Author Topic: EMF pickup from amplifier in I2C line causing glitches. (Now with scope trace!)  (Read 45032 times)

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Offline dmills

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That's cool, and clever it may be, but it's still not perfect. At my sister's place, whenever they use their microwave, the WiFi goes down. It's a pretty new microwave too, and there was no visible damage to the door that might have caused leakage.
And thats fine, the wifi product "Must accept interference", there is not (and never has been) a guarantee that a user of an ISM band will not suffer interference from another user, or that a product may not radiate (within defined limits) power in an ISM band (They are after all unlicensed).

If those door seals are good for 40 - 50dB then a 1KW oven might be radiating 10 - 100mW, not a problem because it is doing it in the ISM band, if it was the ovens power inverter radiating a load of hash via the power cord then that would likely be a problem because it would NOT be in the ISM band. 

Regards, Dan.
 

Offline StarlordTopic starter

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Well it seems we're at am impasse, CJay.  I'm not going to give up on designing electronics, and you're not interested in helping me learn how to design them in such a way that they don't cause interference.  I'm literally sitting here asking how to design filters to reduce the interference from my amplifier, and you're giving me shit because, I guess, you think I'm too stupid to be allowed to design any consumer product.

Btw, all this arguing is eating into what little time I DO have left to redesign this board so it doesn't interfere with everything around it.  Just putting that out there.
« Last Edit: July 06, 2016, 12:37:03 pm by Starlord »
 

Offline StarlordTopic starter

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You should really be plotting the voltage across R1, not that it really changes anything, and yes radians in that datasheet means radians per second.

I tried dropping a multimeter in the schematic to measure across R1, but I can't get the voltage reading to appear in the AC analysis, and the transient analysis isn't plotting the data in a way which is useful.

You said it doesn't really change anything.  Does that mean that -6db drop isn't really there?  And then why is the graph showing a 96db drop at 100Mhz when you suggested that a 1uH indicator + 100nF cap would lead to a much lower drop?  I think you said it would be around 26db?  I can't trust the graph and tweak my component values because I'm getting different results than I expect.
 

Offline CJay

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Well it seems we're at am impasse, CJay.  I'm not going to give up on designing electronics, and you're not interested in helping me learn how to design them in such a way that they don't cause interference.  I'm literally sitting here asking how to design filters to reduce the interference from my amplifier, and you're giving me shit because, I guess, you think I'm too stupid to be allowed to design any consumer product.

Btw, all this arguing is eating into what little time I DO have left to redesign this board so it doesn't interfere with everything around it.  Just putting that out there.

I'm being a b**tard to you because you don't seem to want to realise just how bad this could get for you if you don't do it right.

Don't give up, I honestly don't want that.

 I have a lot of admiration for people who design and get things to market, I've done it in the dim distant past before the stringent EMC laws and know your pain, but you are trying to run before you can walk and are leaving yourself open to immense legal problems.

Nothing you can do or say relieves you of the obligation to comply with the laws of your land but as others have hinted, there are *legal* ways to pass the responsibility on to the end user.

That's not to say you don't have to make every effort to ensure the product is as good as it can be with regard to those laws.

 

Offline StarlordTopic starter

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Well, I installed a free version of Multisim to try it out, but I'm getting nowhere fast trying to simulate an LC circuit with a 4 ohm load.  I think I'm supposed to use the bode plotter, but after creating the LC circuit with an AC voltage source (since I couldn't find something that should produce noise) and attaching the inputs to the bode plotter across the load, and running the simulation, it complained my circuit wasn't grounded, which makes no sense since it I connected both the + and - sides of the AC source to make a complete circuit.  I'm sure this is wrong, but it's really frustrating because it seems like the right thing to do.  I added a ground on the negative side just to get the error to go away, but I'm getting nothing on my bode plot when I run the simulation.  I set F and I to 100MHz and 100Hz and the AC source is 1Hz. 

