Strange problem with a strange solution - what's the real source though? - Page 1
Forum section: electronics / general

author: jewelie
author_rank: Regular Contributor
timestamp: May 05, 2016, 10:00:44 pm
content: TL;DR Erratic performance on laptop trackpad when plugged in with original charger solved by disconnecting house from communal UHF TV aerial!Hi folks!For the last year the trackpad on our Dell Inspiron N5030 has been intermittently erratic. Fed up with it, I decided to try and fix it; I assumed it was the trackpad or the cabling between it and the laptop that was faulty, or perhaps I'd broken something when upgrading it at some point (goodness, a CPU upgrade and an SSD can make all the difference!)I did some Googling for parts and also reports of similar problems and turned up LOTS of results about dodgy 3rd party laptop PSUs emitting RFI that's the route of the problem... so I did some experimenting and yes with the PSU connected there's problems, on just battery it's fine, and the problem also goes when the PSU is connected if I hold a shielded part of the laptop. However, this is the original PSU, not a WunHungLo and I just didn't buy it being the PSU itself in this case......so did some more investigating, just to check...The problem only occurred when connected to around half the sockets on a particular ring main. It didn't matter where the laptop + PSU were, just which sockets on the ring main it was connected to (tested through elemination using extension leads to compare the problem between different sockets with the laptop & PSU in the same place - then moving the laptop + PSU around and repeating the test.)Two sockets on same ring main behaving differently? **BUGGER** Must be iffy connection on ring main.Where? Obvious first guess is where one of the two-where-one-goes dual gang sockets were installed (one by an "electrician", one by my Dad who used to be qualified to do such things, and two by me who's not qualified to do that sort of thing but therefore is REALLY careful to do the job as properly as I can.)Some basic tests later and it seems the continuity measured between sockets around the mains ring is fine (as fine as I am able ascertain with the equipment I have.) So, not the mains ring! This is getting odder and odder... perhaps it's RFI from other devices propagating part way around the ring? All devices turned off. Nope, that's not eliminated it.Hmmmm..... phoned Dad, who suggests to make sure to physically disconnect everything from the mains plugs, including 4-way adapters etc.So, off we went, disconnecting absolutely everything from sockets, all sockets (including the landline/broadband and the communal aerial.)AH-HA! GONE!Cool. So we're getting there! Now to start reconnecting things, one by one, slowly slowy catchy monkey......GOTCHA!With all equipment physically turned off, except for the laptop+PSU, and the communal aerial attached to any piece of (turned off) equipment, we get an RFI problem that messes up the laptop trackpad when the laptop is connected to around half the sockets on the ring main (those nearest the device that's connected to the communal aerial and plugged in but not turned on.) It seems it's NOT travelling "through the air" though but travelling from the communal aerial socket, through the turned off TV, into the mains, and going part way around the ring, and is strong enough to travel a fair way down the extension, through original laptop PSU and bugger up a laptop trackpad!SO....Any thoughts on this mystery RFI from the communal aerial and what I could do about it?Could it be the 4G mobile masts that have been introduced here? Bloody stupidly they're pumping out the new 4G signal slap-bang in the middle of what used to be used for analogue TV here until recently (unsurprisingly there's some expectations of some problems in some areas and there's a service to supply free filtering equipment.)We've just had two pieces of equipment fail early, one high quality ATX PC PSU (general consensus of opinion on here was a premature MOV failure) and now a printer (unknown electrical problem - manufacturer error codes can be bloody useless at times.) Any possibility of a connection to the above that I should be concerned about?Any thoughts how I could investigate further? I don't have a scope, or even decent meters.Any ideas how I could solve it? Somehow decoupling from the communal aerial feed, a bandpass filter perhaps?Warm RegardsJuliexPS Hope this is okay here in general, I didn't know where best to put this?

author: Brumby
author_rank: Unknown
timestamp: May 06, 2016, 12:43:57 am
content: Quote from: jewelie on May 05, 2016, 10:00:44 pmIf the communal antenna has elements tuned for the old analogue TV band, then I would expect it could be doing its very best to pick up the mobile signals and deliver them to everyone connected. Just what you want, eh? This would be even worse if the new mobile masts were along the line of sight between your antenna and the old analogue system transmitters. That would be my first check.QuoteI think you will find that the 'free filtering equipment' would be bandpass filtering. For the price, it might be worth following up.A communal antenna without elements tuned to the mobile signals would be another consideration.The fact that you can (electrically) move around the ring main and get far enough away from the bridging device for the problem to fade indicates to me that the problem is marginal and a small attenuation of the problematic signal could yield an effective solution.

