Electrically noisy radio lacks metal foil area... - Page 1
Forum section: electronics / general

author: ocset
author_rank: Super Contributor
timestamp: July 18, 2020, 01:22:05 pm
content: Hi,I have two DAB radios, a white one which has lots of bad fuzzy noise on it, and a black one which is fine.However, the white one works fine when on batteries instead of the mains adapter.I noticed that the (good) Black one has a metal foil area behind the main PCB (as shown in the attached).The circuit ground is soldered to this metal foil.The (Bad) White one does not have this metal foil. (as shown attached)Both Black and White radios come with 5V,1A mains adapters, but the white one’s mains adapter is only 2/3rds the size of the black one’s. (I guess it also has less common mode filtering inside).Anyhow, Do you think the metal foil in the Black radio acts like a kind of “chassis earth”, and helps in noise suppression?__________---------------------__________________------------Radio part numbers***White one is Model No: ONNDAB001Whttps://groceries.asda.com/product/stereo-systems-radios-alarm-clocks/onn-white-dab-radio/1000003249618***Black one is Model No: DB482BLACKhttps://picclick.co.uk/onn-DAB-Digital-FM-radio-DB482black-portable-163538005419.html#&gid=1&pid=1

author: chris_leyson
author_rank: Super Contributor
timestamp: July 18, 2020, 01:51:15 pm
content: QuoteNot surprising really, you've got an E field antenna (not a very good one), no ground or earth connection on the radio and god knows how much common mode noise generated by the mains adaptor. The foil probably acts as a artificial ground to some extent by capacitive coupling to a local ground and then there is the ferrite. Also, the DAB receiver probably has poor dynamic range and may be blocked to some extent by all of the common mode noise below 200MHz. I blame LED street lights.

author: ocset
author_rank: Super Contributor
timestamp: July 18, 2020, 01:57:42 pm
content: QuoteThanks, yes, my thoughts aswell. As discussed, the white one possibly has less common mode filtering in its mains adapter.......but the good black one with this foil..... It does seem that this foil area is actually acting as a kind of local earth and is helping to suppress the noise.Seems kind of weird that a sheet of foil can act as "earth"...but i suppose the same thing happens in cars and planes......where the metal chassis of the plane or car acts as "earth", and performs this noise suppression function.

author: chris_leyson
author_rank: Super Contributor
timestamp: July 18, 2020, 03:29:47 pm
content: Hi treez, the foil is likely to capacitively couple to the nearest grounded object but at 225MHz it's not going to be a very effective ground plane. Quarter wave is 330mm

author: tunk
author_rank: Super Contributor
timestamp: July 18, 2020, 03:41:47 pm
content: Assuming the plugs are the same, what happens if you switch the mains adapters?

author: ocset
author_rank: Super Contributor
timestamp: July 18, 2020, 09:47:03 pm
content: HiThanks, i just switched the adapters....and it was just as bad.I then switched the adapters back as before.......but removed the electrolytic capacitor from the little LC filter that i wadgered together (as shown attached), and now the white radio is working much better with its original mains adapter....Uuuuugh OK.I have been walking around the room (because it usually goes fuzzy when my body is in a certain place in the room).....but now there seems to be far less places where i can go and cause it to go fuzzy.....now i am only able to make it go fuzzy when i go right up against the white radio.The mysterys of RF.So anyway…it seems that having that electrolytic capacitor in the LC filter that I put in there (as attached) was the problem…….so this seems to confirm that having too big diff mode capacitors in an input filter can actually make the common mode noise worse…this is because the common mode can jump through the big diff mode capacitors and get to the other rail, from where it can cause EM radiation and interference…….would you agree with this?By the way, the only reason I removed the electrolytic was because when swapping the mains adapters, I accidentally reversed the supply….and presumed that maybe the electro cap was damaged…..it just shows that screwing up can sometimes be the best technique.I always assume that noise problems in electroncis are caused by common mode noise and not diff mode...since there is usually a decent diff mode filter so diff mode is never that bad....and also diff mode is at a lower frequency which cant jump about as much as high frequency common mode noise....common mode is highly infectious and can couple to things easily.These findings might all be false...since i live in a busy city centre in a busy flat block and its late in the evening now, and just maybe there is less ambient EM noise around at the moment.---------------------------------------------------------------------------------------------------QuoteSeriously though, I always buy cheap radios, they usually get lost or smashed when I move flat. None have ever been perfect….all can go fuzzy at times. You find yourself moving the wretched things round the table to make the fuzziness stop. And as Chris kindly implies, they are bound to be noisy. I wonder why anyone would ever buy an expensive radio?.....i mean, wont they still be fuzzy at times?....i mean, as Chris says, they have no earth connection……and so common mode filtering will be compromised.

author: coppercone2
author_rank: Super Contributor
timestamp: July 19, 2020, 01:13:17 am
content: there is something on the radio to listen to?is it better in england?and you buy a expensive radio so you don't need to deal with cheap crap breaking?

author: chris_leyson
author_rank: Super Contributor
timestamp: July 19, 2020, 10:04:32 am
content: Maybe the foil balances the antenna capacitance so that capacitively coupled common mode interference generates the same currents in the inner and outer conductors of the coax and the ferrite sleeve helps to attenuate any common mode noise. You could argue that the foil acts as a balanced ground reference for the antenna. In the white radio with no foil, I guess the pcb and maybe the coax outer conductor act as the antenna ground reference but it's probably not well bananced and hence the common mode rejection is not so good.Thanks for the pictures, if I get around to building anything with a similar antenna I think a foil patch is definately worth a try.

author: ocset
author_rank: Super Contributor
timestamp: July 19, 2020, 10:13:08 am
content: Thanks,I removed the electrolytic capacitor from the filter in post #5 above, and now the white radio works fine.The filter in #5 above, is one that i hacked in to the supply cable to the white radio.Why did removing the electrolytic capacitor in the filter of #5 make the white radio work nicely again?When the electrolytic was in that filter, then the white radio worked just as badly as when the filter was not there at all.