I tried to find some Youtube tutorials on how to make this all work and was met with even more frustration because almost none of the damn things have any audio.  There's no one explaining why they're doing what they're doing!

I don't know why this is so difficult. It's a digital amplifier. It's an LC low pass filter / crossover. It's a common thing used everywhere. Why the hell isn't there some audio site or software out there with a graphing calculator specifically for this purpose?

I mean I could just use the 1uH inductor and .1uF cap that was suggested here and hope it works, and maybe replace the caps once I have the boards made to see how different values change things, but I'd really like to see for myself what the graph looks like, and I'd like to try out a 2.2uH inductor with maybe a .22uF cap?  And see how much better that is, and make sure there's no peak.  And I'd like to see how large the peak is and see how things change when I reduce the load from 4 ohms to 2 ohms, because both will potentially be used.
 

Offline StarlordTopic starter

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I found this one page where someone tested a Sure amplifier and made some improvements to it which includes a multisim file:
http://www.trevormarshall.com/class-d-tutorial/

I can see he's set it up quite differently from how I attempted to.  I have no idea why two AC sources are used, but I'm guessing it has something to do with that ground in the center.  I guess maybe it's simulating the push pull that the amplifier does, so the AC on the bottom has it's positive side reference to ground, and is pulling the voltage negative as a result?  Sorry, I don't know how to state it properly.



Anyway, I see the pair of inductors and caps that would be on the amplifier output, and I assume R1 and R2 are the DC resistance of the inductors (why isn't that built into the model of the inductor and set as one of its parameters?) but what's C3 and R3 for?  And why are there three bode plots?  And how did he arrive at those models for those two speakers? 

And why are the bode plotters connected to the circuit in such a strange manner?  In other words, why do they have inputs and outputs, and why are both connected, and why to those locations?

« Last Edit: July 07, 2016, 01:24:24 pm by Starlord »
 

Offline StarlordTopic starter

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Well I read back over the article again and I did find where he got those parameters for the speakers. The link was broken, but I found the datasheet for the 4 ohm driver:
https://www.parts-express.com/pedocs/specs/264-1042-peerless-830983-specifications.pdf

 

Offline Delta

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Simply tweak your circuit so that all the EMI it spews forth is in the ISM bands....  Problem solved!
 

Offline StarlordTopic starter

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So I got that simulation from the audio site working.  At least I think it's working.  Obviously I have no way of knowing if the bode plot it's showing is correct.

However, I did try inputting both the suggested 1uH / 0.1uF and TI's 10uH / 1uF values to use as a reference, and then tried some a few other values for comparison.

For:

10uH / 1uF = 3.4db peak @ 71KHz, -52db @ 1MHz (TI suggestion)
1uH / .1uF = 10db peak @ 522KHz, -9db @ 1MHz (forum suggestion)
1uH / 1uF = 16db peak @ 182KHz, -32db at 1MHz (peak too high? may put amp into overcurrent protection?)
2.2uH / .22uF = 9db @ 255KHz, -25db @ 1MHz (better than the forum suggestion? and tiny 2.2uH inductors that can handle sufficient current exist)
2.2uH / .1uF = 6db @ 372KHz, -18db @ 1MHz (less peaking + uses capacitor value I use everywhere else, somewhat less filtering at 1Mhz range)

I'm not sure if -9db @ 1MHz is a comfortable margin, so I may just go for the 2.2uH inductor instead, possibly with the .1uF capacitor so I don't need to add another part to my BOM.

-9db would only reduce the noise by 1/8th, while -18db wouldr educe it by 1/63rd.  That's a pretty major difference.  Of course -25db would be 1/316th.  But I think 1/63rd will be enough? That should take my +-400mV spikes, and reduce them to +-6mV.

Btw I forgot to mention it earlier, but Mouser offers a free version of Multisim called Multisim Blue, if anyone wants to check that out. It's not perfect, it seems to have only one level of undo which is crazy in this day and age, but aside from that it's leagues better than LTspice.
 

