RF noise by inductor ramping up to full current - Page 1
Forum section: electronics / rf-microwave

author: anvoice
author_rank: Frequent Contributor
timestamp: June 25, 2021, 09:29:49 pm
content: I need to turn on an inductor (a few coils of copper wire) in order to generate a magnetic field. The wire will carry no more than 50A DC, with the current controlled by a knob, so I can go slowly if necessary when ramping up. I am mostly worried about possible RF interference generated by this setup. Is there a good way to calculate what frequency range, and at what power will be generated by the inductor as I get it up to current? I'm thinking of building a Faraday cage to enclose it but need more info about what I'm dealing with.

author: coppercone2
author_rank: Super Contributor
timestamp: June 25, 2021, 09:33:39 pm
content: you can shield inductors, they sell shielded inductorsuse a torroid or R core to reduce emissions

author: anvoice
author_rank: Frequent Contributor
timestamp: June 25, 2021, 10:31:32 pm
content: I should have given more introductory information. This is a custom setup where I need to generate a magnetic field inside the inductor (homebrew NMR) and the inductor is a homemade length of wire formed into several loops. I can't buy a shielded inductor. Instead I need to find a way to shield what I have.

author: coppercone2
author_rank: Super Contributor
timestamp: June 25, 2021, 10:37:47 pm
content: well if you slowly ramp a DC field you are not making much interference, the only problem is power supply noise (is it a quiet power supply) and magnetostatic fields. If you want to restrict the amount the field can change then wire the knob to a circuit which enforces slow rate change so if you spin the knob too fast it won't do nothing, it will be electrically limited by a RC factor.you might want to protect the power supply control loop so it does not accidentally start modulating interference on the power circuit or something, if you have a noisy power supply you need gigantic possibly higher order LC filters for 50A (i.e. bad SCR unit you want to quiet down). This filter will be a behemoth.So you might wanna think about like shielded sense lines depending on the power supply type, rate limiting circuit on the control potentiometer, having a generally good power supply (watch EEVBLOG video on power supply noise measurement).If you want to shield magnetostatic fields then that is a whole different story, i think it might create problems with field uniformitydon't let the coil ring is your primary objective I think, aka trapazoid that is slow

author: anvoice
author_rank: Frequent Contributor
timestamp: June 26, 2021, 10:09:22 am
content: Thanks for the reply. I live in the states so I don't want to run afoul of the FCC or in general create any harmful radio interference. I can try to hack the knob to a slow ramp control circuit, but it's inside a case and will probably be a pain to modify. The power supply is a 50A constant current module (https://www.ebay.com/itm/800W-50A-DC-DC-Boost-Converter-Step-down-Power-Supply-Module-Constant-Current-/202879154954?_trksid=p2349624.m46890.l49286), so it could indeed be noisy. The main problem is I currently have no way of measuring the generated RF noise, and if I don't know the ballpark at least of the range of RF signal I would be generating, I can't even get the proper measuring equipment.

author: radiolistener
author_rank: Super Contributor
timestamp: June 26, 2021, 10:17:05 am
content: Quote from: anvoice on June 26, 2021, 10:09:22 amyou can use usual AM radio receiver to estimate RF noise level. Just walk on LW and MW bands (30 kHz - 3000 kHz) and listen for interference. Higher interference = higher RF emission.Note that inductor connected to DC don't produce RF emission. RF emission will happens when you connect or disconnect current. So, you're needs to listen for a pulse at power switch on/off transition.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 26, 2021, 04:28:35 pm
content: If you are turning up the field with a knob you probably can't do it fast enough to generate significant EMI. Near field effects such as inducing eddy currents or Lorentz forces in the rest of your apparatus are often a bigger concern. Any EMI will be just from the intrinsic noise of your power supply which is hopefully low if you want to use it for NMR.

author: mawyatt
author_rank: Super Contributor
timestamp: June 26, 2021, 05:08:54 pm
content: Quote from: anvoice on June 26, 2021, 10:09:22 amThe TinySA are very reasonable and you might find useful in looking for "RF noise" generated by your setup.Best,

author: coppercone2
author_rank: Super Contributor
timestamp: June 26, 2021, 05:31:50 pm
content: still a slew rate control circuit might help since its naturally a filter, and that would be the most safe method of doing it, but as usual be very careful modifying 50A equipment, if you do buy all high quality connectors, wire, heat shrink, silicone, all on digikey, and do full disassembly of electronics prior to drilling or anything that makes filings (I just had a earth ground fault that I cannot find the reason for, possibly because of 'shop debris' contamination).but as usual checking old equipment might be useful because you could have bad thermal paste, corrosion, old capacitors,etc.. things that can effect both safety and performance.that is your parasitic transmitterand right, a scratchy old potentiometer CAN make EMI I think, if it changes impedance in a nonlinear manner the circuit is being modulated, so cleaning and greasing them will help (its called 'scratching' in the audio world, where old potentiometers make annoying high pitched scratching noises on speakers when adjusted), and you never know when a potentiometer can break, a filter will stabilize it if it doesWhen I took apart old potentiometers for cleaning *the usual ultrasonic and deoxit* I have found very bad contents inside of them, to the point where my house feels dirty until I take apart old potentiometers on ebay equipment lol

