Significant FM radio interference

[woodward]

The A20-OlinuXino-Micro board (Rev.C tested) emits enough RF noise to seriously degrade FM radio reception.

This appears to be a problem with the 6-16V input power supply. To reproduce the problem, take a vanilla a20-OlinuXino-Micro and plug a power supply into this while listening to an FM radio station (no sdcard is necessary).

The reason that I think it is the power supply circuitry, is that the FM interference stops if I unplug the 6-16V supply, while leaving the battery plugged in (BATTERY-LIPO1400mAh), and debian is still up and running, but there is no FM interference.

I have experienced this problem with both of my boards (I only have two A20-OlinuXino-Micros). I have tested in 4 automobiles, and with a little $15.00 FM radio from RadioShack. I have tested with wall power, car 12v power, and a separate 12V battery, each plugged into the 6-16V. I have tested with the radio on the same power supply as the A20-OlinuXino and with them on separate supplies. The problem existed in all tests.

This may not be Olimex's problem. Maybe all of these boards (RaspberryPi, BeagleBone, etc.) do this, and we just need to find a way to wrap them in a shield. I do not know enough about this stuff to make any suggestions. I tried wrapping the whole thing in aluminum foil, but I did not notice any improvement.

I am at a loss. I am hoping to make an automobile product out of the A20-OlinuXino. Any thoughts? Community? Olimex?

Thank you

[olimex]

please use iron metal box (aluminum is not magnetic isolation) which to be connected to GND

[woodward]

Thank you for the response. Enclosing in an iron metal box is definitely not ideal and shouldn't be necessary. There are plenty of Cigarette lighter voltage regulators out there that do not interfere with FM radio and are only enclosed in plastic. Again, it is likely the board's voltage regulator, since the problem does not exist when powered from lipo, only when powered from 12V.

Not interfering with FM radio would likely be a requirement for people making car pc's, which could be a large market.

I don't know anything about EMI. Maybe someone who knows about this stuff could take a look at that circuit and make a recommendation. We could pool money to pay that person (I could throw in
$200 or $300 US). It is completely reasonable that early board revisions would have problems, we just need to find a fix. Rev C seems to be emitting far too much RFI/EMI. Thanks again

[woodward]

Here is a sound file of the problem. You can hear the radio station completely cutting out. I plug in and unplug a 12V wall wart, a lipo is plugged in and debian is running the whole time (neither the lipo nor debian are necessary to see the problem). The radio is battery powered sitting on my desk, and the station is FM. Sorry, the audio is quiet so you have to turn it up.

https://drive.google.com/file/d/0B9Oqle-M8CLFaUNocXJTMTUtLWs/edit?usp=sharing

Again, no problem with a generic 12v input, 5v output regulator, enclosed in plastic, powering some other device.

Not knowing what I am doing, I attached a 100uF capacitor across the 6-16V input and there was a little improvement. I know we can fix this.

[olimex]

I guess the noise is emitted from the inductance of the input DCDC you can shield just this inductance

[woodward]

I just tested on a spanking new A20-OlinuXino-micro Rev.E. There is definitely a problem with the dc-dc emitting a lot of emi. You can see the problem by powering a bare board (no sd card, no battery, no display, no peripherals) via the dc-dc jack and trying to listen to a nearby radio.

[woodward]

I just sprayed an entire $35 can of Super Shield nickel conductive coating inside a completely enclosed plastic shell with only the a20-olinuxino inside, and grounded the sprayed shell to the board's ground; only a couple of ohms resistance at different test points). This is an industrial coating made specifically for this problem. There was only minor improvement. Still unacceptable interference.

Olimex - Will you address this?

It is understandable that problems like this will arise, but this level of noise emission makes the board unusable in a product. Especially if anyone hopes to receive FCC approval for their product. We can put $1000 toward your engineer's time to fix this for the community.

[Lurch]

Have you tried the Cubie-Boards? They use only a 5V supply and may not have that problem.  ITeadStudio also makes an A20 with 5V.  I suspect that Olimex will not redesign the board for just a few users having problems.

[misterzu]

Try to put such a ferrite filter on power cable just near A20 plug: http://en.wikipedia.org/wiki/Ferrite_bead

[woodward]

+1 misterzu, I purchased a bunch of chokes and and a couple of them made a huge difference, unfortunately there is still some interference. I will keep experimenting.

In the mean time, I am going to power the olinuxino's through the otg using a cheap dc-dc 5v regulator, similar to Lurch's suggestion. No interference at all and no metal enclosure necessary. And the lipo battery still charges, so I can cleanly shutdown when power is turned off.

https://drive.google.com/file/d/0B9Oqle-M8CLFSlNvS1JUbVZMZ28/edit?usp=sharing

The choke on this dc-dc converter must be better than the one used on the olinuxino board. It has 330 written on it. Maybe Olimex could swap in that choke for the existing one? Thanks again.

[agustin_nrtk]

I found the solution in Olinuxino A13 board. It´s simple and cheap.

Solder an 0.1uF capacitor in parallel to C126 (electrolytic capacitor at entrance of MP1482DS). That's all.

The problem is that the interference is transferred to the DC source, this capacitor acts like high frequency filter.

In the A20 schematics, the capacitor seems to be C202

Good luck.

[splite]

Hey all !

woodward i'm experience exactly the same problem as yours. Did you find any good solution ?

Here is a screenshot of with A20 on/off (yellow is A20 on, blue is A20 off : BIG DIFFERENCE!)


So far I tried the ferrite around the 12v cable (no improvement), I put a 0.1uf cap in parallel with C202 (no improvement).

I thinking of using a DC-DC 5v converter on USB 5v...My project aim to use ham radio software with RTL-SDR dongle and it's really annoying.

What do you recommend ?

Thanks
Florian

[splite]

I tried to find a solution in using a 12v -> 5v DC regulator to avoid the noise from the power jack alim.
I used the USB_OTG 5v connector but it is not really clear in the doc how much power can goes through this connector. I'm using some peripherals and the whole project is drawing about 1.6A at 5v.

Would it work with this USB_OTG connector ?

Thanks

[splite]

The USB_OTG is doing strange. I'm using an Ipod 5V/1A charger through the USB_OTG. The green led start bright and then switch off. I have read similar behavior on the forum but with no clear answer...
So far the USB_OTG is not working !!!

In my last hope I tried the LIPO_BAT charger. I have a LIPO battery fully charge with 4.2V. When I used it with my A20 Olinuxino everything is powered well. When I tried to use an external power supply set at 4.2V the card don't start...

Does anybody have any suggestion ?

[JohnS]

5V 1A isn't enough.

John