Offline StarlordTopic starter

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Ah crud, I just realized that that tiny resistance the inductors have seems to matter a lot.  A 4.7uF inductor with 200mV of resistance shows a -1.5db drop in the portion of the spectrum that's supposed to be flat. That would mean a 30% reduction in volume.  The 1uH inductor on the other hand with a 47mV resistance only shows a -0.2db drop, which means only a 5% reduction in volume.

Of course, again, I don't know how accurate these graphs are, so perhaps the drops aren't this severe.  But I suspect they are.

Thankfully, I may be able to use a somewhat larger inductor without going too big to get that voltage drop down where I want it: less than 50mV. 
 

Offline dmills

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3dB is generally figured to be at the lower end of the scale for a perceived volume change, with around 10dB (This is frequency dependent) being generally figured to be roughly a perceptual doubling in volume, there is a reason concert sound systems get so stupidly large.....

Regards, Dan.
 

Offline T3sl4co1l

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Simulators are first and foremost, numerical solvers.  They only represent reality to the extent that you, the modeler, have created a model that will produce realistic numerical results.

Ground is absolute, so you have to add a GND symbol (net 0) somewhere.  Preferably on a common net, so all your voltage measurements are relative to it, without having to connect a pair of wires to read the difference.  (In SPICE, there IS such a thing as absolute voltage -- 0 is global ground.  In reality, you always need two wires.)

There are two kinds of components, for example: SPICE standard R, L, C are pure abstract components.  On the other hand, real components, can be modeled as an infinite network of abstract RLC parts!

How far into that network you go, determines how good (accurate over a given frequency range or time scale) your model will be.

The analyses you will be most interested in, are Transient (the "play" button, or select it from the menu), and AC Steady State.

To use Transient, use pulse signal voltage sources to represent your amplifier's switching output.  The rise and fall times will be short, 10ns perhaps.  You specify the pulse width (say 0.5us) and period (say 1us, thus giving about 50% duty at 1MHz -- representative enough), and the voltages when 'on' and 'off'.  You can also assign an AC Magnitude, which does nothing in Transient.

To use AC Steady State, at least one voltage source has to have an AC Magnitude and Phase assigned (otherwise, they default to zero).  This performs an AC frequency sweep, using the assumption that the circuit's DC operating point is linear.  (This won't matter, because your network is linear.  It does matter for nonlinear circuits, like amplifiers, where the DC condition varies with signal level.)  On the sim dialog, enter a range of frequencies, usually a fair amount below and above the cutoff frequency (say, 10k to 100M).  You'll want a decade plot with 200-500 points/dec.  Go to the "Output" tab and pick a few circuit nodes to plot.  (If you haven't named the circuit nodes, they will have default numberings.  Hit OK on the dialog and double-click a wire to see what name it has, and set a user-defined name if you like.)

Once you've picked the outputs, press the Simulate button.  The graph view will open, and you should see something resembling a lowpass filter.

Note that, since the SPICE components are ideal, your filter will also be ideal.  Adding ESR to inductors and capacitors is a good first step.  Inductors also have EPR (i.e., a modest sized resistor in parallel) and C, and capacitors have ESL also.  A real filter will have many humps and valleys at high frequencies (because real components are complicated); in the simulator, you only get as many humps and valleys as you have reactive (L or C) components.

You can also determine power dissipation.  If you do a Transient simulation (from the menu), you can pick outputs.  If you write an expression for power (such as VSRC[ i] * V(VSRC), the power generated by a source VSRC), you will get a plot of that.  You can do this for sources and loads, and subtract to find the difference: losses.