author: anvoice
author_rank: Frequent Contributor
timestamp: June 26, 2021, 08:16:47 pm
content: Quote from: radiolistener on June 26, 2021, 10:17:05 amI don't have an AM receiver handy so I'd probably be buying one for this purpose. Would it make more sense to get a cheap spectrum analyzer, especially since I need to get a pulse? Plus the SA would probably have a higher bandwidth I think.Quote from: ejeffrey on June 26, 2021, 04:28:35 pmThat would be ideal. So you think I might not need a cage? To tell the truth I'm a bit worried about turning the thing on at this point since I won't know whether I'm generating EMI or not, and would rather not find out the hard way after something goes wrong.Quote from: mawyatt on June 26, 2021, 05:08:54 pmI'm very close to purchasing one actually, but was worried about the low bandwidth (100kHz to 960MHz). Do you happen to know if it is fast enough to show a signal pulse?Quote from: coppercone2 on June 26, 2021, 05:31:50 pmThanks for the tip, I'll definitely be careful if I go that route.Many thanks for all the help so far everyone!

author: coppercone2
author_rank: Super Contributor
timestamp: June 27, 2021, 12:43:54 am
content: if you measure it with a dummy load connected to the oscilloscope you can see what its doing with a much less capable radiator then a big coilit might be enough to plug a multi meter AC measurement into it while its connected to a load to see what the AC noise looks like under load, that is your RF energy, just need a quick multimeter

author: boB
author_rank: Frequent Contributor
timestamp: June 27, 2021, 07:09:23 am
content: If you are using a 1/2 bridge to drive and inductor, or even a single transistor, make sure it's not ringing much and that will help. Maybe an R-C snubber across the switch transistorsAlso as was mentioned, keep the switching slew-rate down. Turning on the FET (if that's your switch) with a larger resistance and off with a lower resistance helps.

author: anvoice
author_rank: Frequent Contributor
timestamp: June 28, 2021, 01:10:34 pm
content: Quote from: coppercone2 on June 27, 2021, 12:43:54 amI do have both a scope and a multimeter, but am not sure how to perform the measurement you mentioned. Basically I could connect the power supply with a dummy load to the scope and see how it behaves? I'll try to read up on how to do that.Quote from: boB on June 27, 2021, 07:09:23 amYou mean driving an inductor with a diode half-bridge? I'm driving it with a constant-current 50A power supply module (characteristics not yet measured, could be noisy), and the "switch" is I believe just a potentiometer knob. The control board for the module is actually connected via some kind of port to the power module, so theoretically I could try to reverse engineer the signal it's sending and try to manage it with a microcontroller instead, but it would be my first time doing that so the process would be slow. I can always experiment and see if I need to actually physically modify the PSU and the slew rate control, but I was worried about it generating bad EMI that first time I turn it on.

author: coppercone2
author_rank: Super Contributor
timestamp: June 28, 2021, 01:15:41 pm
content: watch the video he shows you exactly how

author: JohnG
author_rank: Frequent Contributor
timestamp: June 28, 2021, 01:46:04 pm
content: If you are doing an NMR setup, here are some free opinions to consider. I have only worked on subsystems for imaging MRI, which is extraordinarily sensitive to things like ripple and EMI. But since the core operating principle for MRI is NMR, they may apply.You should put the whole thing in a shielded box anyways, because you will have a sensitive receiver and you don't want to pick up other signals from other sources.The power supply you have most likely has limited EMI filtering and will be pretty noisy (I'm somewhat speculating here).The same noise may interfere with your NMR setup. You will most likely see current ripple in your inductor predominantly at the converter switching frequency, and this ripple will appear in the generated magnetic field. Make sure this will not be a problem.Cheers,John

author: anvoice
author_rank: Frequent Contributor
timestamp: June 28, 2021, 03:09:42 pm
content: Quote from: coppercone2 on June 28, 2021, 01:15:41 pmAh perfect will do.Quote from: JohnG on June 28, 2021, 01:46:04 pmThanks for your input! Yep, MRI is just NMR without the "nuclear" so as not to scare people. You're most likely right that the cage will eventually be necessary, but I was hoping that I could at least turn the thing on to test without it. I'll have to measure the ripple and see if it's going to be a problem.

author: profdc9
author_rank: Frequent Contributor
timestamp: June 29, 2021, 01:44:12 am
content: You could get a large ferrite rod and wrap the wire around it. The ferrite rod would not saturate as readily as a ferrite toroid and is used for RF chokes. You would need a pretty beefy ferrite, perhaps something with a cross-section of a few square centimeters.

author: anvoice
author_rank: Frequent Contributor
timestamp: June 29, 2021, 12:37:19 pm
content: Quote from: profdc9 on June 29, 2021, 01:44:12 amI tried looking for ferrite rod but can't find any hollow cylinders... The sample needs to go inside the coil, so that doesn't seem like an option.