100Mhz noise - Page 1
Forum section: electronics / projects

author: madshaman
author_rank: Frequent Contributor
timestamp: March 24, 2013, 09:56:39 pm
content: Hi all. A while back I noticed some subtle 100Mhz noise on my scope while testing the guts of a meter I'd purchased (HP3478A).Well, I've seen the same noise on a *different* scope simply plugged into a function gen via a cheap chinese coax cable. My body seems to couple this noise more strongly by grabbing the cable (doesn't happen with my probes, which are good tek probes).Last time I saw it I shut down the computer and all mains powered devices, I also put all my iDevices into airplane mode. This time I did one more thing which was to unplug my UPS (although by definition it keeps running).Just thought I'd ask if anyone else has had to track down something spewing or possibly mixing some other signal down to 100Mhz and has an idea of what it could be.My next step is going to be physically carrying the UPS and any self powered devices to the basement because I'm a bit stumped. All my main computer and music gear are in the basement and my lab is on the second floor.Could stuff in the basement or from my neighbours by crawling along the mains or ground to the lab?I'll probably rule this out next time I hunt for it, but could the circuitry in a iDevice leak strongly enough to cause this (with the radios turned off)? (Placing my iPhone right up to the coax cable doesn't impact the noise)As an aside, is it feasible to build myself some filters for the mains powering my lab gear to avoid similar issues?Any guesses as to what's causing this (and yeah I killed the lights of course) to help me avoid going bonkers carrying stuff into remote places of the house?P.S. scopes involved are a Tektronix 475 and 2465. Last time I saw it there looked to be a strong 3rd harmonic riding on the 100Mhz, this time it looks like a second harmonic. I'd take this with a grain of salt because I don't have a spectrum analyser and I'm just eyeballing the waveform which is dubious at best.

author: madshaman
author_rank: Frequent Contributor
timestamp: March 24, 2013, 10:08:06 pm
content: Actually, it's more like 90Mhz, here are some pictures (hope they worked).

author: Unknown
author_rank: Unknown
timestamp: March 24, 2013, 10:37:07 pm
content: A frequency of around 100 MHz might suggest FM broadcast radio, although RF typically has a small bandwidth and wouldn't have much harmonics.

author: madshaman
author_rank: Frequent Contributor
timestamp: March 24, 2013, 10:49:02 pm
content: *Palm to face*, don't laugh at me, but being a newb wrt radio electronics, the fact that this frequency didn't ring a bell when I see similar frequencies staring at me from my car radio every day literally made me laugh out loud at myself.Now, I have no idea if the trace in the photos I posted are indicative of a radio broadcast, or if it's even possible to tell (I don't have a receiver, tuner or any RF gear (yet)).Sounds like mystery solved if this is what it is.Out of curiousity:1) Is it common to see radio broadcasts on scope traces?2) Any way to get rid of it short of covering my walls with $$$ of copper foil?3) If I wanted to investigate further, what equipment/methods would I use? Could I easily find out what direction it was coming from?

author: C
author_rank: Super Contributor
timestamp: March 24, 2013, 11:03:37 pm
content: Just to make your day, RFI can be the result of the sum or difference of two frequencies. A poor connection ( a rusty bolt) between something that acts like an antenna for each frequency = mixer. It can be very hard to find. Best hint I can give is, if it shows on your scope, try to make your scope mobile with it still showing. Then you might be able to move scope closer to the source to find it.C

author: madshaman
author_rank: Frequent Contributor
timestamp: March 24, 2013, 11:12:47 pm
content: Quote from: C on March 24, 2013, 11:03:37 pmThanks :-), luckily, if there is an implicit mixer somewhere it's not in my gear, at least not anything in my lab. I see this trace on two separate scopes connected to or probing different equipment. I guess if I rule out the UPS I could use it to power the scope (so I could carry it further than the powercord will allow or having to worry about tripping on an extension cable); seems a bit dodgy but worth a try.

author: C
author_rank: Super Contributor
timestamp: March 24, 2013, 11:26:08 pm
content: I may be seeing things, Second photo looks like you picked two positive cycles in that wave. Do they repeat to two negative cycles and possible two positive again?