Tim
« Last Edit: July 08, 2016, 12:00:17 pm by T3sl4co1l »
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Offline SteveyG

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Ive never used LTSpice before, but I installed it and it looked old and shitty, so I tried setting up a circuit in Digikey's PartSim which I had working in five minutes and graphed the output, but I'm not sure the output I'm seeing is accurate and their parts don't provide many parameters to adjust, so I guess both tools have their serious flaws.  I went back to LTSpice and am now trying to set up a simulation, hoping it actually has a graph and noise function like Partsim so I can see a graph of the falloff, but god this software sucks.   I mean they got nothing at all right about the interface.  Mouse zoom is reversed, and if you point somewhere and zoom in it doesn't zoom into that point, it also works in a mirrored fashion from how it works in Eagle and every other cad package out there.  And when I move a component, the wires become detached, or maybe they were never attached in the first place.  It's a mystery!  How exciting. :(  Oh, and how could I forget that you have to cut to delete, you can't just point and press delete, and for some reason to rotate you have to press CTRL-R instead of just R or simply right clicking while you're dragging the object.  Ugh.  Annnd, I can't find any noise function on the voltage source.  I'm betting this software has no ability to graph the output either.

Tell me again why people recommend this tool so much?  There's gotta be something more modern out there.  This looks like a Windows 3.1 application.  I have literally used simulators written in Javascript that function better than this.  And that even showed the flow of electrons in the circuit in real time.

LTspice is very powerful and isn't difficult to use - electronic simulation tools don't need to show electron flow or run in Javascript to do their job. Do you know how to use SPICE?
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Offline StarlordTopic starter

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LTspice is very powerful and isn't difficult to use - electronic simulation tools don't need to show electron flow or run in Javascript to do their job. Do you know how to use SPICE?

Well I'm sure once you learn it's quirks, you can use it, but I think it's fair to say it's design is antiquated and unintuitive.

And sure, you don't need to see electron flow, but I just mentioned it because when I was learning how to design filters with resistors and capacitors long ago, being able to see what was actually going on with the electrons in real time made it so much clearer to me what the capacitors were doing.  For a beginner, it's a godsend.

Anyway, I got Multisim Blue from Mouser and I found an audiophile site which actually had a simulation model for a class D amplifier output filter, and from there I was able to simulate different values of inductors and capacitors and see how they affected the output.  I'm still not sure how those peaks after the flat region affect the current draw of the amp, but since someone here knew something about that and suggested a certain set of values and said the current use was related to that peak, as long as my peak is less than or equal to the one his suggested values give, I think I'm safe.

I finally settled on a 2.2uF inductor and .1uF cap, because it kept that peak low, but increased the attenuation of frequencies above 1MHz by a great deal more.  And after looking for hours at different inductors, sorting by price and size and quantity available and DC resistance and current handling capacity...  It suddenly occurred to me that I already had a tiny 2.2uF cap on my 5V switching regulator, and when I looked at the specs, it matched up precisely with my needs for the amp.  So it turns out I don't need to add yet another component to my BOM which is nice, given this whole thing is already too expensive.  But they're still gonna be $1 apiece with the placement costs, so, sad face. 

But back on the subject of Multisim... I like it a lot.  Reasonably modern interface, and works exactly as you'd expect it to.  I have absolutely no idea what I'm doing, but at least I can copy and paste and delete and move and rotate components in a way that makes sense, as well as easily connect wires between them.  I don't know how much I'll need to use it for the simple mostly digital circuits I design, but I think it will be my go to sim tool from now on.
 

Online tggzzz

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If the FCC wants products to be tested, then the government, not the small business owner, should bear the brunt of those costs.

Don't be silly.

The reason for many regulations is to stop small businesses having to bear the brunt of the costs. The costs, that is, of being prevented from doing their  business because they are being stomped by interference from someone down the street.

Or maybe you would be happy to have you service disrupted by someone operating a spark-gap transmitter.

I've dealt with the FCC in terms of opening up and setting new types of standards w.r.t. radio interference. The FCC were eminently reasonable, provided we ensured that pre-existing users continued to be able to operate.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline StarlordTopic starter

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http://www.ti.com/lit/an/sloa119b/sloa119b.pdf

On Pg 15, why does it state "Zcm at 250 kHz represents the impedance seen by the amplifier at the switching frequency" when the point on the graph being referenced appears to be at 3.5KHz, and furthermore is not at 51 ohms as indicated, but 45 ohms instead?  Is the X axis of the graph labeled incorrectly?  And did they get the impedance wrong?

