Is Noise cancellation at RF frequencies feasible? - Page 1
Forum section: electronics / rf-microwave

author: rfengg
author_rank: Regular Contributor
timestamp: June 13, 2019, 10:43:36 am
content: My university has benevolently bestowed me with the arduous task of designing an Noise cancellation system to work at RF frequencies........The scenario in a nutshell is like this........we have a -30dbm RF transmitter at 100MHz putting out a modulated signal into an loop antenna connected to a LNA few centimeters away. (don't ask why........its the nature of the beast).Since the system can be installed anywhere and is susceptible to the commercial FM band transmitters and its not possible shield in a Faraday cage due to size restrictions, my uni prof thinks we should be able to actively cancel out any "unwanted" signals by using aseperate pickup probe/antennaand actively cancel any signal other than the "required" 100MHz out . The bandwidth of the receiver connected to the loop antenna is around 20Mhz.However, I cannot get my head around this idea........How can we cancel out two frequencies at 100MHz when they are completely uncorrelated?Some colleagues think that similar to noise cancelling headphones and RF cancellation is also possible.........is this true?If it works for low audio frequencies why would it not work for 100MHz RF frequencies as well?Any advice would be appreciated.

author: radiolistener
author_rank: Super Contributor
timestamp: June 13, 2019, 10:58:50 am
content: theoretically it can works for some fixed point and low frequency. You can receive the signal from powerful transmitter and re-transmit it with antiphase. But the problem is that EM waves are distributed in the space and has a lot of reflections from different objects, the same things will be for the noise cancelation transmitter, so it will be hard to do it.There is also problem is that your noise cancelation transmitter will be so powerful, so it will make interference for the main transmission. So it will affect the main transmitter efficiency. It will just cancel a part of it's powerThe main problem with noise cancellation for electromagnetic fields in such way is that you will need too small delay to re-transmit noise cancellation signal. It is limited by speed of light. You can get the signal from the main transmitter before it is amplified and transmitted. In such way you can win some nanoseconds.If you're need to cancel main transmitter signal for receiver, it can be done by using two antennas and signal processing. Such way is used for noise cancellation from some direction.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: June 13, 2019, 11:21:34 am
content: I don't know if I correctly understood your situation, but depending on what way you look at it, you could consider two antennas like this as the beginnings of a beam-steering array.By combining the two signals with the right phase shift, you will make the entire thing more sensitive to signals in one direction, and less in the other.Another thing you could consider: Use the fact that FSPL is logarithmic. In other words, If I have a source A at 1km, and a source B at 1 m from my receiver, and I then move the receiver 1 meter futher back (Ie, source A is at 1001 meters now, source B at 2 meters), the signal strength received from source A will be almost unchanged, but the signal strength from source B will be significantly lower.I plugged these numbers into the Pasternack FSPL calculator, and found for 100 MHz and 1 dBi gain antennas:FSPL at 1m 10.44 dBFSPL at 2m 16.46 dBFSPL at 1000m 70.44 dBFSPL at 1001m 70.45 dBIn other words, you will see no change in source A, but a significant change (factor of 4 in power!) of source B. Perhaps you can do something with this information to suppress source A? I don't know if the math works out for this though.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 13, 2019, 12:42:23 pm
content: Is the LNA receiving signals correlated only to the -30dBm transmitter, or what? This would be a fine case for a lock-in if so.You can correlate ambient signals and remove them from the input, but in general, the phase (and possibly to a lesser extent*, amplitude as well) will differ at the intended receiver, so you'd have to have some sort of, either a nulling loop, or a radial array of sense antennas surrounding the intended receiver, phased so that radiation incident from any direction gets nulled in the center.To say nothing of vertical components; ideally it'd be a sphere, also, both polarizations. You can maybe make the case that a room with dense floor and ceiling (reinforced concrete?) acts as a shitty waveguide -- if applicable -- so you don't have to worry about that so much. If not, bleh.*They're both just as bad, if the area suffers from multipath -- reflections from dense or conductive walls/floors.Tim

author: borjam
author_rank: Unknown
timestamp: June 13, 2019, 01:09:12 pm
content: There are even commercial products for amateur radio operators.I know two for HF bands:https://www.mfjenterprises.com/Product.php?productid=MFJ-1026https://www.wimo.com/ezitune_e.html