author: madshaman
author_rank: Frequent Contributor
timestamp: March 24, 2013, 11:37:44 pm
content: If you mean do I see the same pattern repeated but negative elsewhere? not that I can see.Those pictures are probably not the best to post, that's actually the waveform riding on the output of my function gen and triggered by it. When I get around to it I'll do one better and try to catch it on my digital scope. It's got a massive sample memory, but only 8-bit ADCs and a 200Mhz BW, I mostly use it for digital circuits.

author: C
author_rank: Super Contributor
timestamp: March 24, 2013, 11:54:23 pm
content: Try setting the function gen and scope on a cart. Move them around on cart for max signal. Then try roting the cart. Think dipole antenna, sides of antenna strong signal, ends is weakest signal. Easer to follow the strong signal but the weak signal from the ends is a better pointer.C

author: madshaman
author_rank: Frequent Contributor
timestamp: March 25, 2013, 01:02:49 am
content: Quote from: C on March 24, 2013, 11:54:23 pmThanks, will try that. Getting it on my MSO-9212 was a total fail (didn't have function gen in the loop, just a loop made from a 100Mhz probe), my faster probes are 10x so that's no good. It's just not sensitive enough at it's limit (5mV/div), I can *sort* of see the same signal with my eye, triggering on essentially nothing, but the *signal* at this level is quantised to maybe 2-3 bits at best, which makes the FFT total garbage. Maybe I can find a way to amplify it...Closest thing I have to a cart here is a rollable TV stand; will try that.

author: C
author_rank: Super Contributor
timestamp: March 25, 2013, 03:58:00 am
content: Saying it a different way, You need a bad noise radio. Your Function gen & scope are working for this now. Some part of the combination is acting like an antenna. Worse case, the power cords is the antenna. If it's not the power cords, then by rotating the combo you are rotating the antenna, and most all antennas have some directional properties to them. A setup on an office chair that has wheels and tilt may be better then TV cart. Quick safe move, rotate and optional tilt with out moving the function gen with respect to the scope which would change the radio is my suggestion.This may or may not help to find the source. I spent a week once trying to find something like this with real fancy equipment. I finally found the problem miles away and was a poor connection between two pieces of medal.If you already understood before the above, Great, if not hope above helps.Good luck hunting,C

author: ftransform
author_rank: Frequent Contributor
timestamp: March 25, 2013, 05:51:46 am
content: This kind of shit would drive me insane and I would probably start ripping wiring out of the walls.Can you maybe rule out the UPS, plug the scope into it and then flip the main distribution breaker to your house? That would save you a lot of effort. I don't see the point in playing wack a mole with a TV cart. After you rule out the house you can put the scope and UPS on your back and climb on the roof and nearby trees!One day I will have a faraday cage...

author: C
author_rank: Super Contributor
timestamp: March 25, 2013, 07:52:39 am
content: If I was going to use a UPS to go mobile as stated, I would try first with the USP disconnected from mobile rig and shutdown. The UPS could be the RFI Antenna or the RFI Source. Off and disconnected would tell a little. Then I would watch what happens each step1. Connect the UPS to wall2 Turn on UPS Input3 Turn on ups output4. Connect a different load to UPSIf none of the above made a difference then5. Connect mobile to UPS.6. Disconnect UPS from wall & remove UPS input cord if possible.Any of the above could show more or less noise on mobile.A pain, you bet Tracking down RFI reading the small hints.Killing the main breaker of house does not rule out all in the house,A mixer is any non-linear device.For example:a wire acting as an antenna for an AM radio station.a wire acting as antenna for an FM or TV station.connect the two wires with a diode or any non-linear deviceYou then have an RFI transmiter.Faraday cage,Think of what happens with two wifi devices in a Faraday cage. instead of a weak signal an few echo's, you have a strong signal with lots of strong echo's.C

author: ftransform
author_rank: Frequent Contributor
timestamp: March 25, 2013, 07:58:58 am
content: Quote from: C on March 25, 2013, 07:52:39 amWell at least it will rule out active sources.