author: Delta
author_rank: Super Contributor
timestamp: May 06, 2016, 02:08:18 am
content: Is the laptop PSU earthed? (Three pin C13 "Kettle lead" / C5 "clover leaf", or a two pin C7 connector to the brick?)I'd be a little bit perturbed that your home doesn't have a good earth.Do you get the problem if only the centre conductor of the communal TV coax is connected to your telly? I'm not convinced by the 4G explanation (stupid though that allocation was), I reckon it might be either the distribution amp or someone's telly connected to it firing crap down the braid... BUT if you've got a good mains earth this should all be shunted away...

author: German_EE
author_rank: Super Contributor
timestamp: May 06, 2016, 07:41:04 am
content: When manufacturing power supplies sometimes corners are cut, either to save space or, more likely, to save money. One of the things often left out is power supply filtering, the class X and class Y capacitors along with the common-mode chokes that you should see on any mains supply. I bet that these are missing.One thing that you can try is wrapping a few turns of the mains lead around a ferrite rod or ferrite ring. Depending on where you are these could be difficult to obtain but if you ask at the local electronics component shops and explain that you have an interference issue then hopefully they will have something useful in stock. Looking for some pictures I even found a UK example:http://www.zen22142.zen.co.uk/Circuits/Power/mainsfilter.htm10/10 for the detective work

author: Brumby
author_rank: Unknown
timestamp: May 06, 2016, 02:54:40 pm
content: Quote from: German_EE on May 06, 2016, 07:41:04 am+1

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 06, 2016, 02:57:58 pm
content: Perhaps a nearby commercial radio station (AM or FM) that's using the house wiring and antenna cable as a makeshift antenna?Tim

author: jitter
author_rank: Frequent Contributor
timestamp: May 06, 2016, 08:28:09 pm
content: Quote from: Delta on May 06, 2016, 02:08:18 amAre you also thinking of a groundloop?

author: jewelie
author_rank: Regular Contributor
timestamp: May 07, 2016, 01:39:18 am
content: Quote from: Delta on May 06, 2016, 02:08:18 amYeah. I don't like being connected to the communal aerial system for that very reason! I'd really like to find a filter to decouple me from DC and lower frequency stuff, but I'm not having any luck - every time I think I've found a bandpass filter, it turns out it's really a low pass filter and carries DC etc. However, the issue I'm experiencing almost certainly IS HF interference (see below) below the TV frequencies, so if I can find a high pass that'll do.And yes, the laptop PSU is three pin, it's properly earthed.Many thanks!

author: jewelie
author_rank: Regular Contributor
timestamp: May 07, 2016, 01:42:41 am
content: Quote from: jitter on May 06, 2016, 08:28:09 pmYes, if nobody else is, I certainly have been. There's potentially multiple issues with having a communal aerial systems.Any thoughts how I should best test for that?

author: jewelie
author_rank: Regular Contributor
timestamp: May 07, 2016, 01:46:25 am
content: Quote from: German_EE on May 06, 2016, 07:41:04 amThank you. If you were impressed by that, you'll be impressed by tonight's investigations.(See below.)With regards to the ferrite ring, I don't currently have any big enough, but it's a good excuse to get hold of some larger ones.Would I need to put it around the mains side (which is earthed, so I don't know if that would confuse matters) or can I get away with doing that around the DC power cable side do you think? There's already a ferrite bead on it I notice.

author: Delta
author_rank: Super Contributor
timestamp: May 07, 2016, 01:47:54 am
content: Does the problem occur if you connect only the centre conductor of the TV aerial coax to your telly?

