Author Topic: EMF pickup from amplifier in I2C line causing glitches. (Now with scope trace!)  (Read 45026 times)

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Offline StarlordTopic starter

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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. :)

Would a 3mA current source really do a much better job than a resistor sized to allow 3mA to flow?

Why doesn't the I2C spec suggest this?  It suggests buffers and switched pullups and other things:
http://www.nxp.com/documents/user_manual/UM10204.pdf



I mean, I think I see how what you're suggesting might work.  I'm guessing that the reason the rise time is curved in that manner is because as the voltage rises, the difference between the pull up voltage and the voltage on the data line becomes smaller, and as a result less current flows, and the signal rises more and more slowly as it approaches equilibrium.

This isn't something I had ever considered before, but it seems to make sense.

That being said, since I would need two of these for each data line, that's four transistors and eight resistors, and with PCB space at a premium I'd be lucky to find room for two transistors, let alone all the rest.

https://en.wikipedia.org/wiki/Current_source

I wonder if that single jfet with resistor would do the job?

Or, I could perhaps find an inexpensive chip that acts as a constant current source?  I've been using one chip that's the size of a single FET that requires only one external current set resistor and provides a constant current supply, which I use for high power LEDs.  I suspect some may exist that are designed for lower currents.  I guess I'll have a look on Digikey.
 
 

Online 2N3055

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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

Tim, guy is a beginner, and not an expert... He admits that and  asks for help.. :-+..
Active pull-up IS a current source... You're right.  :-+
But... I did not suggest he should or must take that chip... I made a point, and Linear has good datasheets and application notes, and I thought it would be a good place for him to get acquainted with the concept... I also said there are many alternatives, including the discrete current source circuit... :-/O

But if I where in hurry, running out of time and was making one of prototype, I WOULD just slap on one of these guys in the circuit, removing pull-up resistors and got it done with. This chip (and all alike ) is characterised for this task and will be pretty much quickest way to do it...

If you ask me how to do it on a large scale  where each penny counts... Well, I wouldn't do anything alike what he did....
I would drive leds differently (i know I do on my products), would make amplifier differently... I wouldn't trust I2C bus to do what he wants to do in a first place..

It all depends on many factors... :-//

Take care !! :)

« Last Edit: July 10, 2016, 10:22:35 pm by 2N3055 »
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Online 2N3055

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Would a 3mA current source really do a much better job than a resistor sized to allow 3mA to flow?

Why doesn't the I2C spec suggest this?  It suggests buffers and switched pullups and other things:
http://www.nxp.com/documents/user_manual/UM10204.pdf



I mean, I think I see how what you're suggesting might work.  I'm guessing that the reason the rise time is curved in that manner is because as the voltage rises, the difference between the pull up voltage and the voltage on the data line becomes smaller, and as a result less current flows, and the signal rises more and more slowly as it approaches equilibrium.

This isn't something I had ever considered before, but it seems to make sense.

That being said, since I would need two of these for each data line, that's four transistors and eight resistors, and with PCB space at a premium I'd be lucky to find room for two transistors, let alone all the rest.

https://en.wikipedia.org/wiki/Current_source

I wonder if that single jfet with resistor would do the job?

Or, I could perhaps find an inexpensive chip that acts as a constant current source?  I've been using one chip that's the size of a single FET that requires only one external current set resistor and provides a constant current supply, which I use for high power LEDs.  I suspect some may exist that are designed for lower currents.  I guess I'll have a look on Digikey.

You guessed correctly... If you look at the shape of that signal, the front slope IS the curve of capacitor charging trough resistor..
If you run it with a current source you will have linear(straight) voltage raise... How fast, depending on current... But a simple current source won't do for this.
Look what datasheet says :

..."During positive bus transitions, the LTC4311 provides slew limited pull-up currents to quickly slew the I2C or SMBus lines to the bus pull-up voltage.
During negative transitions or steady DC levels, the currents are disabled to improve negative slew rate, and improve low state noise margins. "...

So it behaves differently when voltage goes up or down... So, current source, with voltage clamping and also with a circuit that "knows" to charge capacitor with big current (fast and linear rise time), and to go high impedance what chips on a bus are pulling down, so it doesn't violate their specs and have fastest possible fall time...

