on already exist? Interestingly enough, this fits in well with an earlier article we did.
Radio Shack "ProbeScope" ... about $100, plugs into your serial port, uses 9v battery for power ... comes with pretty decent software (16 bit win, but I have seen linux software that claims to support it).
It also has a little lcd screen that will show you the waveform, and will act as a digital multimeter.
I've done this with some of the freeware Windows scope programs out there (I'm a former VLSI hardware designer), but even at 96kHz the latency vs. buffer size issue gets to be a problem when probing around in a circuit. It's marginally acceptable for tuning up tape deck heads and so on, but don't try it for any serious design or repair work.
The other problem is of course sound card inputs are AC, you really want DC coupling. And high impedance.
Bottom line: you're better off with a scope board from the back pages of one of the magazines.
I was looking for something along these lines as well, a couple months ago. The software/sound-card solution is xoscope. I'm afraid I haven't had time to try that.
Another cool site is bitscope.com where you can find a completely open-design for an oscope you can build yourself (or order from them, I think.) It includes all the schematics and such, and the finished unit slides into a bay in your PC, with two slots for probes. How cool is that!
python -c "x='python -c %sx=%s; print x%%(chr(34),repr(x),chr(34))%s'; print x%(chr(34),repr(x),chr(34))"
Game ports don't actually have ADC's on them. The standard design is to discharge a capacitor, then charge it through the resitance of the joystick pot, and tim ehow long it takes to get to a specific voltage. This is related to the resistance. So, you can't use the game port for measuring voltages directly.
Picotech have addons for PCs to convert into a datalogging scope. I've never used one - and note that they tend to connect via the Parallel port. You can't get a whole lot of data through that port, so a 100Mhz scope won't actually show on the screen in real time. This doesn't usually matter, but you do want to bear in mind the delay between the measurement being made, and it actually appearing on the PC monitor.
On the plus side, they normally have data processing facilities that you only find on top of the range digital scopes - things like "trigger before", and FFT's.
I know you want to use your PC, but try to get a real live scope- you'll probably spend less money and get a better measurement device. A sound card would be barely good enough for doing audio measurements, but you run into the problem of an impedance that is too low, with too low a bandwidth. Yeah, you may only want to look at audio frequencies, but realistically, you want a scope with 3-10 times the bandwidth of the highest frequency you'll be looking at.
You don't specify what frequencies you want to work with, but for any sort of digital work I wouldn't suggest that you use *anything* less than a 30 MHz bandwidth analog scope. With that, you'll see a lot of rounding on a 10MHz signal, but you'll be able to see something. Digital scopes are very nice, if they have a decent sample rate. To get the equivalent performance of a 30 MHz analog scope in a digital scope, the digital scope must have at least a 30MHz bandwidth and 100MHz sampling rate (or a good equivalent time setting).
Bandwidth is *very* important. Oscillations can and will happen, and if it is a high frequency parasitic oscillation, a low bandwidth scope will fool you into thinking there is nothing there. You will spend many, many hours trying to debug such a circuit, because your tools will lie.
If you look around (hamfests, ebay) you should be able to find a working old tektronix boat-anchor for between $100 and $200. I've seen plenty of kits available, but they all cost more and have less performance. Since you don't say that you want to build the scope as a learning experience, you want it as a tool. The scope should be your third piece of electronic test equipment, after a multitester and a logic probe. These tools are fundamental. Get something good, whose performance you can trust, that you won't need to debug.
This not meant to be rude, but rather a simple statement of fact. If you have to ask Slashdot you aren't qualified to do it. Analog electronics is Voodoo science, and in the end you will have no idea if the representation on the screen is accurate. Even if you compare certain captured signals to ones grabbed by a high end O'Scope, there will be no assurance that signals of different amplitude and frequency are properly represented.
As an Engineer who was also an Electronics Technician for many years I have seen improper understanding of the nature of Analog cause all kinds of misunderstandings. For example, using a long ground lead or improper input impedance can distort the signal so much that a perfect signal looks FUBARed, or a FUBARed signal looks good. Indeed, there is a kind of Heisenburg Uncertainty to it all. By this I mean that the act of measuring changes the results, of course. I have seen faulty circuits that only work when you are probing them!
Your best bet is to try saving up for one designed by qualified Engineers
Also, a sound card based O'Scope is only going to handle signals in the audio range, and if the card is any good it will do some pre-processing on the inpuit anyway, further distorting the input signal, so that's way out IMNSHO.
Finally, if you do decide to do this just for fun, spend the money to purchase real (quality) probes, and learn to write device drivers if you don't have this skill already. Expect to spend several years coming up with anything that is marginally decent (based on your level of knowledge at this time, as indicated by your post.) Also, be fully prepared to never succeed and ruin a motherboard or two along the way 8^}
Whatever approach you decide upon, Good Luck!