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 13, 2019, 02:19:44 pm
content: Something related on twitter:https://twitter.com/kn8fel/status/1138301489775022080

author: rfengg
author_rank: Regular Contributor
timestamp: June 14, 2019, 07:37:01 am
content: Thanks Tim.Quote from: T3sl4co1l on June 13, 2019, 12:42:23 pm

author: radiolistener
author_rank: Super Contributor
timestamp: June 14, 2019, 07:45:36 am
content: I think it will be much easier to build RF shield around equipment and place it on some distance from transmitter than trying to eliminate effect from high power EM waves with emitting the same power EM waves in antiphase.

author: rfengg
author_rank: Regular Contributor
timestamp: June 14, 2019, 08:04:59 am
content: Quote from: PA0PBZ on June 13, 2019, 02:19:44 pmVery interesting stuff......Andrew, however knows what frequency is interfering with his setup and what he needs to suppress.......in our setup the interfering frequency is unknown.I should admit that I have no idea how he manages to do this.......he says:"2 antennas needed... 1 aims at undesired station, the other aims at desired station. Signals from each line need to be equal in strength and 180 degrees out of phase so they cancel out. This is done with attenuators and phaser in the phasing antenna line."

author: rfengg
author_rank: Regular Contributor
timestamp: June 14, 2019, 08:06:34 am
content: Quote from: radiolistener on June 14, 2019, 07:45:36 amThat's not an option for us......and unfortunately that's what this whole project is about .

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 14, 2019, 08:38:40 am
content: Why is your bandwidth 20Mhz? Can you tell us a bit more about what you are trying to do?

author: rfengg
author_rank: Regular Contributor
timestamp: June 14, 2019, 08:49:38 am
content: Quote from: PA0PBZ on June 14, 2019, 08:38:40 amSure, the bandwidth is limited to 20Mhz due to the front end LNA which needs to have a NF <0.5.Since we did not have experience in building really low noise amps, the one we procured had a B.W. of 20Mhz.I would like to tell you more about the project but the prof would have my badge if I did , as its an industry sponsored research project.

author: ogden
author_rank: Super Contributor
timestamp: June 14, 2019, 09:03:22 am
content: Quote from: rfengg on June 14, 2019, 08:49:38 amWhat is BW of RF signal you want to receive? Are you using preselector filters in receiver? First job in interference mitigation - filter unwanted noise using filters, passing through only frequency band (channel) where transmitter operates.

author: rfengg
author_rank: Regular Contributor
timestamp: June 14, 2019, 09:10:20 am
content: Quote from: ogden on June 14, 2019, 09:03:22 amThere are no pre-selectors and they would hamper the NF of the system but we do have a quartz SAW filter after the LNA.

author: ogden
author_rank: Super Contributor
timestamp: June 14, 2019, 09:18:18 am
content: Quote from: rfengg on June 14, 2019, 09:10:20 amWhat is BW of RF signal you want to receive? At least tell passband width of SAW filter.

author: rfengg
author_rank: Regular Contributor
timestamp: June 14, 2019, 09:42:34 am
content: Really sorry @odgen......have been running around and trying to answer posts at the same time , so I missed your first part.......The BW of the RF signal is approx 1Mhz and the BW of the SAW filter is 2.2Mhz to the best of my knowledge.

author: ogden
author_rank: Super Contributor
timestamp: June 14, 2019, 10:10:01 am
content: Quote from: rfengg on June 14, 2019, 09:42:34 amOk. Good. Now consider situation where near radio station @90MHz is blasting -20dBm into your antenna. You would say 90Mhz is out of my SAW filter band, but I will say your LNA could become saturated by that 90Mhz station. That's why you put preselector filters right after antenna so out of band interferer does not saturate LNA and/or do not affect VGA (if any) level. Seemingly narrow band opens option for frequency-tuned loop antenna which itself can act as first preselector filter.Quote from: rfengg on June 14, 2019, 08:04:59 amHint: very often coax cables are used as delay lines. When you know delay needed, you take velocity factor of the cable and calculate how long it shall be.