Of course it makes sense, it is being used in many forms for many years on other more robust data protocols...   :-+

Original I2C doesn't suggest this because I2C wasn't intended to do what you try  do... and also Philips won't endorse other manufactures... :clap:
You have to learn that datasheets and all materials published by manufacturers are written as much by marketing and legal department as they are by engineers..
Pretty much very correct most of the time, but not complete truth and info.. Many things unsaid and incomplete.

Buffers are also very good, and those you found would possibly solve your problems...

I suggested this active pull-up because it doesn't need ANY CHANGE to your circuit... You connect chip in parallel to existing 10K pull-p resistors and try if it helped..
I would suggest you to try exactly that as a troubleshooting tool, as no other changes are needed..
If it helps it means there is hope. :-+. If it doesn't, you need linear audio amp  :scared:

If it looks better, than you do a small redesign and use those Ti buffers, they look very good and might work too....

Take care!

« Last Edit: July 10, 2016, 10:24:27 pm by 2N3055 »
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Offline Howardlong

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A quick aside, I2C is used on VGA, DVI and HDMI cables for EDID and HDCP. It's also one of the main sources of failure in the HDMI world, somewhat ironic considering the speed of the uncompressed digital video is of the order of 100,000 times faster.
 
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Offline T3sl4co1l

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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".

Geez, I know I'm frank, but that's just being mean...

Quote
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.

I think this was originally suggested (in a past thread), so it's kind of redundant..

Quote
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!

Anecdote: I worked several years at an industrial power conversion company, which literally was directly across from the airport.  Never had a call in history.

Probably the main 480V transformer, steel building, and generally-not-screaming-fast electronics, avoided putting anything in ATC range.  There was even a tech on the shop floor who preferred listening to AM radio, which was only ever intermittently interrupted by operating power supplies.  (Except for certain hijinx, but I will admit to none of it, I swear.)

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Offline T3sl4co1l

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Would a 3mA current source really do a much better job than a resistor sized to allow 3mA to flow?

Yes, about halving the risetime.  Expect something like this:



Quote
Quote
Why doesn't the I2C spec suggest this?  It suggests buffers and switched pullups and other things:
http://www.nxp.com/documents/user_manual/UM10204.pdf

Probably because it's not necessary.  I2C is Inter-Integrated Circuit, i.e., only within a board, or spanning one or two with a short connection (board-to-board header or short ribbon).

It's definitely not suitable under conditions of unshielded cables and IEC 61000-4-3 susceptibility testing.

They also show bridges for other methods.

SPI over three RS-422 pairs is an excellent method for unshielded cables; or RS-232 (with suitable filtering at RX) for slow channels, or most anything (bare logic, USB, etc.) for shielded cables.

The lesson: so many signaling standards exist, for good reason!  There are many sources of interference, and many applications that need more or less bandwidth and reliability.

Quote
That being said, since I would need two of these for each data line, that's four transistors and eight resistors, and with PCB space at a premium I'd be lucky to find room for two transistors, let alone all the rest.

Nah, only two transistors.  Well, you can use the "ring of two" if you like, but resistors are dumber, and almost always good enough.

You can use a circuit like this,



The top half (22 ohm's, 470 and 2.2k) is current sourcing.  You can't use this directly, because when one source saturates (pulls all the way to +V), it disables the other.  In this circuit, it's a feature, not a bug, but you just need two base voltage dividers, that's all.

You'd also use larger resistors, since you don't need the 10-20mA this thing makes!


Quote
I wonder if that single jfet with resistor would do the job?

For a long time, single JFETs were selected, and packaged as diodes -- CRDs.  They're terribly botique, and JFETs are pretty thin on the ground as well.

Better to go with the BJT circuit.

You can even take advantage of the BJT's hFE (i.e., current gain), and use only one resistor (for base bias), no emitter resistor or divider.  This is tricky, though: you need to use hFE-binned transistors (which are available cheaply, though!), and the tempco is awful (you get about double current at maximum temperature, and you need to make sure the transistor won't cook itself in the process).

