What does 385 MHz RF do to a circuit to damage it?

Discussion in 'Wireless & RF Design' started by Joe Black, Oct 7, 2016.

  1. Joe Black

    Thread Starter Member

    Jan 5, 2016
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    I"ll start by saying I'm not an EE nor any engineer for that matter, if you couldn't tell from the question.

    We take a Chinese made Lab peristaltic pump and modify it to be used in the Operating Room for a medical procedure. Don't worry it is harmless and non-critical, so no one is getting sued. Previously the same pump passed the 3rd ed of the 60601-1-2 testing but failed the 4th ed, when it got to the Immunity testing specifically 385 MHz and 450 MHz testing. We did some more modifications, Added a second LED module, which meant a new LED board built and added a stacker board to the PCB that was in the machine already to bypass most of it's functions. All the main board does is run the stepper motor now. We also added a UL listed, medical approved Power supply to help get FDA and CE approval. The two frequencies above would interrupt the machines function and sometimes stopping it altogether, but never killed it outright. We have tried some shielding and ferrite beads on a long Ribbon cable (18") which helped. The Ribbon cable goes from teh LED bd in the case, which is plastic, to the Stacker bd on the main PC bd. We were using a hand-held transmitter, like a walkie-talkie to test it with in house. We sent it back and their first test at 385 MHz killed it and couldn't get it restarted. That's never happened before.

    My question is: "What do these frequencies do to the machine to damage it in this way? And could we have damaged it here with our crude testing?

    It worked fine when we returned it to the testing facility. The output of our transmitter was 5w with an omni-direction antenna (looked like a walkie-talkie). Their test was done at like 2.5w with a directional antenna (Table 9 from the 60601-1-2 regs). What do I look for in the way of damage?
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    The short answer is that there is nothing I am aware of that should cause this behavior.

    But, it is impossible to answer your questions without a complete schematic diagram of the module and the add on circuitry. Drawings and or pictures of the mechanical packaging would be needed to asses the susceptibility of the product.

    I kinda wonder what non EE/non engineering people are doing in this context. Wouldn't it be cheaper and more efficient to get a qualified professional to do this work? It is hard enough when you can see and review the details. It is impossible to answer with a single paragraph description.
     
  3. Joe Black

    Thread Starter Member

    Jan 5, 2016
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    Thank you, PapaBravo, that is a good answer.
    To answer your questions: 1) It appeared to be a simple project, more like using Tinker-Toys. The machine runs fine in the field and has been in use for quite some time with absolutely No problems. The problem came when we decided to get FDA and CE Approvals to widen our markets. The regulations and standards have gotten ridiculous in the past few years so small companies like us, a low volume business, cannot afford expensive work like you suggest, but it doesn't go without merit. The machine had passed these tests before and this was a surprise, totally. We did have a EE design the LED bd and the Stacker bd. It all has worked until now. He's retired and doesn't have testing equipment to trace it and we just can't afford to build the whole thing from scratch and still make any money with it. 2) I really wasn't asking for a solution as I do know enough to know you'd need more than my explanation. As I mentioned, we buy this pump in China and they won't share their schematics or their code, so we had to improvise. They can't get FDA approval due to the added costs. I don't have all of the information you would need.

    We are actually OK with it passing just the 3rd ed, or we can get a conditional Pass on the 4th ed (newest) since the chances of this machine being in close proximity, or any proximity, to either of those two frequencies, is near non-existent. And, if it did happen, no one, patient or operator, would be harmed or in any danger. 4th ed was mainly to address Home use or mobile (EMT/Ambulance) use, which this machine is strictly used in an Operating Room. What I was wondering, and more for my own edification, was why would it kill the machine? What is it that these frequencies do to circuits, or semiconductors or whatever to kill them. it's not a nuclear pulse.