author: rfengg
author_rank: Regular Contributor
timestamp: June 14, 2019, 10:11:48 am
content: Quote from: PA0PBZ on June 13, 2019, 02:19:44 pmCan someone enlighten me on how this works?Andrew tweets that " 2 antennas needed... 1 aims at undesired station, the other aims at desired station. Signals from each line need to be equal in strength and 180 degrees out of phase so they cancel out. This is done with attenuators and phaser in the phasing antenna line."The offending station is at 91.3Mhz and the station he wants to hear is at 91.5Mhz......if you combine these two stations passively 180 out of phase at equal amplitudes, how can you achieve partial cancellation of the offender at 91.3Mhz? How can you cancel out 2 different frequencies (which of course have different phases ) by combining them 180 degrees out of phase?Isnt it better to just use a high Q filter tuned to 91.3Mhz?

author: ogden
author_rank: Super Contributor
timestamp: June 14, 2019, 10:53:21 am
content: Quote from: rfengg on June 14, 2019, 10:11:48 amYou adjust location of both antennas and delay line between them such a way that offending signal is out of phase and ideally - signal of interest in phase. Loop antennas are kinda directional, definitely this will help. Aim one antenna at your signal and another antenna at interferer, then adjust delay line such a way that interferer signal is out of phase, also adjust attenuation of "interference cancelling antenna" for minimum noise level.QuoteIt depends. Anyway it would break whole purpose of his experimentwith phasing.

author: worsthorse
author_rank: Super Contributor
timestamp: June 15, 2019, 04:40:53 am
content: I think you have, potentially, a couple of distinct problems, not the least of which is the current design of the your project system. Simplw noise cancellation systems, at least in amateur radio, are based on one or both of these assumptions: the noise source is directionally distinct from the preferred signal or it is closer and so more powerful. In such systems, the noise cancellation antenna is purposely insensitive or highly directional or both so it will hear the noise source but not, theoretically, the preferred signal. So you have two antennas, your primary antenna that is receiving both the preferred and noise signal and the cancellation antenna that is hearing the noise signal and, at best, a highly attenuated preferred signal.If this is so, you can simply invert the signal from the cancellation antenna and add it to the signal from the primary antenna, which will attenuate the noise signal but not the preferred signal. Having a way to adjust the phase of the inversion is the easiest way to deal with signal path length differences and the like.Timewave's ANC-4 is an example of a commercial unit for the ham radio user. A web search will generate a number of other commercial, kit, and DIY systems of the same sort.And with all of that said, designing your receiver with a filtering in front of and behind the LNA will probably be required. AN LNA with a 20MHz bandwidth without such filtering in a narrowband application and known RF interference sources is a recipe for trouble.

author: radiolistener
author_rank: Super Contributor
timestamp: June 16, 2019, 11:39:48 am
content: Quote from: rfengg on June 14, 2019, 10:11:48 amThat's pretty easy to understand, when you add -1 and +1, the sum will be 0. So, two antiphase signals will cancel each other when you sum them together. Since their sum is always zero, they just didn't exist for receiver and didn't affect other signal reception.The problem here is that EM waves from singal source and from noise source are moving at different directions. So, if you place your antenna at different location, then you will need to adjust phase shift for your noise cancellation system.

author: rfengg
author_rank: Regular Contributor
timestamp: June 17, 2019, 12:31:42 am
content: @radiolistener......thanks for your inputs.While I completely understand that if you have 2 signals say , both at 91.5Mhz and they are combined 180 degrees apart, they would completely cancel each other out.However, if you have one at 91.3Mhz and the other at 91.5 Mhz, how can they cancel out each other at 180 degrees, when they can even never even be made in phase as two different frequencies never have the same phase?I tried to do a simple simulation in LTSpice and when adding up 2 frequencies at 91.5Mhz, 180 degrees apart, they indeed result in complete cancellation; however, when I add up a 91.5Mhz signal and 91.3Mhz signal, the resultant is NOT zero.......Please do help me understand......

