Noise cancellation at RF frequencies? - Page 1
Forum section: electronics / projects

author: rfengg
author_rank: Regular Contributor
timestamp: August 27, 2018, 02:48:23 am
content: Dear All,My RF course instructor has given me something to ponder on and I really dont know how to start........We house very sensitive RF electronics in a Faraday cage to prevent EMI from nearby equipment getting into the sensitive electronics operating at around 100Mhz.My instructor has challenged me to remove the RF shield and then preserve the SNR of the system by detecting the intruding noise with a E/M probe and then using "Active noise cancellation" similar to the principle of Bose ANC Headphones.......I guess the idea here is to pick up the intruding noise with a probe and then invert the noise and add it to the signal so that the noise is now eliminated.I am not convinced this will work at RF frequencies and have a feeling that the instructor thinks so as well, but I am at a loss on how to explain it to him.Any help or thoughts appreciated.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 27, 2018, 04:09:28 am
content: Well, you have one advantage over the headphones: it's narrow band...How many directions is the interference possibly coming from, and how many directions can you constrain?What is the nature of the EUT's susceptibility -- is it sensitive on all sides, or only from a few antennas, cables or openings?Example, if the device merely needs to be accessible from one side, then the other five sides can be shielded, and that makes your job a whole hell of a lot easier than if it's naked and surrounded with whatever kind of antennas this thing will have.In general, you will have a multipole source, driven such that the field in the center is maximally flat (within some specified attenuation limit), giving some interior shielded volume. The shape and size of that volume depends on the number of poles (antennas) and their placement. More elements means better attenuation, in a larger, rounder volume.Helmholtz coils are a similar problem, with magnetic fields: the standard geometry gives a maximally flat magnetic field in the center (i.e., dB/dx --> 0), and a modest volume in which ΔB is small. You can add more coils, to get higher degrees of flatness (more derivatives to zero at the center, or a larger volume with a smaller ΔB), but you can never do a perfect job of it (zero everywhere in a given volume).This is the same problem over, but in three axes, and with propagating waves to consider (since this device is probably on the order of a few wavelengths; it's not wholly near-field as is the case for Helmholtz coils, but also not fully far-field).You might also consider metamaterials, i.e., arrays of resonators (which might be simple PCB loop structures and whatnot). You would need to place them over many wavelengths around the desired volume, but this is a well studied method to "cloak" a volume from external fields and would be the easiest conceptually. Perhaps telling your instructor about the size and cost of this approach will turn them off about the idea, and they'll stick with the Faraday cage?Tim

author: JS
author_rank: Frequent Contributor
timestamp: August 27, 2018, 04:16:11 am
content: In audio, active noise cancelation only works at LF, that's due to the processing time it takes to correct for all factors, the harder it is to correct for the phase of the noise from different sources/sides and that HF is much easier for passive noise control.For EMI, shielding against HF gets harder, as the problematic frequency is reasonably low, so the shielding is reasonable easy to archive, but high enough the electronics are problematic, using active noise cancellation doesn't make much sense.To put things in prospective, in the university, inside the HV lab they have a faraday cage smaller room inside the big room of the lab, as they test pretty fast transient of a few hundred kV, among other tests. They built it with with expanded metal, as they found to be enough for their proposes, much cheaper and easier to deal with than other solutions. There are other labs who have the entire lab inside thick walls of cooper, which is usually overkill, very expensive, and as they have other sources of noise inside the cage might not be as efficient.With this I'm saying you should test a few things out before going nuclear with the solution, you might find an easy and cheap solution to be good enough. Without more details of the environment and the test equipment to protect is a bit hard to give a more precise answer, plus I don't have much experience in shielding for this applications (BW and sensitivity as I suspect you are dealing with) so you might find someone else who could give a better answer to this, but better having more details. Probably not the details on the test being run but what are the levels being dealt with, the expected attenuation, the kind of sources, etc.JS

author: awallin
author_rank: Frequent Contributor
timestamp: August 27, 2018, 04:17:51 am
content: FWIW you might google for papers on military "full-duplex" radios. they send and receive simultaneously on the same frequency with both analog and digital suppression of feedthrough from TX to RX (in excess of 100dB isolation IIRC).

author: rfengg
author_rank: Regular Contributor
timestamp: August 27, 2018, 05:25:11 am
content: Thanks TimQuote from: T3sl4co1l on August 27, 2018, 04:09:28 am

author: rfengg
author_rank: Regular Contributor
timestamp: August 27, 2018, 05:29:29 am
content: Quote from: awallin on August 27, 2018, 04:17:51 amThanks awallinIs'nt the the TX to RX feedthrough in such radios well correlated and hence easier to "cancel" out? My problem is that I have no clue as to what could be the interfering noise source at what frequency and what phase , so it makes the active cancellation allmost a no no.

author: IconicPCB
author_rank: Super Contributor
timestamp: August 27, 2018, 05:51:57 am
content: All you need do is consider how to implement the jw ( jay omega ) operator and everything will come out in the wash.Seriously though.. Do yo u need to shield the volume or just the device under test?

author: ConKbot
author_rank: Super Contributor
timestamp: August 27, 2018, 03:12:00 pm
content: Point out to the professor that the faraday cage does exactly what is wanted? It detects the incoming em waves, induces magnetic field that exactly* opposes the incoming one, to cancel it out and protect what's on the other side.It even harvests the incoming em waves as a power souce.I doubt anyone else can do as good with just passive components.