    And, if it fails or dies again, then I will have to consider getting professional advice, or at least explore it as to the costs. The testing we are required to do has just about made it too costly to proceed. Heaven forbid if we wanted UL/TUV certification. That an additional $35 - $40k. This is one reason medicine costs so much. Small guys like us don't stand a chance, so it's the giants that control the markets and their whopping markups and profits. I'm a capitalist and all for competition, but when the Government stacks the deck in favor of those they claim to hate, the little man (Middle Class/Small Business) fade away. Sorry for the political rant, but the governments actions don't match their rhetoric.

    I appreciate your quick response. It did help more than you may know. Joe
     
  4. BR-549

    Well-Known Member

    Sep 22, 2013
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    What it means is that somewhere in the circuitry.....you have an electrical length that is equal to 1, 1/2, 1/4 or 1/8 wavelength at those frequencies.
    If the only electrical leads for the pump are the power supply leads......I would suspect that. The rf signal could be coming thru those leads and getting rectified in the power supply circuit.

    Try a UHF rf filter at supply input.
     
  5. Joe Black

    Thread Starter Member

    Jan 5, 2016
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    Thanks, That hasn't come up before. So, to be clear, you are talking about the AC incoming power leads, from the PEM to the Power supply, and I guess teh Power switch in the front of the case?

    We did get a significant reduction in the RFI when we stacked a bunch of ferrite beads on the Ribbon cable and build a metal shield around most of the main PC bd. We haven't done anything to the PS or the leads in, or out for that matter. I'll look into that. Thanks
     
  6. BR-549

    Well-Known Member

    Sep 22, 2013
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    I don't have enough information on your pump to confirm. What's a PEM?

    Edit: Maybe consider another pump supplier that will document.
     
  7. Joe Black

    Thread Starter Member

    Jan 5, 2016
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    Thanks, BR, PEM = Power Entry Module in layman's terms the the AC plug.

    Finding another mfr for the pump may be a more daunting task. We have some relationship her and we buy other things from them that we may not be able to get elsewhere. It's complicated, and there is no guarantee that another Chinese vendor would be anymore helpful with what they perceive as proprietary. The plastic case and some other design issues have caused me to work on a design of our own. Everything in it is a shelf-product. I'll seek out professional RFI help them for sure. US manufactures don't sell wholesale. Some will do OEM runs, but we are a low volume business. It's a Low volume market as well. That's why we are using China. They have teh Industrial CE, but not FDA (medical) it's too expensive for them to get. Their market is Labs. It's a good pump, except for 386 MHz and 450 MHz immunity. Thanks, Joe
     
  8. tranzz4md

    Member

    Apr 10, 2015
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    Good shielding and grounding is always appropriate and helpful. There may be reasons to avoid an exposed metal enclosure, but internal shielding along with the aforementioned UHF filters will help. It sounds like building to pass the test is reasonable, and your test using double the wattage may be enlightening and useful, but you've got a very specific problem to tackle.
     
  9. Joe Black

    Thread Starter Member

    Jan 5, 2016
    48
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    Thanks, Tranzz. We do, and I felt we had it licked when we found we didn't need to pass this test at at 350 MHz - $50 MHz, but them the whole thing died very unexpectedly. That's what prompted me to ask about who or what do these frequencies do to actually damage or destroy an electronic component? That's is what i was looking to know. When we tested it, we did get the interference and it would even stop the motor totally, but when we removed the signal it would run again. The Power supply seems fine, the LEDs kept running, it was the motor that wen haywire. The motor is connected to the original Chinese PC bd where the driver is. All of our modifications are in the LED bd and run through the Stacker bd plugged into the main PC bd. The controls for teh motor (switches, etc.) do run through the LED bd and the long ribbon cable back to the PC bd via the stacker bd. We figured the long Ribbon cable was becoming the antenna into the board disturbing the motor and ferrites did help that a lot. But, it died as soon as teh testing lad hit it with their 385 MHz test. I understand the interference, just not the destruction. It could be unrelated. Thanks, Joe
     
  10. Papabravo

    Expert

    Feb 24, 2006
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    My gut feeling is that you are between a rock and a hard place. You don't have the ability to engineer a solution or test it's efficacy. You can try band-aid solutions but in my experience they are almost always a waste of time. If you don't understand what is happening then it is extremely unlikely that you could find a solution by trial and error. It might be cheaper to buy yourself a BSEE degree from a major American University.
     