author: SeanB
author_rank: Super Contributor
timestamp: March 25, 2013, 07:45:45 pm
content: Do you have cable internet? Could be poor connections on the cable box, loose earths or corroded wiring makes an excellent mixer, and couples into the house quite well.

author: madshaman
author_rank: Frequent Contributor
timestamp: March 26, 2013, 01:43:45 am
content: Lot's of great input. Yes ftransform, it *is* driving me nuts, and you guys are gonna have me knocking on someone's door some day holding a parabolic antenna:(door opens): "Yes?"me: "Something in your house is interfering with my equipment..."Nice call on the cable connection SeanB, that's definitely one of a few things shutting the mains off won't rule out.What I really need is a highly directional broad-band antenna and a portable high bandwidth spectrum analyser; I don't know how to build such an antenna yet, and building a spectrum analyser isn't going to happen until I finish my frequency counter, signal generator and, god help me, the oscilloscope.Ever since I've beefed up my lab with an assortment of high precision+accuracy DMMs, and soon a high precision+accuracy time reference, I'm finding myself thinking all sorts of crazy thoughts like filtering and isolating the mains supply to the lab, and yeah, even considering some EM shielding.I'm a music guy, so I understand a bit about room acoustics, I can't see the inside of a faraday cage really being much different, that leads me to think some of the same techniques can be used for EM suppression inside a shielded lab. I.e. using dampening and dispersion, the cage itself would do some dampening, but there has to be an EM equivalent to accoustic green goo or acoustic foam which will convert EM energy to heat over a broad spectrum, keeping the inner walls non-parallel is a good start to eliminate any resonance and an uneven fractal-like conductive surface would be even better.I'm positive this problem must have been solved in labs all over the world. Maybe I'll try an IEEE search :-).I'm going to track this noise down though if it's the end of me, already learned a few things trying to find it (like my DSO (obviously not all DSOs) is crap for this task, both the sensitivity and 8-bit resolution contribute to this; made me also decide to test it's *real* front-end bandwidth once I've built my signal generator, curious as to whether it's really 200Mhz as claimed).I have a feeling that, much like a really intractable software bug in a big system, once I discover what it is, I'll slap my head and say "of course!". Still betting on it being a radio station but trying not to assume that.I love this hobby.

author: kfitch42
author_rank: Frequent Contributor
timestamp: March 26, 2013, 04:43:59 pm
content: Here's the solution ... looks like it might do well for both RF and Audiohttp://www.eevblog.com/2011/09/22/eevblog-202-emc-rf-anechoic-test-facility-tour/Installing one in your house might get just a bit pricey

author: madshaman
author_rank: Frequent Contributor
timestamp: March 26, 2013, 07:24:20 pm
content: Quote from: kfitch42 on March 26, 2013, 04:43:59 pmHoly *!@#&!, I can't believe I overlooked this eevblog video! Fofl, it looks a lot like an acoustic anechoic chamber. Gonna watch while I code, maybe it's possible to build something similar for less cost ^^'.

author: ftransform
author_rank: Frequent Contributor
timestamp: March 27, 2013, 08:49:53 am
content: I feel that a Jim Carrey movie can be made about him as a HAM or EE, going to peoples houses with a parabolic antenna and spectrum analyzer, insisting that they let him analyze their cellphone chargers.I heard of one situation where someones neighbor had a cheap fluorescent light installed over their garage and someones equipment would go nuts when he turned it on.He could get progressively more neurotic as the movie goes on.

author: SeanB
author_rank: Super Contributor
timestamp: March 27, 2013, 06:28:40 pm
content: The foam they use there is made with a very high carbon particle content, so that it is very lossy to RF. We used this as a test bed for airborne radar units as a test fixture, new foam would smoke and burn for the first hour or so of installation, as the binders burnt off from the 5kW pulses. To do direction finding just use a FM radio, and use the signal strength output ( means either build your own with a chipset that does give a signal strength output or at least a RSSI signal), and use a simple FM radio external antenna ( big but pretty good for foxhunting at this frequency, you just mount it on a broomstick and rotate to get bearings) to do the DF triangulation.At 90MHz it probably is either a car FM transmitter or one used to broadcast inside the house for music reception.