author: jewelie
author_rank: Regular Contributor
timestamp: May 07, 2016, 02:45:27 am
content: Quote from: Brumby on May 06, 2016, 12:43:57 amThanks! I've been doing more investigations.Mobile Mast InterferenceThe old UHF TV frequencies used were UHF channels 21-69. With Digital TV that's been reduced to 21-59, with 60 and above (about 800MHz) being used for 4G. The other GSM mobile frequencies used here are 900 & 1800MHz. So the mobile signals are above the TV frequencies (only just so, in the case of 4G.)FiltersSadly, this means almost all the 4G and GSM filters are only low pass that I can find, so even if they filter out 4G they don't decouple me from the communal aerial's DC or low frequency AC. There are a few genuine bandpass filters, but nobody makes them wide enough for our TV signals (which stretch from channel 31 to 59) being in a multigroup channel area isn't typical.I thought I'd found one that was a suitable bandpass, as it said that it would filter TETRA, GSM & 4G. Tetra is around 400MHz and therefore lower than UHF TV signals. When I contacted someone about it, they spoke to the manufacturers, who said it passes DC and is just a lowpass filter, but I don't see how that can be the case, unless it's got a notch filter for TETRA too or something to just carry the DC purposefully to the mast, but that all seems overly complicated and I suspect they're wrong and it's just a bandpass.Further InvestigationsWell.... I said I didn't have any suitable equipment to investigate further, but then I'd forgotten the Raspberry Pi on the window with an DVB-T USB dongle quietly decoding aircraft ADS-B and uploading it to FlightAware24.Hang on.... I've UHF TV channel tuned device that can work as a software defined radio! DOH. Now that's very definitely potentially something I can investigate further with, albeit in a very Heath-Robinson style.I spent a lot of time playing and Googling and learning (e.g. what a DVB-T spectrum looks like) and I think I've found the problem.For comparison, here's an image of the start of the Digital TV DVB-T multiplex here on channel 37 (centre 602MHz). For it to reach -15(dBm?) on the graph requires the RF gain on the thing to be turned up to maximum: 49.6dB.I looked around the UHF spectrum around 400 to 1000MHz and found all sorts of fun and funky things, but nothing too nasty. I could see the 4G very soon after the highest TV channel we use, but it wasn't that much stronger than the DVB-T signal. So I don't think that's it.I then tried looking at frequencies just below UHF TV and eventually found something that looked a bit suspect. At anything other than extremely low gains, it completely saturates the device and gives me all sorts of strange phantom signals and aliases (with peaks that shifted in the other direction to which I was scanning) which confused me at first until I remembered sampling theory etc.To get it not to saturate and produce aliases, and to get it to roughly match the same signal strength of the DVB-T signals, I had to wind the gain all the way down: 1.4dB (sorry, I accidentally cropped the image too much.) So the mystery signal signal appears to be about 48.2dB higher than the DVB-T. And yes, it's really coming from the aerial - if I disconnect it the signal entirely disappears.Result?I went outside checked which direction the aerials here were all pointing, as suggested. There's a great big Police radio mast in the line of sight of the aerial about 400meters away. The Police here use TETRA, Airwave (now Motorola.) According to Wikipedia, it's 390.0125–394.9875 & 380.0125–384.9875 Mhz. The problem massively saturating strong frequencies are around 393Mhz. A-HA!GOTCHA!O0YAY!Warm regardsJuliex

author: jewelie
author_rank: Regular Contributor
timestamp: May 07, 2016, 02:47:08 am
content: Quote from: Delta on May 07, 2016, 01:47:54 amI believe it was occurring already when it's not even quite connected, when holding it up near the socket.

author: amyk
author_rank: Super Contributor
timestamp: May 07, 2016, 02:50:05 am
content: The "free filtering equipment" probably means they're giving out these:https://en.wikipedia.org/wiki/File:A_collection_of_Snap-On_-_Clamp-on_ferrite_beads.jpg