[edit]

Nevermind, I see what they did.  I thought there were two red lines because there's a sharp point at the base of the graph, and I assumed they'd be pointing towards the point of interest and the point they were pointing to was close to 51 ohms.  But the point they're talking about is on the opposite side of the graph, unlabeled.
« Last Edit: July 09, 2016, 02:44:18 pm by Starlord »
 

Offline T3sl4co1l

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Yeah, it just looks weird because the semi-log plot.

Tim
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Offline StarlordTopic starter

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So on the subject of I2C over long cables again, I've got an idea. 

The issue with using less than 2K pullups on my I2C lines is that I have one chip, an ADC, which I'm not sure can withstand the higher currents.

I was looking through the I2C spec sheet, and I saw mention of series resistors in there.  And it got me thinking...  What do you suppose would happen if I used stronger pullups on the microcontroller end of the cable, but put series resistors at the other end? 

For example, what if I reduced the 2K pullups to 500 ohms, and then put 1.5K series resistors at the end of my cable, so the total series resistance between the chips and the 5V bus is still 2K?

Would that in any way improve the rise times, because all that cable capacitance is connected to the strong pull ups? Or would it make no difference because to discharge the cable current has to flow through the 1.5K resistors? 
 

Offline dmills

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Steady state, with the output on that remote chip pulling down, how low will the voltage seen by your controller go (Think potential divider)?

Regards, Dan.
 

Offline StarlordTopic starter

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Steady state, with the output on that remote chip pulling down, how low will the voltage seen by your controller go (Think potential divider)?

Regards, Dan.

Ah.  So if the remote chip is trying to pull the line down to 0V, the voltage divider formed with the pullup will only allow it to go down to 3.75V :(

 

Offline T3sl4co1l

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So on the subject of I2C over long cables again, I've got an idea. 

The issue with using less than 2K pullups on my I2C lines is that I have one chip, an ADC, which I'm not sure can withstand the higher currents.

I was looking through the I2C spec sheet, and I saw mention of series resistors in there.  And it got me thinking...  What do you suppose would happen if I used stronger pullups on the microcontroller end of the cable, but put series resistors at the other end? 

For example, what if I reduced the 2K pullups to 500 ohms, and then put 1.5K series resistors at the end of my cable, so the total series resistance between the chips and the 5V bus is still 2K?

Would that in any way improve the rise times, because all that cable capacitance is connected to the strong pull ups? Or would it make no difference because to discharge the cable current has to flow through the 1.5K resistors?

You might use small series resistors to source-terminate the drivers, to eliminate ringing.  Excessive value will ruin the logic input threshold, because you're making a voltage divider and the driving pin can't pull the line all the way down.

Tim
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Offline 2N3055

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At the very beginning of this discussion, before it became a contest who's right on many moral and legal and political issues :palm:, I mentioned to you something called active pull-up .. I guess you didn't understand so you disregarded it...

ACTIVE PULL-UP !!! :-+

look it up, I don't mean to explain it here, some very good explanations in Googleverse ... If anything can help with extending I2C specs to the limit that's the ticket... :-DMM :-BROKE

A hint from just one manufacturer : Linear tech LTC4311 .. More from others and some discrete circuits too...

It helps cleaning I2C signal edges.... Might help you with your spurious logic transitions on I2C...

Of course, good grounding and filtering RF radiation from amplifier is a must...

So much from me..
Wish you luck..

Sinisa


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Offline StarlordTopic starter

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At the very beginning of this discussion, before it became a contest who's right on many moral and legal and political issues :palm:, I mentioned to you something called active pull-up .. I guess you didn't understand so you disregarded it...

