Is a signal generator an appropriate tool for EMI debugging? (EMI Injection) - Page 1
Forum section: electronics / beginners

author: jfseneca
author_rank: Newbie
timestamp: December 29, 2023, 06:59:44 pm
content: I have a problem where my product's USB hub will restart if the antenna of another product (a radio) and the radio is keyed up. This is w/ a signal centered at 160MHz emnating from the radio.We've had a myriad of EMI issues lately, for whatever reason, on top of the problem statement above. I'm looking for more tools to1) Create noise/EMI without the offending product. The reason being, I don't always have access to the offender or I have to share it.2) I would also like to have a testplan where I can run thru a number of known offensive frequencies and test to see if it causes issues.3) Create EMI in suce a way that it can be replicated in the far field (with an appropriate antenna) or in the near field (with an oscilloscope probe for example to "inject" an offensive signal.So my question(s) to you all, me being at a technician level and not an RF engineer or an engineer of any kind are:1) Could I use a signal generator or arbitrary waveform generator to reasonably simulate noise? So using the above problem statement as an example, if I were to say just create a sine wave at 160 MHz with a bandwidth of 20 MHz (I think the more expensive generators can do a bandwidth?) with an approptiate Vpp, how useful would this be? So I wouldn't have a modulated signal, but it would be the same power and freq. Without the modulation, is this useful?2) How unreasonable is it to use an offensive piece of equipment as a signal generator? Looking to get beat up on this if this is really stupid. So I took the offending radio from the problem statement above and with a series of RF adapters, I was able to get BNC out to an oscilloscope probe. I'm using this to "inject" EMI into the circuit at different points. It's working very well at this point, and I can see the "Weak" points. I tried putting some attenuation in line w/ the RF adapters but it choked off the signal too much to the point where the oscilloscope probe wasnt outputting anything, or the signal was so small I couldn't see it. This radio, this "offensive equipment" I keep referring to is expensive and I don't want to damage it. Or damage it further.3) Final question is, do you all have any EMI debugging equipment tips? I can start googling the terms but I don't know what I don't know... I don't need a writeup, don't want ot waste your term. Just throw out some terms and equipment types and I can do the research.Thanks,

author: wasedadoc
author_rank: Super Contributor
timestamp: December 29, 2023, 07:32:09 pm
content: Please do not be offended but this does not look like a problem that you at technician level should be tasked with solving.

author: jfseneca
author_rank: Newbie
timestamp: December 29, 2023, 07:57:51 pm
content: Not offended and you're right, an electrical engineer or an RF engineer should be looking at this.But it's the hand I'm dealt, and for better or worse It's expected that I can at least capture some data and narrow the problem down to some degree.So if you or anyone can offer advice I'd appreciate it, if not then I'll push this as far as I can and just have to come to terms w/ that.

author: Grandchuck
author_rank: Super Contributor
timestamp: December 29, 2023, 08:07:30 pm
content: There are some very inexpensive (such as Baofeng) two-way radios that could be used as test equipment (of a sort). Not sure if they would help?

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: December 29, 2023, 08:15:10 pm
content: A signal generator (in combination with band-specific amplifiers, couplers and/or antennas) is exactly what is used for all the basic susceptibility tests, The waveform is an unmodulated sine (CW), the steps and the time during which the frequency is held can be found in the pertaining standards. Conducted susceptibility means that the possibly offending signal is coupled into the various external connections (for example cables), radiated susceptibility implies that the signal is applied to the unit under test 'over the air' (i.e. by an antenna).Compared with a real world offending signal, those are rather benign characteristics: the harmonics content is low or zero and as no modulationis applied, timing coincidences in the baseband can't occur.It is exactly what a conformity test at a lab would do, but it does not necessarely reproduce an existing problem.In order to achieve this, you would have to determine the characteristics of the signals impinging on the unit under real conditions and then apply a limited subset of those to the unit. This should only change ONE parameter at a time. For example, waveform changed, second frequency present, modulation applied. You will have to isolate which combination produces the critical interference.An oscilloscope probe is totally unsuitable for the task of coupling an interference into your DuT and you should not abuse it in such a way. First thing you will need is a LISN or a application-specific replacement for it. By this you will ensure that the line impedance(s) are close to the real application and that the possibly interfering signal does not take it's way through your test equipment (power supply, communication partner...) instead of the DuT. 'Near field' can't be approximated by galvanic contact but rather by an inductive or capacitive coupling mechanism.Also, you need to be aware that such testing will produce signals which are possibly interfering with other devices or services and that your test setup can emit signals on frequencies, which are critical and for the use of which you are not licensed. Therefore, such testing should take place in a RF absorber chamber or another type of shielded room.Neglecting those precautions can have severe legal implications both for you and your company!A good link for starting would be:https://interferencetechnology.com/how-to-build-your-own-emi-troubleshooting-and-pre-compliance-kit/Also look at the publications of Dr. Eric Bogatin