  11. tranzz4md

    Member

    Apr 10, 2015
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    Ah, yes. Well, the destruction might be found by a few different means. Calculate the wavelength.
    -Look for straight conductors which are an even fraction or multiple of that, especially which carry low current. - Note however that conductors wich may not normally carry any current, or not be intended to carry current, will still act as antennae, and the induced current can do damage.
    -Look for your "weak links" particularly in "trigger" functions, or under steady load.
    -Look for "unused" pins and traces. These should actually be "grounded" or otherwise connected to ground-referenced conductive paths with overvoltage protection.

    The RF/UHF energy induced upon antennae needs to dissipate, and ground is its circuit counterpart. Whether through induction, or simple resistive means, it will dissipate. You want to direct/control/manipulate that energy to a target which will not affect your equipment/components.
     
  12. tranzz4md

    Member

    Apr 10, 2015
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    Stepping in a bit deeper, look for cumulative effects. Like additive harmonics. The troublesome wavelengths will prove themselves in measurements. You may well have a gap in your shielding. Do you have a microprocessor in there that operates at 700Mhz? What's the stepper freq?, You get my drift here, and I suspect you've looked at the simple stuff like that, but it's maddening how once the trouble is found, the physical measurements fall right in place confirming the find.
     
  13. Joe Black

    Thread Starter Member

    Jan 5, 2016
    48
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    Good points! Actually, we've had two such people involved, maybe they did buy their degrees. The Immunity issue, we had beat until the machine died unexpectedly. It had passed, or gotten through, this test before. Maybe this is something different. I hear some say the frequency test shouldn't damage the machine, but something did. If my repairs don't fix it, then we'll look at the value of professional trouble shooting, though from what some have said, we may not have enough data to effectively troubleshoot it at all. Then we are dead in the water and move on to something profitable. I wish I had gotten that EE degree instead of a BSBA. These machines are all over the world working just fine in labs, and more susceptible to those frequencies that in an operating room. Go figure! Thanks for your reply.
     
  14. BR-549

    Well-Known Member

    Sep 22, 2013
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    It should be easy to determine if you have damaged components. I have no idea of what mods you have made or the engineering of such.

    But any rf tech should be able to see the interference quickly with a scope.
     
  15. Joe Black

    Thread Starter Member

    Jan 5, 2016
    48
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    I have a relative understanding of what you are saying and most of that is beyond my ability and probably costs to locate. If I were, and I may, design one of these from scratch, I will use professional help. There is a similar machine made by the same company. It's basically, the same machine in a metal enclosure, but has LCD screen and many more functions than we need. It costs too much to do these needed modifications. e can't sell enough of them. It passed with flying colors. I assume due to the metal case vs the plastic case on this pump. Good info, Thanks.
     
  16. tranzz4md

    Member

    Apr 10, 2015
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    Papa, you get my respect and lots more around here, but I'm on the troubleshooting side of the spectrum/pendulum,,,, and love it!
    "Buying the degree" makes me chuckle, but would only address a different issue, of course,,, we all well know....

    I'm thinking the OP might be able to tweak, or fine-tune his way through this tight spot, and leave the rocks and hard places in the dust.

    I'm sure we'd all love to see him pull it off.
     