author: C
author_rank: Super Contributor
timestamp: March 27, 2013, 08:14:18 pm
content: SeanBHere is one for you. Great Lakes is a Navy training base north of Chicago. Back in the 70's, one of the things trained on was very high power ship's radar system. They actually had the complete setup. Big radar antenna mounted on the roof of a building near to the lake. For safety they had limit switches that kept the beam pointed out over the lake.One free guess what some student disabled.The antenna did a couple 360's at full power before they realized there was a problem.Light bulbs exploded or burnt out. It even affected operations a o'hare airport.C

author: SeanB
author_rank: Super Contributor
timestamp: March 28, 2013, 04:34:55 pm
content: We just used the Radar dummy load as a pie warmer. I used the computer under test quite regularly ( when it was closed up and the 300 capscrews were back in place to hold the lids on) as a cup warmer, it would brew quite a nice cuppa if you turned off the fan in the test area. I wanted a nice power supply, so designed and built one that used the 28V power supply as power, and I rated it to give 0-24V at up to 40A ( the breaker in the wall) but I needed beefier cabling in the wall, I could only get up to 20A as the wiring in the trunking was 1mm only. In the main panel it was 50mm cable and a 100A breaker, and the main TRU was a 28V 16kA power supply. It never moved off the zero on the ammeters. 3 phase transformer, massive fan cooled bridge and almost no capacitance on the output, voltage regulation by a saturable inductor in the power feed. It probably weighed 30 tons in all, along with the 115V 400Hz rotary converter that was part of the room it was in.

author: madshaman
author_rank: Frequent Contributor
timestamp: March 28, 2013, 04:47:46 pm
content: Quote from: SeanB on March 27, 2013, 06:28:40 pmHmm, I'm wondering if it's feasible to roll your own foam of this type; right now it's a bit blue sky, but I can see myself radio-ioslating my lab at some point. With the long weekend coming up, hopefully I'm going to find the time to track down the offender. I've got an uncommitted digikey order still, so maybe I'll look into building your suggested rig, any suggestion for a slightly less massive antenna?

author: ftransform
author_rank: Frequent Contributor
timestamp: March 28, 2013, 05:27:32 pm
content: Quote from: C on March 27, 2013, 08:14:18 pmahhahaha, please tell us more.

author: kfitch42
author_rank: Frequent Contributor
timestamp: March 29, 2013, 06:22:59 pm
content: I wonder if something like this might help in your hunt:http://www.rc-cam.com/forum/index.php?/topic/3891-20-spectrum-analyzer-for-testing-fpv-systems/For $20 you can't expect spectacular quality (don't see much mentioned about sensitivity/noise...), but might be good enough for DF.

author: madshaman
author_rank: Frequent Contributor
timestamp: March 29, 2013, 07:22:00 pm
content: Quote from: kfitch42 on March 29, 2013, 06:22:59 pmYes, I do believe it would, spaghetti monster praise the internet! That link went right into my "cool" bookmark folder. Thanks!

author: madshaman
author_rank: Frequent Contributor
timestamp: March 29, 2013, 07:36:00 pm
content: Can anyone recommend a good reference/survey/starting-point/you-tube-videos for me to get familiar with the principles of and start learning the specifics of antenna design?