Quote
Or, I could perhaps find an inexpensive chip that acts as a constant current source?  I've been using one chip that's the size of a single FET that requires only one external current set resistor and provides a constant current supply, which I use for high power LEDs.  I suspect some may exist that are designed for lower currents.  I guess I'll have a look on Digikey.

Hmm, possible.  Obviously a high efficiency (switching) type won't work. :)

Tim
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Offline janoc

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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".

Geez, I know I'm frank, but that's just being mean...

No, mean is intentionally causing interference and problems for everyone around, because some fool doesn't give a flying sausage about it. It is like peeing in the pool and thinking it is OK, because nobody will notice and others certainly do that as well.

I am a licensed HAM and fellows like that make my blood boil. Screw everyone else, my business is more important!

Quote
Anecdote: I worked several years at an industrial power conversion company, which literally was directly across from the airport.  Never had a call in history.

Probably the main 480V transformer, steel building, and generally-not-screaming-fast electronics, avoided putting anything in ATC range.  There was even a tech on the shop floor who preferred listening to AM radio, which was only ever intermittently interrupted by operating power supplies.  (Except for certain hijinx, but I will admit to none of it, I swear.)

Tim

Well, if you weren't causing interference then there likely wasn't much reason for that van to show up. Occasional burst of static from switching usually doesn't bother anyone, static is the way things are with radio, especially on HF. On the other hand, if someone is using a 50W amplifier to listen to music for hours and it spews broadband hash all over the bands up to VHF because it lacks proper filtering, that is going to get noticed and attract attention.  With a bit of "luck" that could make a frequency completely unusable for communication.

« Last Edit: July 12, 2016, 08:36:56 pm by janoc »
 

Offline T3sl4co1l

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Quote
Anecdote: I worked several years at an industrial power conversion company, which literally was directly across from the airport.  Never had a call in history.

Well, if you weren't causing interference then there likely wasn't much reason for that van to show up. Occasional burst of static from switching usually doesn't bother anyone, static is the way things are with radio, especially on HF. On the other hand, if someone is using a 50W amplifier to listen to music for hours and it spews broadband hash all over the bands up to VHF because it lacks proper filtering, that is going to get noticed and attract attention.  With a bit of "luck" that could make a frequency completely unusable for communication.

Well, the power conversion was in the 10s of kHz (switching frequency), and 1MW was the largest machine we had on the floor, with 100s kW machines running regularly (hours at a time, throughout the day).  It doesn't take much ringing on the IGBTs/SCRs/MOSFETs to radiate that much RF, at least in the SW frequency range!

Tim
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Offline David Hess

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That being said, since I would need two of these for each data line, that's four transistors and eight resistors, and with PCB space at a premium I'd be lucky to find room for two transistors, let alone all the rest.

Why two per data line?  I do not think you need one at each end.

Quote
I wonder if that single jfet with resistor would do the job?

Or a JFET without a resistor.  If the IC2 supply voltage is 5 volts, then a JFET will not have enough voltage compliance unless you have a higher voltage available.  The current source needs to have a minimum voltage across it to operate and for a JFET, this will be about 3 volts at 3 milliamps.  A bipolar transistor current source can operate down to 0.3 volts or lower.

Quote
Or, I could perhaps find an inexpensive chip that acts as a constant current source?  I've been using one chip that's the size of a single FET that requires only one external current set resistor and provides a constant current supply, which I use for high power LEDs.  I suspect some may exist that are designed for lower currents.  I guess I'll have a look on Digikey.

You cannot use a JFET or current regulator diodes without a higher supply voltage.  Is one available?

The LM334 3-terminal adjustable current source might work and is inexpensive.  They are good down to 1 volts at 5 milliamps but are a little bit slow based on your oscilloscope measurement.

Bipolar transistors in a current mirror configuration are cheaper yet, have better performance, and less voltage drop.  If you match transistor Vbe, then you can get away with no emitter resistors, one resistor, and 1+N transistors for N pullups.  Unmatched transistors will need an additional resistor per emitter.  I use these all time to make impromptu RS-232 level shifters and for critical digital pullups.