  17. Joe Black

    Thread Starter Member

    Jan 5, 2016
    48
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    Tranzz, Thanks Much for the encouragement. While I'm not an EE nor even a techie, I'm a smart person and know what I'm up against. I feel this is winnable with what I know and have gotten from people like you. This could be just a coincidence and the dead machine had nothing to do with the test. i learned something this week. The ribbon cable we are using isn't a "Locking" style. It just pushes into the socket. i wish we had used the type like on RAM sockets with the latches, but we didn't. It plugs into the top of the case into the LED board and hangs there. we had two of these fall out during shipping. I bought a hot glue gun. I don't expect that problem again. Bu the last one that did that, the foot pedal switch (pneumatic switch found on hot tubes and spas) also seemed to fail. There doesn't appear to be any relationship between the two incident. Just a coincidence, I'm assuming. I'm old enough and smart enough to know that Stuff just happens sometimes. Thanks for your support. And, don't be too hard on Papa, he's not probably an entrepreneur.
     
  18. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    Agree with the wire length thing.which is about 25 to 30 inches and the fractions thereof. Cable routing may change the results from unit to unit.
    You may have to shield a certain section. Drastic, but could work RF paint on the entire inside of the cabinet or parts thereof: http://www.lessemf.com/paint.html

    The manufacturer could have made changes that your unaware of. He could have substituted say an RF susceptable OP amp in the signal chain. RF can get rectified and turn into DC.

    See: http://ww1.microchip.com/downloads/...tm_term=&utm_content=MSLD&utm_campaign=AN1767

    http://www.analog.com/media/en/training-seminars/tutorials/MT-096.pdf

    http://www.ti.com/lit/an/sboa128a/sboa128a.pdf

    here's the search: https://www.google.com/search?client=ubuntu&channel=fs&q=op+amp+rf+immunity&ie=utf-8&oe=utf-8

    Do remember that EMI is generally eliminated by twisting and RFI by shielding.

    There is a CMOS phenomena where you can trap charge and render a circuit useless. However, letting the circuit sit for a long time or shorting the power (no power applied) will fix it. I have fixed numerous devices this way. A car clock failed after a jump and a house thermostat kept it's backlight on. The first was fixed by shorting the power to the car (battery removed). The second by turning off the HVAC system for 2 weeks and adding a TVS diode. it's been at least 2 years with no issues. The failure mode is non-destructive.
     
  19. Joe Black

    Thread Starter Member

    Jan 5, 2016
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    That's encouraging! basically, we need two LEDs for readout of the volume dispensed and a control to set the flow rate, which is based on the RPMs of the motor. peristaltic pumps are very accurate with respect to RPMs. My engineer designed a new LED board to handle teh two LED modules and the program to convert RPMs to liter/minute. Since we didn't know the code in the PC bd to reprogram it, he just had a small stacker board made to connect to the main PC bd. every function of teh machine is run through these two boards, except the motor which is still plugged into the main PC bd. I assume because that's where teh motor driver is. There was already a ribbon cable from the LED board to the main PC board. We used that socket on the main PC bd to attach the stacker bd. We swapped out the Chinese power supply with a UL, medical grade version. I grounded the PS and the motor to a chassis ground lug. That's pretty much it. I did rewire it using UL listed wire and put in a new PEM with double fuses and UL listed, as our 60601-1 (Safety) testing will require that.
     
  20. Joe Black

    Thread Starter Member

    Jan 5, 2016
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    Interesting! The spray paint was my first thought, but the UL people will have a fit because I'm not a UL certified applicator and there are numerous tests of the plastic compatibility with the paint that we'd have to show, which would all cost tons more than we even stand to make on this venture. They don't make it easy and much of it is nonsensical. I'm hoping you are correct on that CMOS thing. I hadn't considered that, but am hoping and praying that works. I'm at a point where I'm beyond getting a passing grade on these 4th ed tests. I can do OK with 3rd ed passing, except in Europe. CE won't grandfather us in like teh FDA will. we still have >2 years to figure this out or if this sells like hot cakes, then design one from scratch, which I am working on as soon as I get this passed.
     
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