ACTIVE PULL-UP !!! :-+

look it up, I don't mean to explain it here, some very good explanations in Googleverse ... If anything can help with extending I2C specs to the limit that's the ticket... :-DMM :-BROKE

A hint from just one manufacturer : Linear tech LTC4311 .. More from others and some discrete circuits too...

It helps cleaning I2C signal edges.... Might help you with your spurious logic transitions on I2C...

Of course, good grounding and filtering RF radiation from amplifier is a must...


I appreciate the suggestion, and I did actually look it up and found that particular chip in fact, but they're $3 apiece in low quantities:
http://www.digikey.com/product-detail/en/linear-technology/LTC4311CSC6-TRMPBF/LTC4311CSC6-TRMPBFCT-ND/1965058

I could get two of these instead for the same price:
http://www.digikey.com/product-detail/en/texas-instruments/P82B715DR/296-24729-1-ND/2092551

And stick one one each end of the cable.  With the chip you suggested, if I stick it on the microcontroller end, then maybe the chips can't talk back to it because of the large capacitance between them and the chip, and if I stick it on the chip end, I can't boost the microcontroller pullups to help it overcome the noise because I still have I2C chips at that end of the cable and I'd prefer not to limit myself to parts that support fast mode+.

But if I were willing to accept that limitation, that chip would still probably not be the best solution available, because there's this buffer which is a third the price:
http://www.digikey.com/product-detail/en/nxp-semiconductors/PCA9617ADPJ/568-10211-1-ND/3991867

If I stuck that on the end of my cable with my chips, I could use the stronger pull ups on the microcontroller side, and I wouldn't have to worry about it affecting my MCP3021 ADC which is a 400KHz device of unknown sink capability, or my AS1115 multiplexing LED driver, which also does not specify its sink capability but suggests it can do 1MHz I2C which is not FM+ but maybe implies higher sink capability?

Right now though, since I can implement that last solution without any changes to the main PCB, I'm gonna try it with the amplifier filter and leave the pull ups alone.  I know the system works as long at the noise from the amp isn't being picked up, even with the slow rise times.  I may get a board manufactured with that PCA9617 buffer though so I can test with lower value pullups, and then based on the results I can make a decision about whether I need that right now or not, or if I should manufacture them with just the pullups and then test the TI part on a future board revision.
« Last Edit: July 10, 2016, 01:24:25 pm by Starlord »
 

Offline T3sl4co1l

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Why in the hell... would anyone buy a $3 LTC chip to do what four resistors and a pair of transistors can do for pennies?

A classic current source circuit, tuned for 1-10mA perhaps, will do the same job. :)

Tim
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Offline janoc

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Why in the hell... would anyone buy a $3 LTC chip to do what four resistors and a pair of transistors can do for pennies?

A classic current source circuit, tuned for 1-10mA perhaps, will do the same job. :)

Tim

That sadly assumes that the OP knows what they are doing. Right now I only see a level of ignorance that is only dwarfed by the arrogance of the "designer".
I would still ditch the whole I2C idea over the long wire and put there I2C<->RS485 converters.  However, seeing the "arguments" being put forward about the EMI not having to care about certification, I have my doubts that the OP will be able to pull that off.

@Starlord, if you think you can flount the EMI compliance rules, let me just tell you that I have recently seen a startup selling a certain product with a similar attitude to regulatory compliance to yours. They are not selling it anymore, despite it being their only product and having years of R&D time in it - they were told how much they are going to have to pay in fines by the techs in the EMI compliance lab. Ignore rules at your own peril - I am sure FCC is going to be very entertained by your bizarre theories and arguments why is it ok to break the rules.

FYI, most HF/VHF radios (not CB!) used by amateur radio enthusiasts or by boaters and small planes are 12V affairs radiating few watts (<100) of power. And those certainly have no issues communicating - or causing interference! Cause RFI on an airport frequency and you will have a van with a directional antenna in front of your house before you realize that you even have the gizmo on. Heck, people got located and fined for using a stupid baby monitor that was causing RFI at an airport frequency!
 
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