 

Offline janoc

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Well, the power conversion was in the 10s of kHz (switching frequency), and 1MW was the largest machine we had on the floor, with 100s kW machines running regularly (hours at a time, throughout the day).  It doesn't take much ringing on the IGBTs/SCRs/MOSFETs to radiate that much RF, at least in the SW frequency range!

Tim

Well, but you were obviously not radiating, or at least not causing interference, no? I am not going to argue about whether that system can or cannot cause interference - that's not my point and I definitely know more about brain surgery than 1MW power level machinery (i.e. nothing at all). But rest assured that interference on airport, emergency services or military frequencies is going to get you unwanted attention quickly.

Few random examples:
https://transition.fcc.gov/Bureaus/Compliance/News_Releases/1998/nrci8027.html

or
http://www.theregister.co.uk/2013/08/12/feds_arrest_rogue_trucker_after_gps_jamming_disrupts_newark_airport/

or  (interference by doorbells and other consumer electronics)
http://www.wsj.com/articles/SB10001424052702304434104579378994224188328

 

Offline T3sl4co1l

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Like I said -- it was pretty bad indoors, but the building was probably good enough shielding.

Never underestimate the capabilities of a pile of steel?  :-//

Tim
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Offline StarlordTopic starter

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You can all stop freaking out over interference; I ordered a new prototype with filtering on the amplifier today.  I went with 2.2uH indicators and .1uF caps.  Should attenuate the signal enough that I won't even see it on the scope. 

Wasn't able to do anything about the I2C since nobody could answer me as to whether the ADC could handle higher currents on the I2C lines, not even Microchip.  But since the system worked fine when sitting on my bench and I expect my filter will fix the noise issue, the slow rise times should not be a problem. 

Btw, Microchip's support system is crap.  They require you to register and enter a part number before you can submit a question but their system would not accept any variation on the part number I entered.  I had to tell it I was having an issue with the website instead in order to get it to allow me to submit anything without a part number and aside from a confirmation email four days ago I haven't heard from them.  I should have gone for the slightly more expensive TI part.  In the future, I'll avoid them.
 

Offline CJay

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You can all stop freaking out over interference; I ordered a new prototype with filtering on the amplifier today.  I went with 2.2uH indicators and .1uF caps.  Should attenuate the signal enough that I won't even see it on the scope. 

Wasn't able to do anything about the I2C since nobody could answer me as to whether the ADC could handle higher currents on the I2C lines, not even Microchip.  But since the system worked fine when sitting on my bench and I expect my filter will fix the noise issue, the slow rise times should not be a problem. 

Btw, Microchip's support system is crap.  They require you to register and enter a part number before you can submit a question but their system would not accept any variation on the part number I entered.  I had to tell it I was having an issue with the website instead in order to get it to allow me to submit anything without a part number and aside from a confirmation email four days ago I haven't heard from them.  I should have gone for the slightly more expensive TI part.  In the future, I'll avoid them.

I can answer that, the ADC won't give a damn about the current on the I2C lines, it's not sinking any appreciable current and never will do.

You could make the bus driver capable of sourcing 500 amps and the ADC would still tolerate it as long as the voltage doesn't overshoot.

From your 'scope shots of the signals it looks like whatever is driving the bus isn't capable of providing enough current to charge whatever capacitance there is on the bus fast enough to provide a nice square edge at the ADC bus input.

 

Offline T3sl4co1l

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AFAIK, I2C pin drivers are good for 74HC-sized currents and voltages.  They pull down only, with a bit stronger strength than regular HC.  Almost as if the complementary P and N channel output transistors were replaced by one N channel transistor of the same total die area -- thus, capable of sinking more current for the same voltage drop and thermal limit.

The logic tests (V_OL @ I_OL) are done at something like 4mA, to guarantee < 0.5V, which is well within the V_IL threshold of 0.8V or 0.3*VDD or whatever it is.  The pin itself will be capable of much more, at half- or short-circuit conditions (probably up to ~50mA), which is where line driving capability comes into play -- it only needs to deliver high current for a fraction of a microsecond, as the line discharges.

