Computing stuff tied to the physical world

iPad oscilloscope

In Hardware on Oct 7, 2011 at 00:01

While exploring the different ways to get to grips with the 50 Hz AC signal, I stumbled on video about oScope, an app for iPad and iPhone. It’s just €3.99, and samples with 16 bit @ 48 KHz.

It works via the audio input, which is a decent A/D converter for signals in the audio range. The neat bit is that it can also get its input via a USB audio adapter hooked up using the iPad’s camera adapter kit. And I happened to have all the required bits lying around:

DSC 2671

Woohoo – instant scope!

The horizontal and vertical are set with pinch-and-zoom, and the scale displays in the top left corner. Likewise, setting the trigger you just drag the red trigger line up or down.

Here’s a screen shot:

IMG 0045

(it doesn’t quite come out at reduced size, but on-screen it’s gorgeous)

There’s also what appears to be an FFT power spectrum:

IMG 0044

There’s also a (more expensive) app from ONYX Apps which can sample both audio channels and has a convenient auto-set mode (but no FFT):

IMG 0046

The problem I have with all this is that the noise in my signal is gone. These samples were taken from the same 0.1 Ω shunt setup as in the previous days, so I’m not quite sure why the amplitude is different and why the signal is so noise-free. Perhaps there is some signal processing going in in the iPad.

But a real scope based on touch screen controls and such a large display sure would be phenomenal!

  1. I notice the kit has a ferrite blob on it to filter out RF noise – perhaps this is the key difference in noise?

  2. 48 khz sampling = 24 khz max freq according to nyquist, and you can be fairly sure that there is a filter (rc or otherwise) in the audio input to throw out anything higher than audio range. the spikes are bound to be of a much higher freq…

  3. The power spectrum is interesting – it shows only ODD harmonics which is typical of a non-linearity that retains symmetry around the x-axis. What can cause this? The most likely culprits are the sub-station transformer feeding the house and that little inexpensive plug in. (Driving a/c around the iron B-H curve is non-linear).

    If you can safely “sniff” say the neutral line to ground, I’d expect that power spectrum is much cleaner, pointing the finger at the plug in. Heisenberg strikes this time….

    BTW, I can’t make sense of the vertical scales – the first says 12005mv/div ?? If the second capture is at 10mv/div, the display is ~ 20mv rms, twice that expected from the 0.1ohm shunt. Your metered current will be a little off the 100mA shown since the waveform is not a true sinewave, but not by that much.

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