Soundcard Oscilloscope #2

Moving on from the previous post regarding the homebrew oscilloscope, I needed to build the calibration / signal generator unit to get the thing doing something other than reacting to mains pickup from the palm of my hand.

Mr Build Your Own Oscilloscope’s book has a nice chapter on how to do this. I won’t bore you with a death by photos, so here’s a select few –

The completed board prior to testing

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Peak to peak voltage calibration

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Testing prior to boxing

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Boxed and completed

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I wasn’t overly convinced by the Scope software’s signal generation, which was confirmed by unplugging my probes and watching a perfect sine wave continue uninterrupted on the screen! I’m chalking that up to not having read the manual for the program and a configuration issue because I dug out my other signal/function generator board which is an old Maplin Vellman kit, attached my scope probes and Bobs your uncle, it’s working an absolute treat.

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Now I’ve got a fully functioning oscilloscope, with virtual knobs and dials! Once I get my head around the general functionality of an oscilloscope again (it’s been a long time since O Level Physics) hopefully that will translate across to the hideously complex device which is my DS203 and I can put that good use.

 

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Soundcard Oscilloscope #1

Now this little side project is an absolute bonus on the scale of things and I would thoroughly recommend it to anyone playing with electronics or radio stuff.
There’s a minimum of test equipment you need to play this game and as you progress and your knowledge or lack of it grows there becomes a need to expand and diversify.
When I was trying to get my SoftRock working one suggestion was to monitor what was going on with an oscilloscope. It’s at that point you wish you had all the toys of a full blown R&D department on tap to make things easier!

When I was a kid I remember physics lessons when the teacher used to wheel out oscilloscopes on trolleys for lessons and they were the size of a detached house. That said they were always pretty cool in that they allowed you to see things which were completely abstract and were otherwise just drawn on a blackboard in the hope it would make sense.

A friend of mine who had a brain the size of a planet worked out how to link one to a then cutting edge BBC Microcomputer and get the oscilloscope to trace out keystrokes as they were pressed. Not bad for a 16 year old in the early 80’s! It was at that point I realised I had to have one. I had no useful purpose for one whatsoever but hey, what’s new there?

Things have moved on since then and now for a relatively small amount of money you can buy open source pocket oscilloscopes such as the DS203.

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Moving up in price you can buy USB interfaced PC based units or go the whole hog and buy a second hand fully featured digital or cathode ray tube unit from eBay.

Now I’ve looked at these things over and over with no justification for expending the still not insignificant chunks of cash required (plus a fair amount on postage as a lab oscilloscope would be a suitable substitute on Britain’s Strongest Man for the Atlas Stones Challenge) for something that would get used a couple of times a year.

I very foolishly paid out for a DS203 off of eBay thinking it would be the way to go and then found that the lack of an English manual and a menu system that was so complicated meant I couldn’t use the damn thing! What I needed was something with knobs and dials which I could adjust like the old school ones!

Then one day I was internet surfing when I came across this.

http://homediyelectronics.com/books/soundcardoscilloscope/

It was a website plugging a cheap book (as it only exists as an e-publication via Kindle) on how to build a DIY soundcard oscilloscope probe.
I’d heard about these things before and dismissed the idea until I read further and found that the very simple premise, if utilised with the right piece of software, would give you a PC based oscilloscope that for all intents and purposes had knobs, dials, a screen you could easily read and more importantly a manual that actually explained how to use it!

The book is divide into 4 parts – the probe, a calibration and signal generator unit, how to use your newly built tools with Soundcard Oscilloscope which is available as shareware and a few test projects.

The book’s written on the premise of a very limited knowledge of electronics, computing or even how to use an oscilloscope, which is always a good place to start and at the bare minimum to follow this recipe would cost you a few quid to build a single channel probe. The below schematic shows how simple the circuit which sits between what you want to test and the soundcard is.

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Now admittedly this principle comes with a caveat in that if you stick your probe on a voltage beyond the protection the circuit provides you’ll blow your sound card to pieces at the least and your PC at the worst! The book suggests nothing greater than 30 volts peak to peak should be attached to the probe.

That said, if you respect the limitations you get an oscilloscope for a few minutes labour plugging components into a breadboard or from whealding a soldering iron.

I decided to build the probe first and see what happened.

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This thing had been sitting on my desk for several months waiting to be tested. Why? Well I can safely say I had a degree of scepticism about this project and really didn’t want to blow my recently rebuilt PC to pieces.

As a result my TOUGHBOOK was the designated crash test dummy for this one but even though it’s a cross between The Luggage from Terry Pratchett’s book The Colour of Magic and Arkwright’s cash till from Open All Hours (I’m sure its scheming how to bite my fingers off everytime I close the lid!) I really didn’t want to fry it.

One solution was to protect the PC by running the probe input into an external USB soundcard rather than the on board one. If that went up in smoke no great shakes. I had a Griffin iMic in the bottom of a box, never used and waiting to take one for the team.

So with the iMic attached to a USB port I gingerly plugged my probe in, sparked up Soundcard Oscilloscope and ran the test leads across my hand hoping to see a ripple on the trace. Absolutely jack on the trace. Great!

Now this could have been a problem with my build (doubtful as it was so simple), a level issue with the iMic (again doubtful as it had been duly tweaked) or the iMic doing something strange.

There was only one way to test this hypothesis and that meant giving the TOUGHBOOK both barrels, possibly.

So with every conceivable appendage crossed the probe was plugged directly into the on board sound card and bingo! It worked a treat. The trace below is the mains pickup from placing the probes against my hand.

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The next step is to package the probe board into a neat little enclosure to give it some mechanical protection.

Thankfully Mr Build Your Own Oscilloscope’s book even goes as far as showing you how to drill the enclosure so everything fits perfectly, if you buy the same enclosure as he did, which is a nice touch as trying to box things up to make a professional product can be a real pain.

Suitably armed with his working diagram I headed for the garage. Now the best thermometer I’ve got in the house is the cat and when the urban ninja is hugging the radiator and wearing a look of disdain at any suggestion of going outside you know it’s cold out there! Consulting something with a graduated scale rather than fur showed it was minus 3! I really do need to fit some form of heating in the garage.

Now after saying “isn’t it great to have someone show you where to drill the holes in the enclosure” I discovered a slight snafu in the book. There’s one crucial measurement missing from the diagram which prevents you from simply drilling holes and getting on with stuff. The annotated measurement in red should help anyone wanting to follow the book’s methodology.

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Anyway, after an hour of numb fingers, warmed by several cups of coffee we have this.

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The full specification of this oscilloscope is –

  • Useable bandwidth          20Hz-10kHz
  • Maximum bandwidth      20kHz
  • Maximum input voltage  30V Peak-Peak
  • Coupling                             AC
  • Channels                            1 or 2 depending on PC soundcard
  • Power requirements        Self-powered

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And there we have one completed oscilloscope probe unit!

Step two is to build the calibration circuit and signal generator so the software can be calibrated to give meaningful data.