So, they don't guarantee that you can operate with more DC current, and no, strictly speaking, it's not something you should extrapolate.  The Rds(on) of the transistor is not a flat resistance, so at a higher current (say 10mA), it may drop more than the expected voltage (i.e., over 1.2V, say).  Which may violate V_IH and then you get dubious thresholds and much more sensitivity to noise...

By the way, about that capacitance -- the line capacitance consists of two parts.  One wire is "ground", and the other is SDA or SCL, right?  The greater part of capacitance is between these two wires.  The lesser part is between SCL/SDA and free space -- each time the output pin turns on and discharges that capacitance, it's delivering current directly into free space!  This is the first part of how the line can radiate EMI.  (I2C isn't terribly fast, so this current isn't very high, but it's there.  The second part is by inductance of the line, and the last part is by more complicated resonant effects that depend on the length and position of the cable.)  By the same method, noise in the environment couples into the line, so that it only takes a few volts of incident noise to start throwing your logic thresholds into question.

Tim
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Offline janoc

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By the way, about that capacitance -- the line capacitance consists of two parts.  One wire is "ground", and the other is SDA or SCL, right?  The greater part of capacitance is between these two wires.  The lesser part is between SCL/SDA and free space -- each time the output pin turns on and discharges that capacitance, it's delivering current directly into free space!  This is the first part of how the line can radiate EMI.  (I2C isn't terribly fast, so this current isn't very high, but it's there.  The second part is by inductance of the line, and the last part is by more complicated resonant effects that depend on the length and position of the cable.)  By the same method, noise in the environment couples into the line, so that it only takes a few volts of incident noise to start throwing your logic thresholds into question.

Tim

I think he is only going to see this type of EMI. Right now his edges are so slow, that any interference caused by I2C lines is not going to have a ton of high frequency harmonics, with the parasitic wire capacitance acting as a low pass filter. However, the moment he fixes the drivers on the line and gets nice square edges, the amount of high frequency harmonics will shoot up, with the long cable acting as a nice antenna. Even 100-400kHz I2C can produce harmonics in HF/VHF bands. I hope the cable is going to be shielded.

 

Offline engineer_in_shorts

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Just astounded at the lack of self awareness found some other posts of the OP to find some schematics/layout :
https://www.eevblog.com/forum/profile/?u=101656;area=showposts;start=225

Would like to know what commercial products he has made.
 

Offline janoc

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Just astounded at the lack of self awareness found some other posts of the OP to find some schematics/layout :
https://www.eevblog.com/forum/profile/?u=101656;area=showposts;start=225

Would like to know what commercial products he has made.


I am actually curious as well because reading that the repeating pattern seems to be - "I have dug myself into a hole, I can't afford/don't know how/lack the means to climb out of it, does anyone have any other suggestion how to get out?"

On the other hand, it is certainly a good learning experience, albeit a tad expensive, IMO. But who am I to criticize, I am not designing stuff for sale myself.

 

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Offline StarlordTopic starter

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Just astounded at the lack of self awareness found some other posts of the OP to find some schematics/layout :
https://www.eevblog.com/forum/profile/?u=101656;area=showposts;start=225

Would like to know what commercial products he has made.

Well thanks to you I just had to go through my entire history and delete every schematic and post with potentially identifying information because you want to initiate a witch-hunt.  Guess nobody's going to be able to learn from my mistakes now.  Good job.

I'll tell you who I am.  I'm nobody.  I don't sell "commercial products" in the sense you're thinking of.  I don't work for any big business designing things for them.  I sell stuff in small quantities mostly to hobbyists.  Hobbyists who will be prototyping circuits on breadboards and covering the airwaves with noise of all kinds.  Why don't you take your anger out on Sparkfun or Adafruit, who encourage people to experiment with electronics and thus ruin your CB radio fun?  Or how about Dave himself who also encourages people to experiment by teaching them, or 90% of the people on this forum who are also hobbyists and don't have EMC testing equipment and are also wreaking havoc on the airwaves? 

