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Oscium iMSO-104 Mixed Signal Scope, by Jack Ganssle (Reviewed August, 2011)

The nice folks at Oscium loaned me a nifty mixed signal oscilloscope "plug-in" for the iPad, iPhone or iPod Touch. I put "plug-in" in quotes, as old timers will immediately think of the plug-in modules commonly used on scopes back when Maxwell had only figured out 3 of his equations. In this case, the entire scope, called the iMSO-104, is a tiny module that connects to the iThing's 30 pin connector.

First reaction: the display is simply beautiful. Crisp, clean, simple in the ergonomic Apple way. I get to use a lot of scopes, and the modern high-end versions, too, have wonderful color displays. But those are often crowded with tons of information. Useful information, necessary, even. But simplicity can be stunning, and in the case of the iMSO it is. A free demo version of is available from the App Store.

First, the biggest downer. At $297.99 I think it is priced too high.

But it is cool.

This thing is tiny, running about 1.5" x 2" x 0.3". It's not much more than a flat plastic box with a gold SMB connector sticking out for the analog probe. In operation a blue LED lights to let you know it has established comm with the iThing. And the communications interface is very clean; if you disconnect the unit the app seamlessly goes to a demo mode, and it'll return to normal operation automatically when the hardware is plugged back in.

The iMSO has a single analog and four digital channels, rated at 5 MHz and 12 MSPS. Its sample depth is 240 points. The vertical resolution isn't specified, but examination of a ramp waveform suggests it's 6 bits. As an MSO it has the usual cross-triggering between the analog and digital channels.

The feature set is surprisingly rich. It will do a number of automated measurements, like peak-to-peak, RMS, frequency, etc. Pretty much all of the triggering modes you'd expect are included. It will do FFTs, and includes built-in calibration capabilities. You can vary the horizontal delay and hold-off.

I tested the unit with an iPad 1 and an iPhone 4.

The unit comes with a 1x/10x probe that has just over a foot of cable. I guess the idea is that even with the iThing connected it's all so small one can bring the tool to the work instead of snaking a cable to the scope. There are also 4 short digital leads that end in a connector that plugs into the iMSO.

The unit takes full advantage of the iThing's features. There are no horizontal/vertical controls; one merely does the two-finger squeeze thing to expand or contract the display, which also resets the unit's sweep rate or vertical gain. Touch a balloon to the left of a trace and drag it up or down. Very unscope-like but it works extremely well. Trigger adjustments use the same paradigm: move a balloon on the right up or down to set the trigger level. The cursors, too, use the touchscreen interface.

Screen shots are easier than on any other scope I've used. Just press the screen shot button, and then another to email it to yourself or someone else. The one problem is AT&T's miserable email service; it may take a while for them to deliver the mail.

If you set the gain too high the signal will clip, just like on my fancy bench scope. But unlike that precision instrument, the iMSO helpfully displays a "`Clipping" message.

I couldn't find any accuracy claims, but the vertical measurements were low by about 10% when measuring a 100 Hz 0.5 volt peak-to-peak sine wave. A 5 volt input was accurate to better than 0.5%. But signals above 5 KHz yielded degraded voltage measurements; at 100 KHz a 5 volt sine wave was low by 10%. Frequency measurements were pretty much spot-on up to 500 KHz, and degraded rapidly after that.

The triggering is somewhat erratic; a sine wave jumps a few pixels left and right with each acquisition. I wonder if the apparent 6 bit vertical resolution causes some trigger uncertainty.

The device plugs into the iThing's charging connector and so is powered from Apple's battery. In operation the iPad/iPhone does not go to sleep as long as the app is running. But after 2.5 hours the iPad's charge level declined only from 100% to 78%, and the iPhone went from 100% charge to 67% after 1.5 hours of use.

While the app is beautiful on the iPad, it nearly takes my breath away on the iPhone. Though the screen is, of course, much smaller, the idea of having a complete MSO in my pocket is pretty cool. Pretty soon we'll be able to fill a pocket protector with an entire development lab!

Most of these tiny scopes trade off features for size and price. The iMSO's 5 MHz bandwidth is about par for this class of device. It pales next to the 1 GHz big-bucks scope on the bench here, and is hopelessly inadequate for many, if not most, professional applications. But I can see a hobbyist or student getting a lot of use from it. And, have you ever lugged a big scope on a business trip? The iMSO sure appeals for travelers with low-bandwidth needs. One does wonder how it will fare in the market at nearly $300, when other similar units are available for much less, such as the DSO series from Seeed. I have not used those, yet, so can't make a comparison.

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