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Author Topic: Determining Ambient EMF pollution (Jitter)  (Read 4373 times)
wishfulthinker028
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« on: November 17, 2011, 11:45:08 PM »

So I did some snooping around, and it turns out you can detect ambient EMF using an oscilloscope, this would be exceptionally helpful determining what characteristics (and where) affect the Wacom sensor board.

How to:
  • Simply cut a small piece of tin foil (for the best results), and attach it to the lead.
  • Turn on your oscilloscope and set the sampling speed to some small amount (like 5us or whatever). This will need to be adjusted as you sample the EMF. Also, don't forget about the voltage, set that too some small amount (like 5mV).
  • move the lead to and from parts/areas where you want to sample the EMF, you should see a change in the voltage/time resolution (small spikes in voltage every X time samples).
  • The period between the spikes is the what you'll be looking to observe.
  • To determine the frequency, take the inverse of the period (f= 1/t, f = frequency, t = period), and you will have the EMF pollution frequency.

Why:
I'm thinking that if we can determine the the frequency of the ambient EMF pollution, we may be able to add some sort of low-pass frequency filter to the board, effectively cutting off the jitter, with minimal additional shielding (or a combination of both). Additionally I've seen that the LCD's can be tweaked a bit with their refresh rates, etc, perhaps we can determine a jitter threshold. This threshold may need to be characterized per-product line, per-size, or even per-board (the pens are tune-able so it may just be per-board).

And finally, we can determine what is causing the jitter for sure, and develop a solution//approach to solving this.

I'm bringing my board into work (got some fellow engineers interested) to see if we can't reverse engineer some of the architecture to figure out what's going on inside and see if adding an additional, variable filter may be possible.

Comments, suggestions and speculations are all welcome.

Pictures coming soon...maybe (depends if I can get my hands on a nice scope for personal use or not).
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Tigertron
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« Reply #1 on: November 18, 2011, 02:57:03 AM »

I did try this already but didn't come up with anything usefull. I believe it is not a single frequency but multiple.  It may be a band. It is very low. even at the lowest sensitivity setting on my scope I could only detect very weak signals. It maybe my probe. I don't use it that often. Still I have an XL pad and don't have enough room for it to work on it properly.

Let us know what you find. I for one am interested.
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wishfulthinker028
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« Reply #2 on: November 18, 2011, 05:54:22 AM »

Hmm...have you tried characterizing the EMF in any LCD's? I tried this at a rudimentary scale at work, and saw something like a 60kHz pulse, of course this was without disassembling the LCD or shielding its driver circuits.

I was hoping to be able to find and characterize a pulse like the one described above that could feasibly interfere with the EMF generated by the Wacom board, or even stimulate the pen's LC circuit and give false positioning (jitter), or something like that.

I've come across these resources around the forums before, but what I'm using for information at the moment is:

The Technology section of the wacom wiki says the board emits a B-field at 531kHz, so I feel like it's not necessarily the board itself. Because if an LCD, entirely unprotected with all of it's components together (CCFL, not LED) only emits about a 60kHz pulse, I don't see this interfering with the 531kHz frequency. The pen however, due to its lack of direct power source would feasibly operate at a much lower frequency//voltage. I believe we have a very very sensitive scope at work that could pick up sub milliVolt (or was it microVolts...), I just need to get the OK from the higher ups before I start using company equipment for hobby stuff (should be ok, but some people can be tightasses).

Anyways I'll read up on the patent and see what's going on there.
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Tigertron
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« Reply #3 on: November 18, 2011, 12:12:27 PM »

My testing was done only on a wacom intuos4. And it was limited to just see if I could see anything. Nothing jumped out. If you have access to a spectrum analyzer that works in the kilohertz range that would reveal more than a scope I think.

Once the frequency range is identified it would be helpful to look for interference from the LCD in that range. But many things can be at play. Harmonics and passive reactance can be very trick to isolate.
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wishfulthinker028
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« Reply #4 on: November 19, 2011, 01:39:43 AM »

I think we have a spectral analyzer, will check at work Monday.
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bernard
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« Reply #5 on: November 22, 2011, 02:26:51 PM »

wow! great stuff.  Yeah, spectral analyser is what you need here.

NOTE: the "base" resonant frequency (or carrier frequency) is different for different board "series".  Ultrapad(penenable), Intuos1, Intuos2, Intuos3, Intuos3, Graphire, Bamboo.  All operate at different frequency. 

I would doubt the pen can operate at different frequency, it is tweaked for a precise one.  (there is always the harmonics of course).
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