Or do those hobbyists not count because they're not making money?  It's funny how the moment I mentioned I'm trying to make a meager, VERY MEAGER, living at this all the hate began to spew forth.  Nobody cared when they thought I was just doing it for myself.

Do you know why I pushed back against making changes to my circuit?  BECAUSE I CAN'T AFFORD TO SPEND ANY MORE TIME ON IT.  I'll be lucky to pay my rent next month after I manufacture a handful of these things.  And that handful will only be HALF as many as I'm obligated to produce.  What the hell do you want me to do?  Give the people that pre-ordered these things nothing?   I'm sure that will do wonders for my reputation.  And I sure as hell can't give them refunds.

I'm sorry you don't think I'm wealthy enough to be allowed in your little club because I can't afford thousands of dollars to get EMC testing equipment or have the FCC do it for me.  I assure you though that if my business ever does grow, I'll do things right, if only because at that point I will actually have something to lose. Right now, I have nothing to lose except my shirt. Which I'm about to lose anyway.

Do you think LadyAda did any EMC testing when she was starting Adafruit?  Or whoever the Sparkfun people were?  Hell, I wouldn't even be in this hobby were it not for some italian guys who invented the Arduino, and do you think THEY did any EMC testing on their boards when they got started?  Massimo doesn't even hold an engineering degree.  But me?  By all means, hold a guy with $2K left to his name manufacturing a handful of boards to a higher standard.

Sorry I care more about putting food on the table than a one in a billion chance of a toy that will be used twice a year interfering with some trucker using a technology that's 30 years out of date to chit chat.

Maybe if you care so much about EMC compliance you should have spent all the time you spent arguing with me instead explaining how I could make my circuits more compliant without breaking the bank.  All you've managed to accomplish instead is teaching me that when I am inevitably forced to create a new account and burn the old one, to never EVER mention I plan to make any money or sell anything to anyone.

PS: There's thousands of people on Youtube posting their non-EMC compliant electronics projects.  Lots of people designing guitar pedals and amps without any real knowledge of electronics at all, and selling them.  Why don't you head on over there and tell them all what terrible people they are for trying to learn and start a business at the same time, like me.
« Last Edit: July 14, 2016, 12:51:46 pm by Starlord »
 

Online tggzzz

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Just astounded at the lack of self awareness found some other posts of the OP to find some schematics/layout :
https://www.eevblog.com/forum/profile/?u=101656;area=showposts;start=225

Would like to know what commercial products he has made.

Well thanks to you I just had to go through my entire history and delete every schematic and post with potentially identifying information because you want to initiate a witch-hunt. 

Very revealing. More revealing than the schematics etc.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline StarlordTopic starter

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But who am I to criticize, I am not designing stuff for sale myself.

And if you were, you would be a fool to admit it on this forum, between the people who think if you're making any money you should be paying an engineer to answer your questions (I guess they think as professional engineers they know everything and would never need to go to the internet to ask for assistance), and those who will criticize you for not engineering your product to aerospace standards thus making it so expensive no consumer would ever buy it.
 

Offline engineer_in_shorts

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Never said anything about EMC - don't care.  I was curious about you asking for help on a forum and a number of top posters trying to point your project in a different direction, only for you to ignore them.

And yes i did read the post about your business issues. In fact it is nicely archived here:

http://web.archive.org/web/20141220033446/https://www.eevblog.com/forum/projects/customer-w-unrealistic-expectations-what-would-you-have-charged-for-this-job/?

If you are struggling to put food on the table STOP tinkering with these projects AND GO AND FIND A SALARIED JOB --- IMMEDIATELY. If you have one and it's not enough money/hours then go and find another TODAY.  Trust me, it's for your own health both physically and mentally.

 
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Offline CJay

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Wow.... :palm: :palm: :palm: :palm: :palm: :palm: :palm: :palm: :palm: :palm: :palm:
 

Offline janoc

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@Starlord, I think you need a serious attitude change to be taken seriously and not as a whiny kid with a large sense of self entitlement.

Nobody is initiating any witchhunts. It is your attitude that is a problem, not that you are making mistakes. Start to take criticism as what it is - an opportunity to learn - rather than a personal attack.

