Electronics > Projects, Designs, and Technical Stuff
EMF pickup from amplifier in I2C line causing glitches. (Now with scope trace!)
T3sl4co1l:
--- Quote from: Starlord on July 10, 2016, 08:05:03 pm ---Would a 3mA current source really do a much better job than a resistor sized to allow 3mA to flow?
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Yes, about halving the risetime. Expect something like this:
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--- 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
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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.
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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?
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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.
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Hmm, possible. Obviously a high efficiency (switching) type won't work. :)
Tim
janoc:
--- Quote from: T3sl4co1l on July 11, 2016, 09:44:00 am ---
--- Quote from: janoc on July 10, 2016, 07:32:16 pm ---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".
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Geez, I know I'm frank, but that's just being mean...
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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
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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.
T3sl4co1l:
--- Quote from: janoc on July 12, 2016, 08:35:25 pm ---
--- 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.
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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.
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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
David Hess:
--- Quote from: Starlord on July 10, 2016, 08:05:03 pm ---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.
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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?
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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.
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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.
janoc:
--- Quote from: T3sl4co1l on July 12, 2016, 08:46:10 pm ---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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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
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