Computing stuff tied to the physical world

Getting an oscilloscope

In Hardware on Dec 20, 2011 at 00:01

(Note – a better title would probably have been: “How I picked an oscilloscope”, since YMMV!)

Oscilloscopes are the “printf” of the electronics world. Without a “scope” you can only predict and deduce what’s happening in a circuit, not actually verify (let alone “see”) it. Here’s what an oscilloscope does: on the vertical axis, you see what happens, on the horizontal axis you see when it happens. It’s a voltmeter plus a time-machine.

Most modern oscilloscopes are digital. One advantage is that they can store the observation, long enough for us sluggish humans to look at the captured signal and ponder about it. For things which happen only rarely, that is crucial. But even when you’re examining things that are periodic, like the shape of a waveform, the scope gives you time to think regardless of the time scale of the event.

If you’re into soldering, then you really need at least one multimeter. Any one will do, even the cheapest one. If you’re into electronics, to the point of trying out new circuits, then you should consider getting an oscilloscope. And lastly, if you’re into pushing limits of any kind with these circuits, then you must have an oscilloscope. Let me add for completeness, that if you are only working with digital chips, interconnecting them in your projects but not really operating at the upper range, then you might want to get a Logic Analyzer first. A logic analyzer is similar to a scope, but only cares about (multiple) 0/1 signals, not actual voltage levels – their analog input circuitry is simpler than scopes, but they usually need to sample over much longer periods of time to be useful.

In this 3-part post, I’ll describe how I started out and where I ended up, with also a bit of “why” thrown in.

My first purchase, 3 months after I started with JeeLabs, was a DSO-2090 USB oscilloscope front end for a PC:

DSC 2820

It samples at 100 MSa/s and is quoted as having a 40 MHz bandwidth. Realistically, these figures tell me that it’ll give a good view of sine waves up to 20 MHz, and square waves up to say 3 .. 5 MHz. It cost me €239 at Conrad.

Such a “USB scope” does all the analog stuff in the box, and then pushes the digitized data over USB to the host PC to do the rest of the work, including presenting an oscilloscope-like display on the screen. The DSO-2090′s software is fairly good (Windows-only, not very convenient for me).

First off, let me say that I’ve got over 2 years of excellent mileage out of this thing. It’s 100x better than no scope.

The limitations I ran into were as follows:

  • It’s tied to USB, and hence needs to be close to the computer or notebook. This wasn’t always convenient for me. A second aspect is that the whole thing is not “galvanically isolated” – signal ground is USB ground. For my recent 230V AC mains experiments, that simply wasn’t practical anymore.
  • Emulating a scope on screen, while tempting due to the available screen real-estate, is not as great as I had thought. It’s downright tedious to rotate a knob on-screen using a mouse, and if you don’t, then you have to figure out a bunch of keyboard shortcuts (and remember them next time around!).

Which led me to get this tiny unit as add-on – a DSO Nano (v1), sized and shaped a bit like a mobile phone:

Dsc 2123

At US$ 90 from SeeedStudio, I didn’t expect this 1-channel 1 MSa/s scope to replace my DSO-2090, it was more a way to get a very portable unit, and a convenient little box on the desktop.

The DSO Nano trades screen size (dropping back to 320×240) for battery-powered unthethered operation. There are now more capable (and more pricey) models with 2 analog channels.

It’s a neat little box, but I underestimated the fact that the controls are even more limiting, and that a 1-channel unit very much reduces the number of things you can do with it. It’s probably fine for audio work, but with a scope, capturing the right signals at the right time is crucial, and a 1-channel unit doesn’t offer many options for triggering. In the end, I decided that the unit was not for me, and have since re-sold it.

Two years pass…

Yes, it actually took me about two years to realize that I wasn’t getting nearly as much out of my DSO-2090 as I ought to. I had also bought a Logic Analyzer around the same time (a ZeroPlus LAP-16032U), but it too was mostly sitting on the shelf collecting dust, again because having a USB device with Windows software connected to my Mac in the wrong place wasn’t truly convenient.

Last October, I decided that it was time for a change – stay tuned for the rest of this story in tomorrow’s post.

  1. “It’s downright tedious to rotate a knob on-screen using a mouse”

    Emulating real-world objects to the detriment of usability is one of the greatest sins in GUI design. I thought this had been learnt long ago, so it’s sad to hear that this sort idiocy is still showing up.

    http://homepage.mac.com/bradster/iarchitect/phone.htm

  2. JCW I want to thank you for a such an excellent set of articles about Jeenodes and so many other very interesting applications and extensions you have provided this year. I have been an avid reader. The way you write , I feel I am right there with you as you try out the various ideas, feeling your frustrations, humour and excitement of new discoveries. The best thing for me is that your experimental stuff seem to be cutting edge stuff, and your expertise is way more than my own, so I get the benefit of much faster progress than if I did it myself. Keep up the great work mate. For you and your family, and all at Jeelabs, may you have a very special Christmas.

  3. But… If you miss knobs & buttons, what prevents you from hooking some to a JeeNode and “fix” the scope sw? :) Surely cheaper… At least if you use open hardware for the analog frontend.

    • Heh… I did actually consider that. But I’ve since come to the conclusion that there’s a lot more to a “real scope” than that. I suspect that a USB link would be too slow to get the same responsive behavior from a separate analog front end.

      Maybe open source hardware will one day offer the required functionality and speed, but from what I’ve seen so far they can’t yet match a Rigol DS1052E w.r.t. price/performance.

      See also this video on Dave Jones’ EEVblog (from 2009).

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