author: C
author_rank: Super Contributor
timestamp: March 29, 2013, 08:34:20 pm
content: madshaman,In the past I had access to all the fancy tools to find sources of RFI. I found that unless you can quickly rig a better tracker,(IE get the fancy stuff to show the RFI) to go with what is working. I could get better results faster using a pile of crud that showed the RFI vs fancy RFI tracker that did poorly. In your case, it's money spent on might be better vs time with what you have.If moving your scope & function gen around in the room and rotating it 360 shows no differences then it will not work and you need to try something and have wasted a few minutes of your time, but if it does then you have more money for the good things.C

author: C
author_rank: Super Contributor
timestamp: March 29, 2013, 09:29:29 pm
content: Antenna'sThere is some good info on the net and a lot of very bad as far as antenna's. I saw a news item the other day where an Italian took a good wifi dish antenna sliced it from edge to center to create a spiral dish if that's the word. He has a radio link setup with regular wifi dish an beside it his spiral dish. States he now has two wifi links that are independent. I can see it working, a lot of antenna designers will probably say "no way" until they see it for themselves.You have probably seen the saying " electronics is F**** in magic" well antennas are a magic all their own. I have often seen a description of how an antenna works be wrong. Also sometimes, something that works, is very hard to enplane.So here is something to get you started with. If you look at a sign wave on your scope, antenna is trying to make something that may look like that but is very weak, large enough for electronics to see it. A dish is like a mirror, while most antenna's are trying to act like an oscillator. The idea being that it takes very little energy to oscillate and in the process of trying will make the signal larger. You want the peaks at the antenna ends while you hold it at the zero point. With light you use lenses and mirrors, an antenna does the same normally with wires, rods, pipes acting like the lenses and mirrors.Good old wikipedia is not a bad start point, then ham sites. If you want to see large, google "cdaa" and to to wikipediaC

author: madshaman
author_rank: Frequent Contributor
timestamp: March 29, 2013, 10:53:13 pm
content: Thanks C. This is usually why I ask for recommendations of resources. I'm not an expert at anything myself but when I look for a text or other source material I try to find what's considered the best of the best. The instruction is usually more coherent and is, shall I put it kindly, on the other end of the spectrum from "I'm getting free energy by electrolysing water".A couple of my favourite authors are Jim Williams and Douglas Self, but personalities are independent of truth in science, and any good reliable source is good. More often with ACM articles than IEEE (I'm a computer scientist by trade) I find hidden brilliance in random papers, sadly wishing these people knew how good they were.Re antennas: what you've said I pretty much understand from basic principles and my experience with audio, computer simulations of the wave equation, and to a laser degree lasers, however, at ~= 100Mhz I'd have a resonator of 3 metres in length; hardly portable. I know that there are antennas that operate efficiently at fractions of a wavelength, I know there are antennas that are efficient over a broad band of wavelengths and I have a basic understanding of things like a parabolic reflector. I know there are "fractal" antennas.I don't mind stuff that "seems" magical, but I like a good solid explanation of why the magic works (preferably with equations and their derivation). I don't mind stuff that works but isn't fully understood why it works, just so long as that's really the case and not just lack of knowledge.When I build stuff, I like to be able to design the thing myself from the ground up, or at least *fully* understand an existing design, and that requires really knowing what's going on. I'm a bit of a goof and let my imagination run wild, put when it comes to a task, I'm pretty serious, even for fun projects.Hopefully that gives you an idea of the kind of resource I'm looking for, and I will hunt around!It's probably obvious my next thought was to buy one of those DVB-T TV dongles, turning much of the task into a software problem, leaving me with the task of building an antenna for it and maybe rig myself some kind of broadband direction finding tool, eventually making it portable (not tied to the PC or laptop)I'm putting up new shelving in my lab right now, so I haven't gotten back to the noise hunt yet, but I am going to follow the common sense suggestions first. I am thinking to the future though for the next time I need to hunt for a noise source, it would be nice to have a quick and dirty tool to help me out :-)

author: C
author_rank: Super Contributor
timestamp: March 29, 2013, 11:26:30 pm
content: If read between the lines so to speak, nothing says a very short or very long wire will not work as an antenna for _____Freq. Less than perfect just means less range, weak signals not heard. Some things you can counter act, others you live with. Those rubber duck antennas you see are very very poor, but if you can stand shorter range and the transmitter can protect it's self, rubber duck = short range while better antenna = more range. It could also mean better antenna = longer on battery.C