author: DimitriP
author_rank: Super Contributor
timestamp: June 17, 2019, 01:35:38 am
content: Noise is noise. It's not another "signal" on a specific frequency. Some noise can be at a specific frequency , then we don't call it "noise" but a heterodyne, IMD product, "birdie" anything but noise.AS for the project at hand.FM,(as assumed from the 100MHz frequency-correct me if Im wrong-I was once)receivers are pretty much immune to "noise".The issue seems to be this:-30dbm suspicion of a signal at 100MHz exists.Some monster signals above and below 100MHz exist as well.So ....reject everything below 99.9 MHz and everything else above 100.1MHz. Is it practical? probably as practical as expecting a -30dbm signal source to not be interefered with.Now on to actuall "noise cancelation". (mostly useless for FM)There is a transmitter, a receiver , and two antennas - we'ill call them signalAntl and noiseAnt ) and a black box(or white, purple or brushed aluminium).The transmitter transmits the receiver receives through the black box.Into the back box (or purple or... you get the idea...) are connected the two antennas.One, the signalAnt is positioned to receive the signal of interest (and unfortunately whatever other junk is on the air at the time)The other noiseAnt is positioned in a magical position so that it manages to receive "the other junk" but not the signal of interest.In the black box, the signals are added out of phase so "only" the signal of interest is left (in theory)Going back to that suspicion of a signal at -30dbm With the right amount of power in an FM receiver the capture effect takes over and you don't need to be "canceling" anything unless the offending signals are causing the light bulbs in the ceiling to light up.And where is the fun in not answering with another question or two (ok a few)?How arbitrary is the -30dbm signal?If FM modulation being used?How far will the receiver be from the transmitter?Can you use directional antennas?Is the receiver "special" in some way?

author: _Wim_
author_rank: Super Contributor
timestamp: June 17, 2019, 08:46:43 am
content: Depending on how your reference signal is modulated, a lock-in amplifier can be the way to go. Some general information about lock-in amplifiers:https://www.thinksrs.com/downloads/pdfs/applicationnotes/AboutLIAs.pdfOff course, like others have been saying, to ensure your LNA is not saturated, first a preselector is probably needed. If not possible, the gain of the LNA needs to be lowered to avoid saturation, otherwise a lock-in will not be able to recover the desired signal.Typical commercial lock-in amplifiers have an adjustable LNA built-in to the unit, so that could be an option as well. Ebay regularly has lock-in amplifier at bargain prices, but most are not RF but low frequency.An example of a commercial RF lock-in can be found here:https://www.thinksrs.com/products/sr844.html

author: DimitriP
author_rank: Super Contributor
timestamp: June 17, 2019, 09:32:07 am
content: QuoteSR844 ... from $8450They might take $50 off if you buy two

author: IconicPCB
author_rank: Super Contributor
timestamp: June 17, 2019, 12:03:29 pm
content: I made an in band rejection system based on phase and gain cancellation system for an active antenna,Range of operation HF to 1GHz.I could set up an interfering carrier in the space of an analog TV channel such that the TV receiver would loose reception and then cancel the interfering signal and recover TV reception.

author: radiolistener
author_rank: Super Contributor
timestamp: June 17, 2019, 02:26:18 pm
content: Quote from: rfengg on June 17, 2019, 12:31:42 amdifferent frequencies cannot cancel each other, they can make interference and the most bad thing is that one frequency may have too high level and overload receivers input. While the second frequency may have too weak signal and may be loss in background of very loud signal from the first frequency.The ability to listen for a weak signal near to powerful signal is known as Dynamic Range of receiver. More dynamic range means more ability to listen weak signals near to powerful signal.

author: borjam
author_rank: Unknown
timestamp: June 17, 2019, 03:13:38 pm
content: Anyway, why aren't you filtering _before_ the LNA input? Is the interfering signal on the same frequency as the signal you wish to receive?I must be very thick today but the whole thread doesn't make much sense!

author: DimitriP
author_rank: Super Contributor
timestamp: June 17, 2019, 07:24:41 pm
content: QuoteRoot cause:QuoteInstead of solving the problem, this thread is focused on making someone else's "solution" work.

author: IconicPCB
author_rank: Super Contributor
timestamp: June 17, 2019, 08:32:14 pm
content: Your professor is right.It is quite possible to do it in RF domain ( solution may depend on the frequency ) but You should also look into hHeaviside function in conjunction with Laplace transform ( hint 1/s) and how to apply it to sampled time domain ( solution limited by A/D converter performance.

author: frogg
author_rank: Regular Contributor
timestamp: June 27, 2019, 01:51:06 pm
content: do some paper reads on "self interference cancellation" or "full duplex radio".the difference between your application and a classic self interference cancellation system is that you're injecting an unwanted signal as your interference, as opposed to your own transmission (self). Generally speaking, your architecture should look pretty much identical to a self-cancelling system.since the SNR of the unwanted signal is probably lower than a typical self-transmitted signal, you should be able to make reasonable compromises, at least to prove the technique.the biggest challenge will be phase control.this is not a filtering problem, per se.http://csl.stanford.edu/~pal/talks/duplex-princeton.pdf