author: jfseneca
author_rank: Newbie
timestamp: January 02, 2024, 03:04:45 am
content: @GrandchuckThat's a good idea, something cheap like that I'd be okay if it broke. It wouldnt be the same modulation as my offender but it would be "Realistic" without having to try and match the modulation with a function generator.@Neomys SapiensQuoteI think will continue to look into the signal generator and antennas, then. As you said, the characteristics of the signal(s) would be benign, but I could at least then run a suite of basic tests against other known 3rd party radios that we and our customers use.QuoteBy googling terms like "EMI injection" in several different ways, I get no results matching what I attemptted to do before the holiday: using a scope probe at the antenna output of the offending radio to inject signals into pins or ports. So based on this, I can see that this is not an industry standard thing and isn't the norm. Despite that, I'd like to talk more about it and fully understand why it's a bad idea.My thinking was that, I knew waving the offender over my device caused the glitch. My device is small, handheld actually, so trying to understand exactly what is upset with near field probes and the antenna was a challenge. The whole device is saturated by the offender. So with clues from DMESG I could assume it was either the microcontroller rebooting or the other divce (havent mentioned this yet) connected to the DUT could momentarily not communicate with the DUT. So my reasoning (dumb as it might sound) was to inject the noise into the I/O at the top of the DUT to see which pin or line was the problem. So that that particular line could be filtered in some way, since I already knew the frequency and bandwidth of the offender. By using the oscilloscope probe method, I was able to narrow down the ingress point. There are two knobs, two buttons, and a headset jack on the DUT. Pressing the scope probe to the knob pins (rotary encoders) reboots the device, but it doesnt happen on the headset jack or on the buttons. I was checking these locations because my boss had assumed it was the headset port, before he left on Christmas vacation.I'm explaining all that to say I (seem to) have gotten more conclusive results by using the scope probe. So I want to understand, is the danger that I'm a) damaging the scope probe? b) damaging the offender? c) damaging the DUT or d) that testing in this way is really inconclusive and that I may have garnered some false positives? Should I instead use attenuation and weaker antennas to try and pinpoint noise ingress, rather than contact?QuoteCould you explain this a little more please, or give examples of such a mechanism?Thanks,

author: Andy Chee
author_rank: Super Contributor
timestamp: January 02, 2024, 03:10:04 am
content: Quote from: jfseneca on December 29, 2023, 06:59:44 pmGoogle the term, "common mode interference".

author: radiolistener
author_rank: Super Contributor
timestamp: January 03, 2024, 11:03:52 pm
content: You're needs powerful RF power amplifier (10-100W) in addition to the signal generator. The signal generator output is too weak to produce noticeable interference.Power amplifier needs to have protection from high SWR load to avoid burning it.And also you're needs some antenna to transmit signal from power amplifier. For better result you can use large coil with capacitor. It will works like LC circuit. You're needs to tune it at signal generator frequency frequency.Use fluorescent or neon lamp to test your antenna, it should glow in your hand near antenna. If lamp is glowing, then it works, so now you can put your DUT instead of neon lamp for testing

author: vk6zgo
author_rank: Super Contributor
timestamp: January 03, 2024, 11:11:07 pm
content: Quote from: wasedadoc on December 29, 2023, 07:32:09 pmIn the cold, hard, world, Techs are regularly tasked with such problems.EEs go away for 6 months to think about it, the "Suits" can't wait, so back it goes to the Tech to sort it out.

author: vk6zgo
author_rank: Super Contributor
timestamp: January 04, 2024, 12:37:08 am
content: Many years back, my old work was confronted with a problem where an interfering carrier was appearing at the output of Vision Distribution Amplifiers used to feed PAL analog video to the input of our TV transmitters, causing a vertical line in the picture.Hanging a 7L12 equipped 7000 series Tek across the VDA output revealed a nice 800kHz carrier.I found that adding a coiled-up length of coax to the VDA input would attenuate the 800kHz, but not eliminate it.We then found a toroid in the "odds 'n sods" drawer, which, if the long length of coax was wound on it, would solve the problem.We had to fix the problem with several VDAs, so the EE headed off to buy some toroids which would be effective at 800KHz.I was a bit dubious about this, as the toroid we found looked like it was for higher frequencies, & the other ones in the drawer which seemed to be low frequency ones, had no effect.At this point, my immediate Boss "got a bee in his bonnet" & had me dragging the SA around the paddock with a super-long power cord attached, looking for an 800kHz source.Needless to say, nothing was found.Ok, something had changed in the local RF environment, but what?Quite recently, a brand-new FM Broadcast site had been built next door, but that was around 100MHz, & we had no problems with the two FM stations co-sited in our building.While looking at a list of FM Broadcast frequency allocations, I noticed that the Licensing Authority had decreed that station allocations should be separated by800KHz!Yes, several of the transmitters next door were on adjacent frequency slots.It appeared that the strong RF fields from next door overpowered the VDA's common mode rejection, & the input stage acted as a mixer, with 800KHz as the only product within the amplifier's passband.Looking at the input circuit, it appeared to use a network consisting of an inductor & a large capacitor to chassis to revert to unbalance input at low frequencies----maybe we could do something similar for 100MHz!To this end, we tried various low capacitor values in parallel with that network to see if it attenuated the interference, ending up with a value of around 67pF which attenuated the interference to the point that it was indiscernible, whilst having no effect upon the wanted video signal.All of the above is to bring up the point that, perhaps, there is nothing inherently wrong with the devices, but maybe the input cabling to the affected device, or its case are inadequately shielded.

author: vk6zgo
author_rank: Super Contributor
timestamp: January 04, 2024, 01:12:42 am
content: A bit more.USB stuff is "as broad as a bull's backside" bandwidth wise, so the exact frequencies probably don't matter.The USB leads you can buy vary considerably in their shielding so when the 160MHz transmitter keys up, you might, depending upon your cable quality, inject a very low level of RF or a very high level.Without knowing anything about the devices you are having trouble with, it is hard to offer anything other than general advice.Perhaps you could try "clip on toroids on your USB cable.