Guns don't kill people; Physics kills people! - John Lithgow as Dick Solomon on Third Rock From The Sun
Game ports don't actually have ADC's on them. The standard design is to discharge a capacitor, then charge it through the resitance of the joystick pot, and time how long it takes to get to a specific voltage. This is related to the resistance. So, you can't use the game port for measuring voltages directly.
Erm, that's single-slope integration, that's an ADC! Many voltmeters for instance use a variation called dual-slope integration that charges a cap for a fixed time and then times the discharge back to zero. How else can a digital device measure voltage "directly"?
You are correct. I have this and do use it on occassion. Its certainly not a tektronix, but its really great for tinkering. A real nice perk with this unit is that although it connects to a serial port, near the connector it transfers the signal optically, so you dont have any ground loop problems to worry about, nor do you have to worry about nuking your mobo - which lets you poke around freely without worrying (except about the board you are poking around on)...
It's really a nice quality geek toy, and it does have a small silly lcd display right on the unit so it does not always require a computer unless you want to look at something other than a semi accurate representation of the waveform.
.sig wanted: Must be concise, funny, and display my cleverness.
If you're at all serious about building up your stash of test gear, the PC is just about the last place you want to look. Nothing it can ever do, even with add-in boards, will ever equal the quality of equipment that was made for the purpose you're trying to accomplish.
Others have pointed out that you should check out ham radio/electronic swap meets for a good used O-scope. This is an excellent idea. As far as that goes, I have some suggestions.
The Tektronix 7000 series, more notably the 7603, 7704, 7904, and 7104 are all some of the longest-lived and most durable instruments that Tek ever built. My personal unit is a 7904 (500 MHz basic bandwidth on the mainframe) with a whole raft of plug-ins. If I recall my specs correctly, the 7603 mainframe is rated at 100 MHz, the 7704 at 250, the 7904 at 500, and the 7104 at 1 GHz.
Keep in mind those are MAINFRAME bandwidths (top-end response for the mainframe IOW). You'll need appropriate plug-ins to take full advantage of that. Anyway, the 7000 series mainframes need a minimum of one time-base and one vertical amplifier plug-in to be useful. I would settle for nothing less than a 7A26 (200 MHz dual-trace) vertical module, and a 7B85 (time base) module. If you can, find a 7B92A for the time base and a 7904 or 7904A for the mainframe.
If you need REALLY high-frequency response on the vertical side, look for a 7A19 (single-trace, 50-ohm input, 1 GHz response). Bear in mind that you'll only get the full bandwidth from such in a 7104 mainframe (although you can use it in other mainframes at reduced bandwidth).
A word of caution on the 7104 mainframe, assuming you should choose it: Its CRT is more delicate than those of other Tek scopes, thanks to its extremely high writing speed, so be VERY careful to check it for screen burn.
If a portable 'scope is more to your liking, look around for a Tektronix 465, 475, 475A, or 485. Hard to go wrong with those, especially if you get one that has the DMM option.
Other sources for used Tek equipment include, of course, Ebay.
OTHER TEST GEAR: I suggest HP or Fluke for a multimeter. Fluke makes good stuff, especially older hardware, but HP's is easier to get calibrated. My own benchtop meter is an HP3456, backed up by a Fluke 8520.
POWER SUPPLIES: A good bench power supply is critical to any lab. HP, Kepco, and Lamda all make excellent units. Figure out what you're going to be working with and shop around.
As for more specialized stuff, I can't comment intelligently without knowing what specific sub-field you want to go into. There are different specialty instruments for RF, digital design, microwave, power supplies, etc.
Perhaps the best advice I can offer is ALWAYS get the best stuff that you can possibly afford. NEVER sell yourself short where your own test gear is concerned! Doing so -will- come back to haunt you later on.
Bruce Lane, KC7GR,
Blue Feather Technologies
Because I couldn't find anything good that required a PC, I went out and just bought a used oscilloscope. I bought a used oscilloscope off of e-bay for ~$400 (you can find cheaper ones easily). What I did was I would search daily for oscilloscopes that worked, included probes (they are really expensive), and most importantly had a "buy now" option. I found one that a guy put up that the "buy now" was really low (considdering the scope would be over $1500 brand new). The other trick is to MISPELL things. If you search for oscilloscope, oscilliscope, osciliscope, etc... you'd be amazed what you can find. Because it's mispelled, most people don't see it and therefore don't bid on it. That way you can get things cheaper.
My last suggestion would be to go around to schools, electronic repair shops, etc. and ask if they have any old oscilloscopes you could buy cheap. If you explain your situation you'd be suprised how nice people can be.
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