author: jitter
author_rank: Frequent Contributor
timestamp: May 07, 2016, 07:32:43 am
content: Quote from: jewelie on May 07, 2016, 01:42:41 amFirst how I found out about groundloops the hard way:Way back when I was still living with my parents, I had an audio/video system in one corner of the room and a PC in the other. All mains sockets were of the three prong earthed type. The TV, video and audio components were all class II double insulated devices with two prong mains leads, the PC was a class I earthed device, having the three prong plug.The TV was connected to a communal aerial (aka "cable TV") type. The TV audio was connected to the amplifier resulting in a grounding to aerial shield of all devices connected to the amp through the RCA leads.So far so good, since none of these devices had mains earth, a groundloop was impossible, until I decided to run the audio signal from the PC soundcard to my amplifier. As soon as I hooked it up, there was massive hum in the speakers, even with the volume set way down.Since I had heard about groundloops already, I also knew the solution: break the loop. The groundloop that I had created was: aerial ground>TV chassis>amp chassis>PC chassis> safety earth.To test my hypothesis I disconnected the ground lead from the PC and the hum was gone. Because of safety reasons, I didn't want to keep it that way, so I bought a 1:1 audio transformer sold in the car hifi business (designed to break groundloops in a car audio system). Later I ran the SPDIF signal (digital) output of the PC to the hifi system with a self built coaxial to optical converter.In the home a groundloop may consist of mainly mains frequencies and multiples, and of course noise from SMPSs. In a car it's mainly EMI from the ignition. It's why the RPMs of the enigine can be heard in the sound of the car audio system when there's a groundloop.The reason why you are having problems on some sockets and not on others may have to do with the surface of the groundloop. The bigger thebetterworse.The groundloop I had was huge as I was living in the attic with long lengths of wire from both the mains and the aerial. The protective earth of the mains system was connected to a long rod driven into the ground under the house and the aerial system would most likely have been earthed in the distribution box at the beginning of the street.How to test for a groundloop? I think the fact that running the laptop off batteries or disconnecting the aerial prevents problems from occurring strongly indicates that this might be a groundloop at work.You say you don't have a DMM, so maybe it's time to get one. For this job a cheapie that can do resistance measurements/continuity will do.Failing any measuring equipment, I would probablytemporarilydisconnect the third prong from the mains lead to the laptop charger. If the cause of the problems is indeed a groundloop, then they should disappear despite the laptop charging. But please note that disconnecting the mains earth from that lead should only be done for this experiment, leaving it disconnected creates an unsafe situation should the insulation in the charger fail. The current will then find a way through the aerial system to earth and it is not designed to do that.If it does indeed solve the problem, the long term safe solution would be to buy a good quality replacement charger with atwo prongmains plug. These are the so called double insulated type, they will also carry the double square symbol to indicate that. A random example of such a charger/adapter would bethisone.

author: jewelie
author_rank: Regular Contributor
timestamp: May 07, 2016, 08:05:49 am
content: Quote from: amyk on May 07, 2016, 02:50:05 amIt'shttp://at800.tv/who're handling it, and to be fair, given the nature of the devices out there, I suspect it would be a genuine high Q (if that's the right term) / fast falloff low pass filter - there's different ones available depending on how tight you need the filter to be (as used UHF channels go up to 59 and 4G starts at 60.)

author: jewelie
author_rank: Regular Contributor
timestamp: May 07, 2016, 08:25:24 am
content: Quote from: jitter on May 07, 2016, 07:32:43 amThank you for your story.I had similar experiences in my younger days and eventually also bought a 1:1 audio transformer (although I hated what it did to the sound.) I helped with engineering at a student radio station in my university days, so got very familiar with audio issues like ground loops too.Quote from: jitter on May 07, 2016, 07:32:43 amWhen I said I didn't have suitable testing equipment, I meant I don't have a spectrum analyser, an oscilloscope or a posh meter. Of course, I have a DMM, a couple in fact.Dave would hate them I'm sure, but they do me fine for what I use them for, and I'm probably not stupid enough to do anything with them that would put them in a position to potentially explode spectacularly (at least, not with me near them at the time anyway.) I do have a couple of very old poorly Fluke meters (original 73 and 87) coming soon from my Dad too, which I will attempt to repair if I can.I will repeat the test, but I seem to recall about 20VAC between the screens of the incoming communal aerial socket and my socket, depending on what equipment was turned on at the mains.Quote from: jitter on May 07, 2016, 07:32:43 amI may try that, although I suspect it'll make little difference given that I think neutral is connected to ground here. Also, I'm confused how a fault current could go directly through that path to aerial screen, because (other than testing with the SDR / DTV dongle) the laptop isn't connected in any way to the aerial?Quote from: jitter on May 07, 2016, 07:32:43 amIronically, of course, that's exactly the opposite to what is normally recommended to people who see this touchpad jitter due to interference problem, but I shall keep that in mind (and the product link) perhaps to combined with a toroid in future.Thanks flower!