author: madshaman
author_rank: Frequent Contributor
timestamp: March 30, 2013, 12:19:59 am
content: Quote from: C on March 29, 2013, 11:26:30 pmIf I get what you're saying, I should imagine an antenna a bit like a band pass speaker enclosure (or any resonator really, laser cavity, flute, etc.), where the antenna length isn't an integer multiple of the wavelength I won't get maximum gain and in the worst case near 0 gain where the length is an odd integer multiple of half the wavelength. That leads me to think there must be software which given an antenna model, returns the frequency response of the antenna (obviously I could get this easily for trivial ideal antenna models myself using Mathematica). Anything open-source (I'm not a simulation guy, but a half simulation, half real world guy ;p)? Like anything domain specific I imagine commercial software must have ridiculous prices like $50,000 per seat.

author: C
author_rank: Super Contributor
timestamp: March 30, 2013, 01:03:22 am
content: I do not know any free software for this, I have not looked, A while back I was looking a site where they were talking about some commercial software. From what I was reading, it sounded like that software would have said the spiral dish would not work.It is a shame, but some of the early people's designs are unheard of today. You find a lot of cap tuned radios. If you dig, you can find some old ham radios that are inductor tuned. I have worked with both and no tuned ones, but I have only seen one that did both when tunning it.In the different category was an IF band pass filter. There was a long L shaped U in the bottom of the chassis. At one end was a coil paralleled by a cap connected to an amp output, with the coil paralleled by a cap connected to an amp input at the other end, Normal almost. There were no other electrical connections between the ends. Spaced along this L shaped channel were (coil paralleled by a cap) on insulators and no connections. They covered the U with a medal cover that had small holes you could uncover to tune each coil/cap pair.If my description is good, can You understand how it worked?C

author: madshaman
author_rank: Frequent Contributor
timestamp: April 02, 2013, 02:43:07 pm
content: C, I have sort of an idea, but I'll admit my mental picture of the setup is a lottle hazy. Was imagining it worked much like a recorder does.

author: madshaman
author_rank: Frequent Contributor
timestamp: April 02, 2013, 02:48:08 pm
content: Also, with what I've got on my plate right now I might *never* get around to it, but if this kind of simulation software doesn't exist, I'm tempted to try to write it.If I do start giving it a whack (atm the timeline is "real soon now"), I'll be sure to post some video of prototypes. Don't expect anything until at least the fall, already have 6 full fledged projects on my bench.Anyone care to give me a couple million dollars so I can quit my job and tinker full time?

author: C
author_rank: Super Contributor
timestamp: April 02, 2013, 04:29:14 pm
content: Think backward.In stead of finding someone to give you money,Be Mean,Thinker up something so that,They are begging to give you moneywhich could equalGoing to work to Play.C

author: madshaman
author_rank: Frequent Contributor
timestamp: April 10, 2013, 11:20:05 pm
content: Quote from: C on April 02, 2013, 04:29:14 pmWheels have been turning ever since!

author: vk6zgo
author_rank: Super Contributor
timestamp: April 14, 2013, 07:14:33 am
content: A 100Mhzhalf wavedipole is actually around 1.425m long,as there is a correction of around 0.95 applied to real antennas.Full wave antennas are not commonly used.If,as you believe,there is some 200MHz involved,you can halve the dimensions.Back in the day,you could probably wait till the "wee small hours" of the morning when the FM stations went off air,& see if the signal went away.If there is enough 100MHz signal to see on an Oscilloscope,buy a cheap portable radio & tune around--you should hear it go silent as you tune through the carrier.Crappy little radios are much more sensitive than an Oscilloscope!

author: madshaman
author_rank: Frequent Contributor
timestamp: April 18, 2013, 08:21:14 pm
content: Quote from: vk6zgo on April 14, 2013, 07:14:33 amThanks, I will try! Been absorbed in other stuff recently including a minor lab reno :-), I've yet to get back on the noise hunt... Now, if I can actually *find* a cheap FM radio any more these days..

author: vk6zgo
author_rank: Super Contributor
timestamp: April 19, 2013, 02:50:24 am
content: Some cellphones have a built-in FM radio function.Never used it,don't know if it actually works!