I am a semi-hobbyist myself, sometimes building props and contraptions to use at work, but mostly playing in my own little sandbox for my own education and fun. However, I am also aware of the laws and regulations that govern my hobby. I also don't have an EMC lab on hand or a ton of expensive test equipment - but heck, I do the darnedest to try to limit the possible EMC problems where I can, because I know what sort of issues I can have with that. It is fundamentally in my own interest.

The law doesn't make a difference between a "little guy selling to hobbyists" and a multi-billion company like Apple, neither do the fines you can get if you are careless. Apple probably won't care too much about a few hundreds or thousands EUR/dollars/whatever fine, they would be more concerned about the bad publicity something like that would entail.  However, for me, as a hobbyist, having to pay that out of my own pocket certainly would matter!

Whether or not you are trying to make money is really irrelevant here, the point of it was that if you are trying to sell something, it is going to attract quite a bit more scrutiny than a teen building a wireless bug in their garage for fun. Not to mention that your future clients are likely not expecting to be fined by FCC (or whoever is your regulatory authority) in the future because of using your product. That's a good way to end up getting sued, IMO.

The argument about Lady Ada and Sparkfun is not really relevant neither, because they are selling kits and components, not complete products. These don't have to be EMC tested, it is the responsibility of whoever integrates them into a final product. It doesn't make sense to test a component without the enclosure or other parts of the final system because they all can affect the performance - e.g. a metal grounded case acting as a shield vs. a plastic one which does not. And even then both Sparkfun and Adafruit are actually applying for FCC testing these days, even though they don't really have to.

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Maybe if you care so much about EMC compliance you should have spent all the time you spent arguing with me instead explaining how I could make my circuits more compliant without breaking the bank.  All you've managed to accomplish instead is teaching me that when I am inevitably forced to create a new account and burn the old one, to never EVER mention I plan to make any money or sell anything to anyone.

Oh puh-leeze. You have been told and explained many times what you have to do, but you shoot down most of the proposed solutions right away. Or you lack the understanding or means to implement them. And that is somehow the fault of everyone else but you.

And the deal with the new account - you somehow think that if you do that people will miraculously start solving your problems for you, because obviously the only thing standing in the way is them knowing it is @Starlord who is asking these questions?

You are asking people to do free consulting for you, refuse to provide information, shoot down the proposed solutions, think that rules don't apply to you for some reason and show incredible arrogance in general and now think that making a new account will somehow change this? Do you think that everyone is an idiot?

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PS: There's thousands of people on Youtube posting their non-EMC compliant electronics projects.  Lots of people designing guitar pedals and amps without any real knowledge of electronics at all, and selling them.  Why don't you head on over there and tell them all what terrible people they are for trying to learn and start a business at the same time, like me.

A guitar pedal with a few opamps or linear power amp is typically not going to spew interference all over the place, unlike your class D power amplifier switching in the megahertz range and connected using long wires. If you don't see or understand the difference between these, I think you really should first educate yourself more before you dig yourself into an even bigger  hole than you are in already.


« Last Edit: July 14, 2016, 01:36:59 pm by janoc »
 

Offline StarlordTopic starter

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I think he is only going to see this type of EMI. Right now his edges are so slow, that any interference caused by I2C lines is not going to have a ton of high frequency harmonics, with the parasitic wire capacitance acting as a low pass filter. However, the moment he fixes the drivers on the line and gets nice square edges, the amount of high frequency harmonics will shoot up, with the long cable acting as a nice antenna. Even 100-400kHz I2C can produce harmonics in HF/VHF bands. I hope the cable is going to be shielded.

I suspected that might be the case, as I'm aware of how if you try to compute a Fourier transform on a square wave you'll find an infinite number of frequencies would be required to recreate it perfectly.

Anyway, I'll keep that in mind if I eventually decide to put proper drivers on the I2C lines.

But I would ask why if shielding is so necessary with data transmission over CAT5 cable, why most CAT5 is not shielded?  Does it operate at frequencies that aren't in important bands?  Or does the differential nature of the signal cancel out the noise?  If I used a differential I2C driver chip, would I not need shielded cable?
 


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