author: jitter
author_rank: Frequent Contributor
timestamp: May 07, 2016, 09:06:43 am
content: Quote from: jewelie on May 07, 2016, 08:25:24 amAh, okay. The only need for the DMM is to check continuity.I'm expecting the - output of the charger to be connected to mains earth on the input side. If that's the case there should be continuity between those two pins and a groundloop is possible. Hence my advice to get a double insulated charger should a groundloop turn out to be the cause of the problems.QuoteAnother indication of a groundloop. A groundloop in itself is not a problem, but it becomes one if there's a noise source in it. With such a voltage differential (even though it's measured with a high impedance DMM), I'm pretty sure that's the case.QuoteThat's the case here too, but it doesn't matter. The charger has an output that is isolated from the primary side. In an earthed charger, that isolation isn't particularly strong and it needs the earthing should that insulation fail. That earth lead introduces a path for a groundloop to occur, though.In a double insulated charger the insulation is strong enough to do without the earth lead, giving true isolation from neutral and mains earth.QuoteMeasure continuity between aerial shield and USB shield. My guess is they're interconnected like on my TV aerial shield and RCA shield were interconnected. If so, that's the path.QuoteYes, but EMI/RFI can have many strange ways of rearing its ugly head.The groundloop messes with your signal reference (aka GND). Once that happens your signal is polluted (like the groundloop hum on a hifi) and it may become undefined leading to logic lockups and other rubbish with your pad.Also mains earth must not be seen as a vacuum cleaner for unwanted electrons. It's just not how it works.A double insulated charger can be very good at EMI/RFI suppression too. IF a groundloop is the cause of your problems, then it's actually more logical NOT to have the earthed charger.

author: SeanB
author_rank: Super Contributor
timestamp: May 08, 2016, 02:12:00 pm
content: You can get an antenna isolator for the TV set, which has 2 1n capacitors to break the braid and the centre conductor. Otherwise grab a length of TV coax, a F splitter ( the one that is only 3 connectors together) and 3 F connectors, and make a quarter wave trap to notch out the offending frequency. Trim to match.https://en.wikipedia.org/wiki/Quarter-wave_impedance_transformer

author: AF6LJ
author_rank: Unknown
timestamp: May 08, 2016, 07:16:08 pm
content: Let me get this straight;When you have everything plugged in except for the television coax; all is quiet.If you turn on everything with the coax disconnected (still) all is still quiet.Here try this;Get a a two or three snap on chokes.Put two on the TV coax and reconnect it. That might make the problem go away.If that doesn't work take the DC cable going to the laptop and loop it through one of those snap on chokes two or three times (whichever will fit and within 10cm of the DC plug going into the laptop and see if it goes away.This sounds like a simple the EMI being conducted into the room and coupled over to the laptop power supply.I hope this helps..

author: wraper
author_rank: Unknown
timestamp: May 08, 2016, 07:39:54 pm
content: Sounds like you have earth connection problem in your sockets. Probably wire somewhere is broken and half of the sockets share the "earth connection" but are not actually earthed. So any crap on that "earth" gets onto laptop ground end messes with touchpad.

author: jewelie
author_rank: Regular Contributor
timestamp: July 19, 2016, 05:57:23 pm
content: Hi allJust to let you know how this panned out...Tried chokes, an inline isolator in the aerial connection, a TETRA filter, nothing made any difference.Eventually tried to use a an AM/MW radio and noticed horrendous noise across the whole band. Walked around and found it was loudest near the mains cupboard supply to the communal aerial system. Turned aerial system off, MW radio back. Turn it back on, and it started again.So the aerial had turned into a rather powerful MW transmitter! :-PReported the aerial fault. Engineer came out and it turned out others had had issues with the aerial too. Spoke with the engineer. He replaced the mast head amp and filters and the distribution amp. He thinks it was something to do with 4G at 800MHz (our highest channel here is 790MHz-795MHz) - there was no filtering in place and the install pre-dated 4G (800MHz) and TETRA (393MHz).Problem gone.

author: AF6LJ
author_rank: Unknown
timestamp: July 19, 2016, 06:23:18 pm
content: Makes good sense...The masthead amplifier was being overloaded...Works for me.

author: jitter
author_rank: Frequent Contributor
timestamp: July 19, 2016, 07:20:54 pm
content: So, EMI/EMC after all...Good to know that the problem is gone.Reminds me of the analogue cable TV area when they put up abig digital transmitterfor wireless digital TV and radio nearby. All of a sudden some of the analogue channels on the cable were interfered with. All the cable TV companies did was advise people to get better shielded coax cables... Now everything has gone digital, it's no longer an issue.